REESE LIBRARY 
 
 UNIVERSITY OF CALIFORNIA. 
 
ELECTRICITY flND MflGNETISM 
 
 LESSONS 
 
 NATIONAL SCHOOL OF ELECTRICITY 
 
 PREPARED UNDER THE IMMEDIATE DIRECTION 
 OF THE FACULTY 
 
 QENERAL COURSE 
 
 SECOND EDITION, REUISED 
 FIFTH THOUSAND 
 
 PUBLISHED BY 
 
 CHICAGO SCHOOL OF ELECTRICITY, 
 
 335 Dearborn Street, 
 
 CHICAGO. 
 
 1896. 
 

 Copyright, 1895 
 
 DONOHUE & HENNEBEKRY, 
 
 PRINTERS, BINDERS AND ENGRAVERS. 
 
 CHICAGO. 
 
INTRODUCTION. 
 
 YOUNG men and women, and older ones too, who have time 
 and money at their disposal, who have been blessed with a 
 sufficient preparatory education, and who desire to learn, some- 
 thing about electricity either for pleasure or profit, may do so by 
 entering and taking the very comprehensive courses at the colleges 
 and technical schools. But there are many of our brightest mechan- 
 ics, artisans and business men who have not had the benefits of a 
 sufficient primary education, especially in mathematics, to enable 
 them to pass the more or less exacting entrance examinations required 
 by the technical schools and colleges, and these and many others also 
 may not be able to part with the time or money necessary to secure 
 these benefits. It is to these classes of people that the National 
 School of Electricity especially appeals, and to meet their require- 
 ments, several conditions must be met. 
 
 ist. The course of instruction must be of an eminently practical 
 character so that the knowledge acquired may be immediately utilized. 
 
 2d. The course must be furnished at times and places con- 
 venient to participants during their leisure hours ; as they cannot go 
 to school it is necessary that the school be brought to them. 
 
 3d. As it is not always expedient to have graded courses, it is 
 lecessary to bring the course of study within reach of those of even 
 >ry limited education, yet have it include all that may be essential 
 lor the interest of advanced students. 
 
 4th. In order that the theories and practice of electricity may 
 be taught together, it is necessary to employ actual working apparatus 
 for the purposes of demonstration at all stages of the work. 
 
The lessons of the school were prepared to meet these conditions, 
 by Prof. Dugald C. Jackson, with the assistance and suggestions of 
 other workers of the faculty of the school. The course of lessons has 
 now been thoroughly revised and many additions have been made 
 where the advances of the science of electricity and its application 
 have made it desirable, and some alterations suggested by the experi- 
 ence of the instructors of the school, have been incorporated. As 
 a large new edition of the lessons is now being issued, and a consider- 
 able demand for complete sets of the lessons bound in permanent 
 form has arisen among the members of the classes of the school, this 
 volume is issued in response to this demand. The volume contains 
 the lessons of the general course of the school as they are used in 
 the instruction of the classes. 
 
 NOVEMBER, 1895. 
 
CONTENTS. 
 
 LESSON I. PAGE. 
 The Nature and Properties of Electricity 1 
 
 LESSON II. 
 Machines for Generating Electricity by Friction and by Electric Induction 9 
 
 LESSON' III. 
 Electric Batteries, or Appliances for Generating Electricity by Chemical Action. 15 
 
 LESSON IV. 
 Electric Batteries, or Appliances for Generating Electricity by Chemical Action 
 
 (Concluded} 21 
 
 \ 
 
 LESSON V. 
 The Nature and Properties of Magnetism. Magnetic Fields 27 
 
 LESSON VI. 
 The Magnetic Effects of Electric Currents, and Magnetic Circuits 35 
 
 LESSON VII. 
 Ohm's Law of the Flow of Electricity 42 
 
 LESSON VIII. 
 Heating Effects of Electric Current. Miscellaneous Effects of Electric Currents. 51 
 
 LESSON IX. 
 Galvanometers and Voltameters 58 
 
 LESSON X. 
 Measurement of Electrical Resistance 66 
 
 LESSON XI. 
 Every-day Measurements of Electric Currents and Pressures 74 
 
 LESSON XII. 
 Every-day Measurements of Electric Power. Condensers and the Measurement 
 
 of their Capacity 84 
 
 LESSON XIII. 
 Electrolytic Deposition of Metals 91 
 
 LESSON XIV. 
 The Electric Telegraph.... 102 
 
 LESSON XV. 
 Multiple Telegraphy 110 
 
LESSON XVI. 
 The Telephone 118 
 
 LESSON XVII. 
 The Construction of Telegraph and Telephone Lines and Instruments 125 
 
 LESSON XVIII. 
 Testing Lines for Insulation and Conductivity and the Location of Leaks and 
 
 Breaks 138 
 
 LESSON XIX. 
 Principles of Continuous Current Dynamos and Motors 140 
 
 LESSON XX. 
 Principles of Continuous Current Dynamos and Motors: Their Construction, 
 
 Care and Attendance 155 
 
 LESSON XXI. 
 Arc Lighting and Arc Light Machinery 167 
 
 LESSON XXII. 
 Incandescent Lighting and Power Transmission: Two, Three and Five Wire 
 
 Systems of Distribution for Electric Lights and Motors 177 
 
 LESSON XXIII. 
 Construction of Electric Light and Power Circuits and Their Testing 191 
 
 LESSON XXIV. 
 Testing Electric Light Circuits, and the Distribution and Measurement of Light. 205 
 
 LESSON XXV. 
 Electromagnetic Induction 213 
 
 LESSON XXVI. 
 Alternating Currents ., 220 
 
 LESSON XXVII. 
 Alternating Currents and Alternating Current Machinery. (Concluded} 228 
 
 LESSON XXVIII. 
 Miscellaneous Applications of Electric Motors 241 
 
 LESSON XXIX. 
 Electric Railways 256 
 
 LESSON XXX. 
 Methods of Handling and Controlling Railway Motors and Generators , 266 
 
 LESSON XXXI. 
 Model Electric Plants 279 
 
 LESSON XXXII. 
 Underwriters' Rules, Etc 297 
 
 LESSON XXXIII. 
 Electric Welding, Forging, Etc. Electricity Applied to the Kitchen 304 
 
 LESSON XXXIV. 
 Electro Therapeutics 316 
 
The National School of Electricity, 
 
 LESSON I. 
 
 THE NATURE AND PROPERTIES OF ELECTRICITY. 
 
 The exact nature of the electricity which makes itself evident 
 in so many ways has never been determined. Many surmises or 
 theories have been advanced, but none have yet been able to fully 
 stand the test of close examination. By experimental evidence, 
 which has been gathered for decades, we have been able to deter- 
 mine the laws which govern the action of electricity, though we do 
 not know its constitution, very much as we know the results of the 
 laws of gravitation, though we do not know what "gravity" really is. 
 
 The derivation and use of the word "electricity" has itself had 
 a development parallel with that of the experimental development of 
 the science which bears its name. Springing from the Latin name 
 for amber, electricus or electrum, the adjective "ELECTRICAL" 
 was first used by Dr. Gilbert in a book published in 1600 to desig- 
 nate the attraction for light bodies like chaff and bits of .paper 
 which amber and similar substances exhibited when briskly rubbed. 
 The original discovery of this electrical property is often attributed 
 to a Greek philosopher named Thales, who lived about 600 years 
 before the Christian era, and whose writings contain the earliest 
 records of its observation which have come down to us. It is prob- 
 able, however, that a knowledge of this peculiar property of amber, 
 and possibly of other bodies, was one of the well-guarded secrets of 
 the priesthood of that day. 
 
 From the word electrical came the word "ELECTRICITY." Since 
 the day Dr. Gilbert first applied the word electrical to a particular 
 phenomenon, our knowledge of all the sciences has widened, and 
 with the widening has come an equal advance in the knowledge 
 which was represented to the ancients by that one peculiar property 
 of amber and similar bodies. The term electricity is, therefore, 
 applied not only to one little branch of a great science, but covers a 
 vast field of facts which are supposed to be based on the same under- 
 lying cause. 
 
 The action of electricity led many experimenters after Gilbert 
 to the belief that it was a fluid not perceptible to the senses. Our 
 
own great philosopher and statesman, Benjamin Franklin, assumed 
 it to be a fluid, and bodies which exhibited electrical manifestations 
 were thought by him to contain either more or less than a natural 
 amount of the fluid. A Frenchman named DuFay and an English- 
 man named Symmer considered electricity to be composed of two 
 fluids, which were contained in neutral bodies in equal amounts. 
 When by any means this equality was disturbed in a body electrical 
 manifestations occurred. 
 
 We will not at this time further discuss the nature of electricity, 
 but will pass on to a consideration of its properties. The study of 
 these properties will be divided into two classes the first, in which 
 STATICAL ELECTRICITY, or electricity at rest, is considered, and 
 the second, in which CURRENT ELECTRICITY, or electricity in 
 motion, is considered. There is no well-defined division between 
 these, and the laws governing the two classes are practically the 
 same. In general, however, the first class, or static electricity, 
 includes the phenomena known by the ancients where electricity is 
 produced by rubbing or by the influence of one ELECTRIFIED 
 body on another. The second class includes electricity produced by 
 the electric batteries and dynamos which are so well known today. 
 The first class is of comparatively small importance and will receive 
 only such attention in the earlier lessons as is necessary on account 
 of its bearing on the second class. 
 
 If a rod of sealing wax, amber, or other resinous substance be 
 rubbed with dry wool or fur, it immediately gains the property of 
 attracting to itself light bodies, such as pith. After these bits of pith 
 have been in contact with the rubbed body for a short time, they 
 usually fly off as though repelled, and they also seem to repel each 
 other. The rubbed body when in this condition may be found by 
 proper examination to be covered with an apparent layer of electricity, 
 which is called a CHARGE, and the pith balls which have touched it 
 are also said to be charged. If now a glass rod be rubbed with silk 
 it will show the same properties as the resinous substance. If the 
 glass rod be brought close to the pith balls which have been in con- 
 tact with, and repelled by, the resin rod, it will strongly attract 
 them, and in the same way the resinous rod attracts the pith balls 
 which have been charged by contact with the glass. We are there- 
 fore shown the existence of two kinds of electricity, which are called 
 vitreous or POSITIVE electricity, and resinous or NEGATIVE electricity, 
 depending on whether they are produced by rubbing glass with silk, 
 or resinous materials with wool. The action of the pith balls also 
 shews that bodies charged with 0ne kind of electricity repel those 
 charged with the same kind, but attract those charged with the opposite 
 kind. Charged bodies are also said to be EXCITED or ELECTRIFIED. 
 
 Other similar manifestations of electricity may be easily produced. 
 For instance, if a well dried sheet of paper be laid on a table and 
 briskly rubbed with a rubber eraser or a coat sleeve, it will adhere to 
 the table and when it is slowly raised by one corner, small sparks 
 
may be seen to pass between it and the table. On dry days it is 
 sometimes possible for a person to gather a charge on his body by 
 shuffling across the carpet; this charge may be sufficient to produce a 
 spark if the finger be presented to a gas-fixture or to another person. 
 Again, if a charged body be held near to the face a peculiar cob- 
 webby sensation may be felt on account of the attraction of the 
 small hairs of the cheeks by the charge. 
 
 If the wool used to develop a charge on the sealing wax by 
 rubbing be now tested by bringing near it a charged pith ball, it will 
 also be found to be charged the charge being positive. In the 
 same way the silk which was used in rubbing the glass may be found 
 to be negatively charged. This is in accordance with a fact which has 
 been experimentally proved, that whenever a charge of one kind is 
 developed, an equal charge of opposite kind is also developed. When 
 two dry bodies of different materials, which do not have the power 
 of conducting electricity, are rubbed together, they always become 
 charged with opposite kinds of electricity. If one of these bodies be 
 rubbed with a third material its charge may be changed. The kind 
 of charge which appears on a body of one material when rubbed 
 with another material depends altogether on the nature of the two 
 materials. For instance, as we have seen, when glass is rubbed with 
 silk, the glass becomes positively charged and the silk negatively 
 charged. If a stick of sulphur be rubbed with silk the order is 
 reversed and the silk becomes positively charged, while the sulphur is 
 negatively charged. It is possible to arrange a table of materials 
 placed in such an order that when any two materials named in the 
 table are rubbed together the one that stands earliest in the table will 
 ordinarily become positively charged and the other negatively charged. 
 The following table is so arranged. Its correctness may be easily 
 tested by experiments: 
 
 i. Fur. 7. Wood. 
 
 2. Wool. ' & Metals. 
 
 3. Some resinous substances. 9. Sulphur. 
 
 4. Glass. 10. Some resinous substances. 
 
 5. Cotton. 1 1 . India rubber. 
 
 6. Silk. 12. Gutta-percha. 
 
 The reason for this difference in materials is not known, and in 
 fact slight differences in the constitution or the surface of the mate- 
 rials may cause them to change their relative positions, so that sim- 
 ilar tables given in various books do not all agree. 
 
 If a piece of metal be held in the hand and rubbed, no apparent 
 charge can be discovered on it. This is because the metal has the 
 power of readily conducting electricity, exactly as it has the powei 
 of conducting heat, and the electricity therefore all flows away into 
 the body of the operator, or through his body into the earth. The 
 same thing is true of any of the substances named in the table if 
 they are dampened with water, because water has the power to a 
 limited degree of conducting electricity. Consequently experiments 
 
in statical electricity cannot be readily made on a damp day or when 
 the materials are damp. 
 
 If the metal be fastened in a handle of dry wood or hard rubber 
 and again rubbed, it will become charged. This is because the wood 
 or hard rubber does not have the power of conducting the electricity 
 to an extent which is here appreciable and it therefore cannot 
 escape. 
 
 Materials which readily conduct electricity are called conductors, 
 and thoss which either do not conduct it at all or only conduct it in 
 a very small degree, are called non-conductors or insulators. An inter- 
 mediate class which have the conducting power to a considerable 
 degree are often called partial conductors. 
 
 The following table gives a list of materials placed approxi- 
 mately in the order of their conducting powers: 
 
 1. Metals. 7. Various oils. 13. Vulcanite. 
 
 2. Charcoal and graphite. 8. Dry wood. 14. Paraffine. 
 
 3. Acids. 9. Silk. 15. Porcelain. 
 
 4. Salty solutions. 10. India rubber. 16. Glass. 
 
 5. Plants and animals. n. Mica. 17. Dry air. 
 
 6. Pure water. 12. Shellac. 
 
 We ordinarily restrict the term conductor to the metals. The 
 materials in the table numbered from two to six may be called par- 
 tial conductors, and the last eleven materials may be called insula- 
 tors. Of all the materials named dry air may be said to be the only 
 one which has absolutely no conducting power under ordinary con- 
 ditions, though that of glass, porcelain, etc., is exceedingly small. 
 
 The cause of the difference in the conducting power of the 
 various materials is not known and will probably not be known 
 until the exact constitution of electricity is determined. By means 
 of the great conducting power or conductivity of metals electricity 
 may be conveyed from place to place. If, for instance, two blocks 
 of metal connected by a wire be mounted on insulators, then, if a 
 charge be given to one, part of the electricity will flow along the 
 wire to the second block, electrifying it. A conductor which is sup- 
 ported on insulators in such a way that electricity cannot escape from 
 it is said to be insulated. 
 
 A body may be charged or electrified by the influence upon it 
 of a charged body. Thus suppose a brass ball be insulated and 
 charged, and then be brought near an uncharged insulated brass ball. 
 The second ball will be found to be charged, if it is tested by bring- 
 ing a charged pith ball near to it. A charge which is thus devel- 
 oped by the influence of a charged body on a NEUTRAL or uncharged 
 one, is said to be developed by INDUCTION. If the brass ball on 
 which a charge is thus INDUCED, be carefully examined, its two sides 
 will be found to hold opposite kinds of electricity (Fig. i). The 
 side of the second ball which is away from the first ball will hold the 
 same kind of electricity as the latter, and the side which is near the 
 first ball will hold the opposite kind. This is in accordance with the 
 
law of attraction and repulsion between the different kinds of 
 electricity, given in this lesson. For example, if the first ball (A 
 in Fig. i) be positively charged, the side of the second (B in Fig. 
 i) which is away from the first will be positively charged and the 
 near side will be negatively charged. This is the condition shown 
 in the figure, where the plus or positive sign, +, represents a posi- 
 tive charge, and the minus or negative sign, , represents a nega- 
 tive charge. Now if the second ball be touched for an instant when 
 it is very close to the first, the positive charge will immediately flow 
 away into the operator's body on account of the repulsion of the posi- 
 tive charge which is on the first ball. The negative charge on the 
 second ball will remain on account of the attraction of the charge on 
 the first ball. If the second ball now be removed from the influence 
 of the first ball it will remain negatively charged, the charge spread- 
 ing all over it. If the two balls be now brought into contact the 
 two charges will combine and the two balls will become neutral. 
 The latter shows that an induced charge is equal in quantity to the charge 
 which induces it. This fact is strictly true, but in many cases the 
 induced charge is divided among several bodies which are near a 
 charged body. The induced charge is to be found wholly on one 
 body only when it completely surrounds the charged one, or is very 
 much nearer it than any other bodies. 
 
 The object of using brass balls in such experiments is simply to 
 obtain convenient and inexpensive conductors. Any other materials 
 will give similar results, but in the case of poorly conducting bodies 
 it is more difficult to perceive the results on account of the difficulty 
 presented to the distribution of the electricity under the influence of 
 induction. 
 
 The means of detecting a charge thus far mentioned has been 
 through the attraction or repulsion of charged pith balls or other 
 light objects. Various other means may be used, all of which are 
 dependent upon electric attractions arid repulsions for their indi- 
 cations. 
 
 Devices or instruments for determining the presence of an elec- 
 tric charge are called ELECTROSCOPES. The simplest electroscope is 
 a charged pith ball. A very sensitive one is made by attaching two 
 narrow strips of ordinary gold leaf to the end of a brass rod and 
 hanging the leaves in a glass bottle to insulate them and protect 
 them from injury (Fig. 2). If a charged body be brought near the 
 top of the rod which is connected to the gold leaves, the rod and 
 leaves are electrified by induction. If the charged body be a rubbed 
 glass rod which is positively charged, as in Fig. 3, a negative charge 
 will appear at the top of the conductor and a positive one in the 
 leaves (compare the case of the brass balls given above). In this 
 case, since the two leaves have charges of the same kind, they will 
 repel each other and separate (Fig. 3). The gold leaves are so sen- 
 sitive that they are likely to be torn by the force of their repulsion 
 if a heavily charged body is brought too close. 
 
FIG. 2. 
 
 FIG. 3. 
 
 If while the glass rod is still held near the electroscope the brass 
 rod of the electroscope be touched by the hand, the positive charge 
 in the leaves will at once flow off into the operator's body on account 
 of the repulsion of the charge on the glass, and the leaves will drop 
 together. The negative charge will remain in the electroscope rod 
 on account of the attraction of the charge on the glass (compare 
 brass balls above). Now if the glass rod be taken away the nega- 
 tive charge will spread all over the electroscope rod and gold leaves c 
 and the leaves will again separate. It can be easily proved that the 
 charge on the gold leaves is now negative by bringing a charged 
 glass rod near the top of the electroscope, when the charge will be 
 attracted out of the leaves and they will fall together. Or, if a 
 negatively charged rod of sealing wax be brought near the top of the 
 electroscope, the charge in the instrument will all be repelled into 
 the leaves and they will separate farther. 
 
 With this simple device it is possible to detect very small charges 
 of electricity. The electroscope may of course be directly charged 
 by contact with a charged body, but the leaves are likely to be torn 
 by the violence of the action, unless the charge is quite small. 
 
 We are now in a position to see the reason for the attraction 
 which rubbed amber, rubbed glass, and other charged bodies, have 
 for light objects. Since electric induction acts between any charged 
 body and any other body which is reasonably near, the effect of the 
 charged body on a light object is first to charge it by induction. The 
 positive and negative charges induced on the light object are equal 
 in quantity. One of them is attracted and the other is repelled by 
 
o 
 
 B 
 
 FIG. 1. FIG. 4. 
 
 the original charge. That which is attracted is nearest the original 
 charge, so that the force of attraction is greater than the force of 
 repulsion. The condition is illustrated in Fig. i, where a positive 
 charge is seen at a on the large ball. This induces the negative and 
 positive charges b and c on the small ball. Since b is considerably 
 nearer a than is c, the attraction between a and b is materially greater 
 than the repulsion between a and c. The small ball is therefore 
 attracted towards the large ball. If the balls come in contact the 
 small ball receives a part of the positive charge belonging to the 
 large one, and they at once separate on account of tha repulsion of 
 the two positive charges. 
 
 The attraction between a charged body and an uncharged one, 
 or between two charged ones, always exists, though the pull or push 
 exerted by the charges is usually sufficient to move the bodies only 
 when they are very light. 
 
 The actual force of attraction or repulsion exerted between any 
 two bodies depends upon the product of the quantities of electricity 
 in their charges, their distance apart, and the material which is 
 between them. If they are surrounded by air the push or pull which 
 two charged bodies exert on each other increases directlv with the 
 product of the quantities of electricity which they hold, and decreases 
 directly with the square af the distance between the bodies, provided 
 the bodies are small compared with the distance between them. 
 
 The unit quantity of electricity is called a COULOMB, after a 
 French experimenter who lived about the beginning of this century. 
 As a rough analogy with the measurement of water or gas, we may 
 say that a coulomb of electricity is the equivalent of a gallon of water 
 or a cubic foot of gas. 
 
The reason that the force exerted between two charged bodies 
 depends on the product of the two quantities of electricity, is that 
 each coulomb of electricity on one body attracts or repels every 
 coulomb on the other body with a fixed force, and therefore the total 
 force of attraction or repulsion depends on the number of coulombs 
 on one body multiplied by the number on the other body. 
 
 Instruments for determining the quantity of electricity which is 
 held in a charge on a body by measuring its attraction for another 
 charged body, are called ELECTROMETERS. These instruments are 
 valuable for many purposes and will receive more attention in later 
 lessons. 
 
 If the two charged bodies be immersed in a liquid such as water 
 or oil, or be separated by solids, the force exerted between them is 
 decreased. The amount of the decrease depends upon the nature of 
 the separating material, and is apparently due to a difficulty in the 
 attractive force making its way through the material. 
 
 The last of the peculiar properties of electricity which we need 
 consider before taking up the various generating machines, is the 
 location on a body which a charge always takes. We often hear the 
 statement that electricity flows only on the surface of a wire. This 
 is entirely untrue. When electricity flows or moves it passes through 
 the substance of the conductor.. In the case of electricity at rest, 
 however, the case is different. When electricity is at rest it never 
 enters the substance of a body, but stays strictly on the surface. It is 
 important that this difference in the action of electricity in motion, 
 or current electricity, and electricity at rest, or statical electricity, 
 be remembered. Again, statical -electricity not only stays on the 
 surface of a body, but it tends to stay on the outside surface. Fig. 
 4 shows this by the position of the pith balls which are suspended 
 on the inside and outside of a hollow brass cylinder. The cylinder 
 being charged, the outside pith balls which are in contact with it, 
 at once diverge on account of a charge which they receive from the 
 cylinder. This shows that the outer surface of the cylinder is 
 charged. The inner pith balls, which are also in contact with the 
 cylinder, remain entirely inert, showing that there is no charge on the 
 inner surface. This is true whether the charge is given to the 
 cylinder from the inside or outside, and is to be expected on account 
 of the known repulsion of like charges or parts of a charge. The 
 different parts of a charge try to get as far away from each other as 
 possible, and therefore go to the outer surface of a body if its conduc- 
 tivity is sufficient to permit it. A brass cylinder is here used so that 
 the electricity may readily follow its tendency to move to the outer 
 surface if it be applied at the inner surface. 
 
 By virtue of the fact that a charge tends to stay on the outer 
 surface of a body, it is possible to entirely screen an object from all 
 electric force by completely surrounding it with a conducting cage. 
 This is done in making electrometers, when it is desirable to screen 
 the working parts of the instruments from outside electric forces. 
 
 Copyrighted, 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON I. 
 
 Points for Review. \. How much is known about the real constitution of 
 jiectricity ? 
 
 2. What is the origin of the word electricity ? 
 
 3. What are the two kinds of electricity called, and how is their existence shown ? 
 
 4. What is the law of attraction and repulsion of he two kinds of electricity ? 
 
 5. Can a charge of one kind of electricity exist alone ? 
 
 6. If a body becomes positively charged upon rubbing it with some material, will it 
 always become positively charged when rubbed, regardless of the material used in rub- 
 bing it ? 
 
 7. What precautions must be taken in handling metals in order that they may be 
 charged by rubbing ? 
 
 8. What are the meanings of the terms electric conductors, insulators, and electric 
 conductivity ? 
 
 9. What materials are the best conductors, and what the best insulators ? 
 10. How may electricity be conveyed from one place to another ? 
 
 11. What effect does a charged body have on an uncharged conductor which is 
 
 brought near to it? 
 
 12. Why are light bodies attracted by charged bodies? 
 
 13. What is the purpose of the electroscope? 
 
 14. How can the kind of electricity in a charge be determined by using an electro- 
 scope? 
 
 15. Upon what does the force exerted between two charged bodies depend? 
 
 16. What is the force equal to when the charged bodies are surrounded by air? 
 
 17. If the bodies are surrounded by liquids or solids how is the force affected? 
 
 18. What is the unit of measurement of a quantity of electricity called? 
 
 19. What is the purpose of the instruments called electrometers? 
 
 20. Upon what part of a conductor does an electric charge always remain? Why? 
 
 II. 
 
 MACHINES FOR GENERATING ELECTRICITY BY FRIC- 
 TION AND BY ELECTRIC INDUCTION. 
 
 We will now take up the various machines for the generation 
 of electricity. From what has preceded, it is evident that a simple 
 machine may be made for the generation of electricity by an 
 arrangement for continously rubbing glass with silk or other simi- 
 lar material, with some device added for collecting the electricity 
 which is developed. A German, named Von Guericke, first built 
 such a machine. In this a large ball of sulphur was revolved. 
 When any person pressed his dry hands upon the sulphur ball the 
 friction generated electricity and his body became charged. Later, 
 a glass cylinder or plate and a rubber of silk or leather came into 
 use. 
 
 In such machines the charge upon the glass is usually col- 
 lected by induction. A row of points, called a comb, attached to 
 an insulated brass block is presented to the charged surface of the 
 
glass (Fig. 5). The positive charge on the glass causes the far side 
 of the brass conductor to become positively charged and the row of 
 points to become negatively charged. The particles of air surround- 
 ing the points become negatively charged and are repelled off to the 
 positively charged glass. This leaves the brass conductor with a posi- 
 tive charge, and the negative charge of the air particles neutralizes 
 the positive charge of the glass, which is therefore ready to be again 
 excited as it again moves around to the rubber. The action is con 
 tinuous while the glass is revolved. 
 
 By sprinkling the rubber with a conducting powder or 511 
 amalgam, made with mercury, the negative charge of the rubber 
 may also be drawn away. If the positively charged brass conductor 
 is then connected by a wire to the rubber, a continuous flow of elec- 
 tricity will pass from the brass conductor to the rubber. If there is 
 a* small break in the wire the electricity will jump across it in the 
 form of a spark. 
 
 The friction of a jet of wet steam passing through a wooden 
 nozzle, and many other plans, may be used to generate electricity 
 in a similar way by friction. 
 
 By the frictional methods the quantity of electricity generated 
 in a reasonable time is comparatively small, and machines operating 
 by induction may be used. The simplest device of this kind is called 
 an electrophorus. This consists of a plate of sulphur, vulcanized 
 rubber, or similar material, and a metal plate or cover with an 
 insulating handle (Fig. 6). Rubbing the sulphur or rubber with 
 flannel electrifies it negatively. When the cover is set down it 
 touches the base at only a few points on account of its roughness, 
 and it becomes electrified by induction (Fig. 7). The negative 
 induced charge maybe allowed to escape into the operator's body by 
 touching the cover with a finger, as explained in the first lesson. 
 The cover remains with a positive charge which may be used to 
 charge other bodies. 
 
 The process of charging the cover maybe repeated again and 
 again without affecting the charge on the base, but the latter will be 
 slowly dissipated through dampness in the air. 
 
 What is known as a Holtz electric machine may be roughly 
 described as an automatic electrophorus. This consists of two parallel 
 plates of glass, one of which is mounted to rotate, with certain induc- 
 ing and collecting devices (Fig. 8). The following is a brief expla- 
 nation of the action of this machine: at opposite points on the sta- 
 tionary plate holes or windows are cut and over these are pasted 
 pieces of paper called sectors. These are given opposite charges by 
 means of rubbed rods of glass and sealing-wax or by other means. 
 In front of the revolving plate opposite each sector, is a comb. The 
 charges on the sectors act indirectly on the combs and the conductors 
 attached to them, so that the knobs that terminate the conductors 
 
 10 
 
. 6 
 
 are charged with opposite kinds of electricity. The electricity which 
 is attracted into the combs, flows off onto the revolving plate exactly 
 as was explained in the case of the cylinder friction machine, and 
 charges it as shown in Fig. 9. The charges on the revolving 
 glass are carried around under the opposite combs and act inductively 
 on them, and are then neutralized by the charges on the streams of 
 air particles passing off the combs. If the two knobs be placed in 
 connection a flow of electricity passes through the conductors out of 
 the combs onto the revolving plate, which is thus kept charged as in 
 Fig. 9, and the current of electricity continues as long as the 
 plate is revolved. If the plate is revolved with sufficient rapidity a 
 spark will jump from knob to knob, thus completing the circuit even 
 when the knobs are a considerable distance apart. 
 
 Fid. 7 
 
 ii 
 
In starting the machine it is really sufficient to charge only one 
 of the sectors, as the other will then become charged through the 
 action of the machine. It is not necessary to go fully into the action 
 of these machines or into that of various devices to increase their 
 effectiveness and make them self-exciting. 
 
 The action of these machines may be compared, as a rough but 
 handy analogy, to pumps for circulating water or gas through a sys- 
 tem of pipes. The machines act as though they were machines foi 
 pumping electricity. 
 
 Suppose a water tank to be placed in the basement and another 
 in the garret of a house, and a pump be connected in the pipe lead- 
 ing from one to the other. If the tanks are full of water and the 
 pump be started, water will be drawn from the lower tank and sent 
 into the upper one, which will overflow, and unless the water is 
 caught it will run down to the ground. If an overflow pipe be car- 
 ried from the upper tank to the lower one, the overflow will run 
 back into the lower tank, and the water will be singly circulated 
 by the pump through the system of pipes between the two tanks. 
 
 This is similar to the conditions of an electrical machine when 
 the positive and negative terminals are connected together or sparks 
 are passing between them. 
 
 Now, if the overflow pipe is stopped up and drip pans be arranged 
 so that water from the upper tank cannot run down into the lower 
 one, the pump will soon empty the lower tank, after which it may 
 continue to run, but it cannot pump any water and no stream will 
 flow through the pipes. 
 
 In the same way if the two conductors of an electric machine are 
 not connected, and are too far apart for a spark to pass between 
 them, the conductors will be strongly charged with opposite kinds of 
 electricity, but then the action of the machine in circulating 
 electricity will cease until a path is provided for the current to 
 flow. 
 
 The quantity of water circulated by the pump depends upon the 
 pressure which it produces, and upon the size of the pipes, and a sim- 
 ilar rule holds for the circulation of electricity by an electrical 
 machine. The volume of the stream of water may be designated as 
 a certain number of gallons or cubic feet per second. In the same 
 way the volume of a current of electricity may be designated as one 
 which conveys a certain number of coulombs per second. An 
 electric current carrying one coulomb per second is called a current 
 of one ampere, and the volume of electric currents is always given in 
 amperes. This name was given in honor of a great French scientist 
 whose name was Ampere. 
 
 To pass a stream of a certain number of gallons per minute 
 through a certain pipe, demands the application of a certain pressure 
 to overcome the frictional resistance. In the same way it requires a 
 
certain electrical pressure to pass a given electrical current through 
 any conducting wire, on account of the resistance which the wire 
 offers to the flow of the electricity. The resistance to the passage of 
 electricity, or the electrical resistance, of any material, is the recipro- 
 cal or opposite of its conducting power. The greater its conducting 
 power, the less is its electrical resistance. 
 
 We usually speak of water pressure, or the pressure of gas, in 
 pounds per square inch, or in feet difference of level, or head. The 
 corresponding unit of electrical pressure is a volt, which was named 
 after Volta, a great Italian scientist. 
 
 Return again to the pump and tanks. When the pump is set 
 in motion it sets up a difference of pressure which may be measured 
 by a gauge, and this starts the water to flowing if it has an outlet. In 
 the same way we may look upon electrical machines as setting up a 
 difference of electrical pressure (which may be measured by a proper 
 electrical instrument), and this starts the electricity to flowing if it 
 has an outlet. 
 
 This leads us to the necessity of considering a positive charge 
 of electricity as electricity at high electrical pressure or high potential, 
 and a negative charge as electricity at low electrical pressure or low 
 potential. 
 
 When a point of high electrical pressure is connected by a con- 
 ducting wire to a point of low pressure, electricity will flow from the 
 point of high electrical pressure to the point of low electrical pres- 
 sure until the pressure is equalized, unless the pressure is continually 
 kept up by a machine; exactly as when two tanks standing side by 
 side are filled with water to different heights, if they be connected 
 by a pipe water will flow from one to the other until its level is the 
 same in both. 
 
 In using these comparisons it must be remembered that we do 
 not touch upon the true nature of electricity, which is unknown, but 
 only upon the laws of its action which have been experimentally 
 determined. Also, that while water and gas may be directly perceived 
 by our senses, electricity is absolutely impalpable that is, it cannot 
 be perceived by the senses, and the only way in which we may 
 recognize it is by its various effects. 
 
 Before leaving the question of electrical machines working by 
 friction and induction, it is well to call attention to the great pres- 
 sure of the electricity generated by them. This is shown by the 
 sparks which may be caused by them to pass through the air, or 
 even to pierce wood, glass, or other solid insulators. These effects 
 may be called miniature lightning effects, for lightning is simply 
 caused by the passage through the air of a current of electricity 
 under enormous pressure. Thunder is like the crackle of the spark 
 from an electrical machine greatly magnified. 
 
While the electrical pressure generated by these machines is 
 very great, the quantity of electricity generated is quite small, and for 
 commercial purposes in which a considerable volume of electricity is 
 needed, other methods of generating the current are used. An 'ex- 
 planation of these will come in the following lessons. 
 
 Copyrighted, 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON II. 
 
 Points for Review. 1. How can a machine be made to continuously generate 
 electricity by friction? 
 
 2. How does an electrophorus work? 
 
 3. How can a machine be made to generate electricity by induction? 
 
 4. What are the units of measurements of electric current and pressure called? 
 
 5. How much electricity is conveyed by a unit current in each second? 
 
 6. If a conductor charged positively be connected to one without a charge, what 
 happens? 
 
 7. If a conductor charged negatively be connected to one without a charge, what 
 happens? 
 
 8. v If a conductor charged positively be connected to one charged negatively, what 
 happens? 
 
 9. Why are friction and induction machines not generally used in general commer- 
 cial service? 
 
 10 What is the relation of lightning to the sparks given by an electrical machine? 
 
 LESSON III. 
 
 ELECTRIC BATTERIES, OR APPLIANCES FOR GENER- 
 ATING ELECTRICITY BY CHEMICAL ACTION. 
 
 One of the effects of chemical action is to give out heat. When 
 wood or coal is burned the carbon of the burning material combines 
 with oxygen of the air, and heat is given out as the result of the 
 chemical combination, which we call combustion. In the same way 
 if zinc be dissolved in sulphuric acid, the acid combines with the- 
 zinc and heat is given off as the result of the chemical combination. 
 This heat represents a certain energy or capacity for doing work. It 
 has been found that under certain conditions the energy thus repre- 
 sented by chemical action may be converted iuto an electric current, 
 and taking advantage of this we get electric batteries. Electric 
 currents produced by chemical action were first observed and studied 
 about the end of the last century by Galvani and Volta, both of 
 whom were Italian scientists. Volta will be recognized as the man 
 from whose name comes the word volt, the name of the unit of 
 electrical pressure. 
 
 When two plates of different metals are placed so that they do 
 not touch each other in a liquid (Fig. 10) which is inclined to attack 
 them chemically, one of them becomes positively charged and the 
 other negativaly charged with electricity. The charges are so minute 
 that they cannot be distinguished by the electroscopes previously 
 explained, but a delicate electrometer will distinguish and measure 
 the charges. If the metals be connected by a wire a current flows 
 
 " 
 
through it from one plate to the other, and 'this may be readily dis- 
 tinguished by its effects, which will be explained later. The positive 
 or high pressure plate is the one which is attacked least readily by 
 the liquid. 
 
 A cup containing two plates thus immersed in a liquid is called 
 an electric battery cell, and the plates are called the poles or electrodes 
 of the cell. It is usual to speak of the pole which is at the higher 
 pressure, and from which the current flows through the wire, as the 
 positive pole; the other pole is then called the negative pole. The 
 difference of electrical pressure between the poles is called the 
 electromotive force of the cell. The phrase electromotive force means 
 that which tends to move electricity, that is, a difference of electrical 
 pressure. This phrase is often abbreviated into E. M. F. We will 
 generally speak of it, however, as the electrical pressure of the cell. 
 An electric battery is often called a galvanic or voltaic battery, and 
 the electricity produced by a battery is often called galvanic or 
 voltaic electricity, although the electricity is exactly the same as that 
 produced in any other way. These terms are applied in the same 
 way as the terms spring water and well water, for instance, are 
 applied to pure water which is drawn from a spring or a well, though 
 the water does not differ from pure water drawn from other sources. 
 
 When the poles of a cell are connected by a wire it is found that 
 an electric current not only flows from the positive to the negative 
 pole through the wire, but it continues through the liquid from the 
 negative to the positive pole. If the current is followed from any 
 point in its flow, it will be found to return through a complete path 
 to the same point, exactly as water is circulated by a pump through 
 a system of pipes. A continuous current of electricity is therefore 
 said to flow in a complete path or circuit. A complete circuit is often 
 called a closed circuit. The current inside the cell then is driven, 
 by the effect of chemical action against a difference of pressure, just 
 as water is raised against a difference of pressure by a pump. Out- 
 side of the cell where there is no restraining action besides that o^. 
 the electric resistance of the connecting wire, the current follows its 
 own tendency to flow from the point of high pressure to that of low 
 pressure. We have seen that electricity is conveyed in a similar 
 manner through a complete circuit by the action of friction and 
 induction machines. When two insulated conductors at different 
 electrical pressures are connected by a wire a brief current flows, just 
 as a current of water flows through the pipe connecting two tanks in 
 which the water stands at different levels, but the current ceases as 
 soon as the pressure is equalized. In order that a continuous current 
 may be produced a difference of electrical pressure must be continuously 
 supplied in a closed circuit. 
 
 The magnitude and direction of the difference of electrical pres- 
 sure between the poles of a battery cell depend upon the materials 
 
 16 
 
IINC PlfiTt 
 
 FIG 1O. 
 
 CARBON WtiTC. 
 
 CARBON PJ.XTE' 
 ^ />0AO(/3 C(V> 
 
 Z/NC W.AT& 
 
 - ZINC 
 
 2&. 
 
 17 
 
in the plates and the nature of the liquid. For instance, if zinc and 
 copper be the plates of a cell containing sulphuric acid, the electrical 
 pressure of the cell is about nine-tenths of a volt and the copper 
 plate is the positive pole. If two cells containing sulphuric acid as 
 the liquid be made using zinc and lead for the plates of one and lead 
 and copper for the plates of the other, the lead is the positive plate 
 in the former and the negative plate in the latter. The electric 
 pressure developed in each of these would also be less than in the 
 case of the zinc-copper cell. In cells which are to be obtained from 
 dealers the negative pole is nearly always of zinc, but the metal 
 composing the positive plate and the composition of the liquid vary 
 greatly. The positive plate is generally made of copper, carbon, or 
 platinum, and the liquids consist of various acids, sal-ammoniac, 
 caustic potash, etc. 
 
 If a number of cells, such as the zinc-copper cells described 
 above, are connected in a series with the zinc pole of one connected 
 to the copper pole of the next, the zinc pole of this connected to the 
 copper pole of the next, and so on (Fig. u), then the total difference 
 of electrical pressure between the free copper and zinc poles is equal 
 to the sum of the pressures developed by all the individual cells. 
 When a battery is thus connected up so that the pressures developed 
 in the individual cells are all added together the cells are said to be 
 connected in series. 
 
 The electrical pressure of a cell depends only upon the nature 
 of the plates and the liquid, and is entirely independent of the size of 
 the plates. This can be easily proved by making two cells out of 
 tumblers containing dilute sulphuric acid, in one of which are placed 
 narrow strips of copper and zinc, and in the other are placed broad 
 strips of the metals. If these are connected in series with the free 
 poles joined by a wire, a current will flow as shown by the vigorous 
 chemical action which causes bubbles to gather on the copper plates. 
 If one of the cells be now reversed, so that the copper pole is con- 
 nected to the copper pole of the other, no such action will occur, 
 showing that the electrical pressures which tend to send currents in 
 opposite directions are equal and neutralize each other. 
 
 If the two poles of a zinc-copper cell, such as we have been 
 considering, be connected by a wire, a vigorous chemical action goes 
 on at first, but it gradually decreases in intensity and finally appears 
 to stop altogether. This effect may be plainly shown by connecting 
 an electric bell in the circuit of the cell. When the circuit is first 
 completed the bell will ring loudly, but it will soon weaken and 
 after a time cease to ring altogether. If the cell be then examined a 
 layer of bubbles will be found upon the copper plate. These bubbles 
 are composed of hydrogen gas which is liberated from the sulphuric 
 acid by the chemical action in the cell. The effect of these hydro- 
 gen bubbles is two-fold. First, they tend to set up a counter electric 
 
 18 
 
pressure in the cell, or a pressure which is opposite in direction to 
 that due to the regular action of the cell, and thus the effective pres- 
 sure of the cell is reduced, and second, the layer of bubbles presents 
 a hijgh resistance to the flow of the current. A cell which is made 
 inactive by a layer of hydrogen bubbles is said to be polarised, and 
 the effect is called polarisation. 
 
 In order that a cell may be capable of working continuously 
 some plan must be adopted to keep it from polarizing, or, as it is 
 often called, to keep it depolarised. This may be effected in three 
 different ways: ist, by mechanical action; ^ by direct chemical 
 action, which absorbs the hydrogen; 3d, by electro-chemical action, 
 by which the hydrogen is exchanged "for a metal which is deposited 
 upon the positive plate. 
 
 The first method of depolarizing requires that the hydrogen 
 bubbles be cleared off the positive plate as fast as they are deposited 
 upon it. This may be done by continuously stirring the liquid or 
 blowing air into it. If the positive plate be well roughened the 
 hydrogen bubbles will not stick to it so closely, but many will float 
 off to the surface of the liquid and escape. This plan was used in a 
 cell commonly called Smee's cell, which was used commercially 
 many years ago, but it was not very successful. 
 
 If some substance be added to the liquid of the cell, which will 
 combine with the hydrogen as quickly as it is formed the polarization 
 will evidently be avoided. This is the foundation of the second 
 method of depolarizing. Various substances may be used for this 
 purpose, but dioxide of manganese, bichromate of potash, chloride of 
 lime bleaching powder, ancT nitric acid are used most commonly. 
 The well-known bichromate battery, which is often used to run 
 small motors, ignite the gas in gas engines, and for similar purposes, 
 is a zinc-carbon battery, with a liquid composed of sulphuric acid, 
 in which bichromate of potash is dissolved. When this cell is in 
 operation polarization is prevented by the immediate combination of 
 the hydrogenjwhich is liberated from the sulphuric acid with the 
 bichromate offpotash. Carbon is used for the positive plate in this 
 cell because tiae bichromate of potash will attack and destroy copper. 
 In the bichromate battery the zincs are generally arranged so that 
 they may be lifted out of the fluid when the cells are not in use, 
 because the fluid eats up zinc when the circuit of the cell is open. / 
 From this comes the name///^^? battery. 
 
 When nitric acid is used as a depolarizer it cannot be allowed 
 to come in contact with the zinc, which it attacks vigorously, conse- 
 quently it is confined in a porous earthenware cup within which is 
 the positive pole of carbon or platinum. Fig. 12 shows such a 
 cell complete, and Fig. 1 3 shows the same, thing in cross-section. 
 The earth enware/wtf^ cup is sufficient to prevent the liquids from 
 mixing, but after it has become well soaked it does not present much 
 
resistance to the passage of a current. The cells, which are made tip 
 with nitric acid for the depolarizer, are only useful for furnishing 
 current for experimental purposes and for that purpose they are 
 are much more expensive than dynamos. They have, therefore, 
 practically gone out of use. The commonest forms of cells of this 
 type are those known as Bunsen's and Grove's cells. The action of 
 nitric acid as a depolarizer is quite similar to that of bichromate of 
 potash, though it is more powerful. 
 
 When dioxide of manganese is used as a depolarizer it is gener- 
 ally broken up into small lumps and put into a porous cup surround- 
 ing a positive plate of carbon. When sal-ammoniac dissolved in 
 water is used as the liquid in this form of cell, it makes the familiar 
 Leclanche battery (Fig. 14), which is used so frequently for ringing 
 door bells and in similar service. Sometimes the dioxide of man- 
 ganese is pulverized and mixed with shellac, after which it is 
 pressed into small bricks, which are placed upon either side of the 
 carbon positive plate (Fig. 15), as in the "prism" Leclanche battery. 
 /The depolarizing effect of dioxide of manganese is not sufficiently 
 , powerful to prevent a cell from becoming polarized if used con- 
 ' stantly. Consequently Leclanche cells are only satisfactory in service 
 which is intermittent like ringing door bells, where the circuit is 
 open a considerable part of the time and the battery rests without 
 chemical action. Leclanche cells are called open circuit cells on 
 account of the small chemical action which goes on in them when 
 the circuit is open and because they are not satisfactory in continuous 
 service. 
 
 Copyright 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON III. 
 
 Points for Review. 1. Upon what action do electric batteries depend for their 
 operation? 
 
 2. What is electric pressure often called? 
 
 3. Why fs electricity which is generated by electric batteries called galvanic or 
 voltaic electricity? 
 
 4. Does it differ from electricity generated by other means? 
 
 5. In what direction does the electric current flow through a battery cell? What is 
 its direction in the circuit outside of the cell? 
 
 6. What is the path through which electricity flows called? 
 
 7. How may a continuous current of electricity be produced? 
 
 8. If a number of battery cells be connected in series, what is the total pressure 
 generated? 
 
 9. Upon what does the pressure produced by any cell depend? 
 
 10. What is polarization? 
 
 11. How may polarization be avoided? 
 
 12. How is a Leclanche cell made up? 
 
 13. Why are Leclanche cells called open circuit cells? 
 
 IV. 
 
 ELECTRIC BATTERIES, OR APPLIANCES FOR GENER- 
 ATING ELECTRICITY BY CHEMICAL ACTION. 
 
 (CONCLUDED.) 
 
 The third method of depolarizing introduces more complicated 
 chemical reactions, but which we need not go into in much detail. 
 Through the use of this method cells are constructed which give 
 excellent results in continuous service, and which are, therefore, 
 called clo$&d 'circuit cells. One of these is probably the most com- 
 monly used battery of any form. This is the ordinary gravity battery, 
 or copper sulphate battery which is used so much in telegraphy. 
 
 The original form of cell from which the gravity cell came is 
 one in which the active liquid is sulphuric acid, in which is 
 immersed the zinc or negative plate. The copper plate is immersed 
 in a solution of ordinary copper sulphate, or blue vitriol (sometimes 
 called blue stone). The two solutions are separated by a porous cup. 
 In general terms the chemical action which occurs when the battery 
 is in action is as follows: The sulphuric acid attacks the zinc, and 
 sulphate of zinc is formed. At the same time hydrogen is liberated 
 from the sulphuric acid and goes towards the copper plate, where it 
 would be deposited if it were not for the copper sulphate which sur- 
 rounds the copper plate. When the hydrogen gets into the copper 
 sulphate solution, it goes into combination and copper is separated 
 from the solution and deposited upon the copper plate, which is 
 therefore kept bright and in good working condition. 
 
 21 
 
During the operation of the cell the chemical action which has 
 been briefly explained causes a change in the character of the solu- 
 tions. The sulphuric acid changes to a solution of sulphate of zinc, 
 and the copper sulphate changes to sulphuric acid. If the sulphuric 
 acid is replaced by a dilute or weak solution of zinc sulphate, a cur- 
 rent is set up, as before, and the chemical action is similar, but the 
 copper sulphate is converted into zinc sulphate. In order that the 
 depolarizing action may continue during the life of the cell the 
 strength of the copper sulphate solution must be kept up. This is 
 done by putting crystals of copper sulphate or blue vitriol in the cell 
 so that they may be dissolved. Fig. 16 shows a cell of this battery 
 in its original form, in which it is called DanielPs battery. In the 
 figure the zinc plate is shown within the porous cup at the right 
 hand of the battery jar, and the copper plate is at the left hand of 
 the jar. Alongside of the copper plate is a perforated copper cage 
 in- which may be put the copper sulphate for renewing the solution. 
 
 The suphuric acid or zinc sulphate solution of this cell is ordi- 
 narily much diluted or weakened by water, while the copper sul- 
 phate solution is kept quite strong or saturated. When in this con- 
 dition the solution of zinc sulphatej&lighter than the other and will 
 float upon it, just as oil floats on water. Consequently if the copper 
 surrounded by the solution of copper sulphate be placed in the 
 bottom of a battery jar, a weak solution of zinc sulphate or sulphuric 
 acid may be carefully poured on top, and the solutions will only mix 
 very slowly. The zinc may be hung from the top of the jar in the 
 upper solution (Fig. 1 7). This constitutes the gravity battery, so- 
 called because the solutions are separated by gravity through the 
 difference in their densities, instead of by a porous cup. 
 
 In setting up such a cell it is usual to put the copper in the 
 bottom of the jar surrounded by crystals of copper sulphate. The jar is 
 then filled with water to near its top and the zinc is immersed in the 
 upper part of the liquid. The cell may be placed on short circuit for a 
 time and it will work itself into good operating condition, or a little 
 sulphuric acid or zinc sulphate solution may be carefully poured into 
 the water and the cell will at once be in condition. 
 
 If a gravity cell be allowed to stand upon open circuit the two 
 solutions will slowly mix by diffusion. When any of the copper 
 sulphate solution reaches the zinc a black deposit of oxide of copper 
 is made on it. This puts the cell in such condition that it will not work 
 satisfactorily until the zinc has been cleaned. When the cell is in 
 operation the copper sulphate is changed into zinc sulphate so 
 rapidly that it gets no chance to mix with the latter. A gravity 
 battery, therefore, is only satisfactory in a service which keeps it con- 
 stantly working. 
 
 There are various other types of batteries in which the third 
 method of depolarizing is used, but which are not in sufficiently gen- 
 eral use to make their description desirable here. 
 
 22 
 

 
 FIG. 16. 
 
 FIG. 17. 
 
 FIG. 18. 
 
 FIG. 19. 
 
 23 
 
In nearly all battery cells some chemical action by which the 
 zinc is wasted goes on when the circuit is open. This may also pro- 
 ceed while the circuit is closed without adding to the useful current 
 of the cell. Such wasteful chemical action is called local action. It 
 is usually caused by metallic impurities in the zinc, which form with 
 the zinc little electric batteries by the action of which the zinc is 
 worn away in spots. A similar action is also caused in some cells by 
 differences in the density of the liquid at various parts of the cell. In 
 this case the zinc near the top of the liquid is ordinarily wasted 
 away, and may be entirely eaten off. 
 
 To avoid local action the zinc may be amalgamated, that is, its 
 surface may be alloyed with mercury. For this purpose the zinc is 
 cleaned by dipping into a dilute acid solution and it is then rubbed 
 with mercury, which makes a pasty alloy on the surface. The 
 Aunties in the zinc do not readily form an amalgam with mercury 
 arid are therefore covered up, while pure zinc is brought to the sur- 
 face. Zinc for battery plates is also sometimes cast with a small 
 percentage of mercury in its composition. 
 
 The amount of metal usefully consumed in a cell depends 
 directly upon the number of coulombs of electricity which are per- 
 mitted to pass through it. The amount of hydrogen gas, copper, 
 or other metals liberated from the liquids also depends upon the 
 number of coulombs of electricity which are passed through the cell. 
 This may be stated as a general law of electro- chemical action, that 
 the amount of chemical action in a cell depends directly upon the 
 amount of electricity which passes through it, and therefore the chemi- 
 cal action is the same in all cells of a number connected in series since 
 the same amount of current will flow through them all. 
 
 The weight of a metal in grammes (metric measure) which is 
 dissolved or deposited when one coulomb of electricity passes through 
 a cell, is called the electrochemical equivalent of the metal. 
 
 Electric batteries in which a metal is directly consumed by 
 chemical action for the generation of an electric current, are called 
 primary batteries. In nearly all primary batteries the metal which 
 is consumed is zinc. The law of electrochemical action already 
 stated shows that no current can be produced without an equivalent 
 consumption of metal, just as an appreciable amount of heat cannot 
 be given out from a fire without an appreciable consumption of coal 
 or Tfrood. The consumption of zinc in a battery to furnish electrical 
 energy in the form of an electric current is similar to the burning of 
 coal under a boiler to furnish steam power. It can be readily seen 
 that zinc makes an expensive iuel, though the consumption of a 
 pound of zinc in a battery produces several times as much energy as 
 is produced by the combustion of a pound of coal in the furnace of a 
 boiler, so that batteries in which zinc is consumed cannot be used 
 commercially to furnish electricity where currents of great magni- 
 
tude are required, as in electric lighting. For such purposes the 
 battery can never compete with the dynamo driven by a steam 
 engine, unless a cell be invented in which coal may be economically 
 consumed in the place of zinc, and the heat due to its combustion 
 be thus directly transferred into electrical energy. If this is ever 
 done the electric battery will displace the steam engine, but batteries 
 in which zinc is consumed can never economically furnish current for 
 light and power. 
 
 In many domestic operations, such as ringing electric bells, 
 regulating dampers, etc., primary batteries hold an important place. 
 In telegraphy and telephony, and other commercial applications on 
 a large scale in which a comparatively weak current is required, 
 they are used in great numbers. They are also used in electro-thera- 
 peutics and similar applications. For many domestic purposes the 
 work required of a battery is intermittent and so small that a 
 constant electromotive-force cell is not required. Consequently many 
 batteries are made of simple zinc-carbon cells in which the liquid is 
 a solution of sal-ammoniac. These cells are just like L,eclanche cells 
 without the dioxide of manganese depolarizer. The carbon plate 
 is generally made with a large surface so that the polarization is not 
 very rapid. 
 
 If a gravity cell be worked until its zinc is nearly used up and a 
 current be then passed through it from the copper plate to the zinc- 
 plate, metallic zinc will be deposited on the zinc plate by the chemi- 
 cal action due to the current. The current which separates the zinc 
 from the liquid is passed through the cell against the electric pres- 
 sure naturally developed by the cell, and energy must be expended in 
 order that the current may flow. This energy is stored up during 
 the process in the deposited zinc, and may be returned when the zinc 
 is again dissolved through the operation of the battery in the ordinary 
 manner. 
 
 Alternate discharging of the battery by taking current, and 
 consequently energy, from it through the consumption of zinc, and 
 then again charging it by expending energy in the cell by sending 
 current into it and depositing zinc, may be kept up indefinitely. 
 Each time the cell will give out nearly as much energy due to the 
 consumption of its zinc as was given to it in depositing the same 
 amount of zinc. 
 
 A battery in which energy may be stored through the forced 
 chemical action called charging and from which this energy may be 
 then withdrawn through the natural action of the cell, is called a 
 storage battery. Storage batteries are also called accumulators or 
 secondary batteries. 
 
 Commercial storage cells are usually made with lead plates im- 
 mersed in dilute sulphuric acid. 
 
The chemical action which goes on in these during charging 
 and discharging roughly consists in transferring oxygen which 
 exists in oxide of lead on the plates from one plate to the other. It 
 is desirable that the plates be capable of holding a large amount of 
 oxide of lead in order that the cells may be of large capacity, and they 
 are therefore made with corrugations or perforations in which the 
 oxide may be fixed. The perforated plates are called grids. 
 
 Sometimes the plates are made up for use by filling the perfora- 
 tions in the plates with a paste consisting of lead oxide moistened 
 with sulphuric acid. This process is called pasting, and plates made 
 up thus are often called pasted plates, or Faure plates after the name 
 of the inventor of the method. Sometimes the oxide is formed by 
 frequent charging and discharging of the cell. This process is called 
 forming, and plates of this kind are called PI ante plates, after the 
 original inventor of the lead plate storage battery, who used this 
 method. 
 
 Figure 1 8 shows a lead plate storage cell in a glass jar, and fig- 
 ure 19 shows one in a wooden box lined with rubber. In order that the 
 cell may have a capacity for a large current, a number of positive and 
 negative plates are put alternately in one jar and are connected in 
 parallel that is, the plates are connected so that the current capacity 
 of the cell is equal to the sum of the capacities of the various plates, 
 but the pressure of the cell is the same as that of a cell made up of a 
 single pair of plates. 
 
 The positive plates of a lead plate storage battery usually have a 
 brownish color and the negative plates a greyish color. The electri- 
 cal pressure produced by a lead plate cell generally varies between 
 1.8 and 2.3 volts at different conditions of the charge. 
 
 Commercial storage batteries are made with other liquids than 
 sulphuric acid and other than lead plates, but they cannot be given 
 consideration here as they have not come into common use. 
 
 Copyrighted, 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON IV. 
 
 Points for Review. 1. For what kind of service are closed circuit cells adapted? 
 
 2. What is local action? 
 
 3. How may local action be avoided? 
 
 4. What is the law of electrochemical action? 
 
 5. What is the electrochemical equivalent of a metal? 
 
 6. Why are primary batteries of the ordinary commercial forms unable to compete 
 with dynamos driven by steam engines in furnishing electric currents for electric light 
 and power? 
 
 7. For what purposes are primary batteries particularly adapted? 
 
 8. What is the difference between a primary battery and a storage battery? 
 
 9. Of what are the common commercial storage batteries made? 
 
 10. Why are a large number of plates connected together in parallel in the usual 
 storage cells? 
 
 LESSON V. 
 
 THE NATURE AND PROPERTIES OF MAGNETISM. 
 MAGNETIC FIELDS. 
 
 The true nature of magnetism seems to be very closely con- 
 nected with that of electricity and it will probably not be exactly 
 known till the exact nature of electricity is determined. The word 
 magnet probably comes from the Greek word for the country of Mag- 
 nesia, which is a small division of Ancient Greece, where a deposit 
 of magnetic iron ore or lodestones (also called loadstones) was known 
 to the Greeks. Some of the properties of magnets were known many 
 centuries before the Christian era. It is said that the Chinese used 
 a device similar to the compass to guide their way across the plains 
 of Tartary as early as 1,000 B. C., and some say much earlier, but in 
 Europe the use of the compass did not become general until the 
 thirteenth century of the Christian era. The attractive power which 
 magnets have for iron is mentioned by many early writers Plato, 
 Euripides, and Thales, the Greek philosopher mentioned in Lesson i 
 (page i), all speak of the lodestone or magnet. Dr. Gilbert, who 
 coined the word electrical (Lesson i, page i), made a great many" 
 experiments with magnets and magnetic materials. Dr. Gilbert 
 seems to have been the first to notice that the attractive power of 
 magnets is greatest near certain points, or poles as he named them. 
 
 Pieces of iron ore composed of oxide of iron, which is called 
 magnetite or magnetic iron ore, when in a pure form, sometimes 
 have the peculiar property of attracting pieces of iron and they are 
 theri called lodestones. The property held by the lodestone is called 
 magnetism, and the body having the property of magnetism is called 
 a magnet. The action of magnets led some of the earlier experi- 
 menters to look upon magnetism as due to a magnetic fluid, but this 
 Idea has been proved to be wrong. It is found that pieces of steel 
 
 27 
 
FIG. 20. FIG. 21. FIG. 23. 
 
 which touch a lodestone or other magnet, become magnets. Magnets 
 thus made are sometimes called artificial magnets, and lodestones are 
 called natural magnets. When pieces of soft iron touch a magnet 
 they also become magnets, or are magnetized while in contact with 
 the magnet, but when separated from the magnet the magnetism of 
 the soft iron disappears. This is called temporary magnetism, while 
 the magnetism of hard steel which remains permanently is called 
 permanent magnetism. 
 
 If a magnet be suspended on a pivot or a thread it w r ill be found 
 to point nearly north and south, and if it is pivoted at the center the 
 north end will dip down as though it were heavier than the south 
 end. A small, elongated magnet thus suspended is called a magnetic 
 needle (Fig. 20). If a magnetic needle be turned from the direction 
 which it naturally takes when free to swing horizontally on its pivot, 
 it will at once return, swinging to and fro until it settles down in its 
 original position. The pole of a suspended needle which points to 
 the north is called the north pole and the other pole is called the 
 south pole. This tendency of a magnetic needle to set itself north 
 and south is the foundation of the compass, which essentially con- 
 sists of a magnetic needle mounted over a dial. It is usual in com- 
 passes to counter-balance the needle, or pivot it so that it will hang 
 horizontally, but dip needles are sometimes constructed of magnetic 
 \ieedles mounted on horizontal -pivots (Fig. 21). When a dip needle 
 is turned north and south its north pole turns down towards the earth 
 as already explained. 
 
 If a pole of a magnet be brought near a magnetic needle it will 
 be fcnmd to attract one pole of the needle and repel the other pole. 
 The north pole of the magnet may be determined by noting the way 
 it stands when suspended by a thread, and it will be found that its 
 north pole always repels the north pole of the needle and attracts the 
 south pole of the needle. The south pole of the magnet acts in exactly 
 an opposite manner. This action shows that there are tivo kinds of 
 magnetic poles and that poles of the same kind repel each other and 
 poles of opposite kinds attract each other. This is quite similar to the 
 
FIG. 22. 
 
 FIG. 24. FIG. 25. 
 
 law of the attractions and repulsions of electric charges given on 
 page 3 of Lesson i . 
 
 Magnets made from straight bars of steel are called bar magnets 
 (Fig. 22), and those made from bars of steel bent into horseshoe form 
 are called horseshoe magnets (Fig. 23). The north pole of a magnet 
 is often called the positive or plus (-J-) pole, and the south pole is often 
 called the negative or minus ( ) pole. Since the positive pole turns 
 towards the north it is sometimes called the north seeking pole, and 
 the negative pole is in the same way sometimes called the south 
 seeking pole. 
 
 If the experiment with a magnetic needle, described in the para- 
 graph above, be repeated, but a bar of soft iron be used in the place 
 of the magnet, it is found that either end of the iron bar attracts 
 either pole of the needle. If the iron bar be laid with one end 
 near the pole of a magnet it may be shown to be magnetized by mov- 
 ing a magnetic needle around it. The needle will show by its action 
 that the end of the iron bar which is near the magnet pole has be- 
 come a pole of sign opposite to that of the magnet, and the farther 
 end of the bar has become a pole of the same sign as that of the mag- 
 net. The bar is said to be magnetized by induction. The magnet- 
 ism in the iron bar becomes stronger as it is brought closer to the 
 magnet pole, and is greatest when the iron is in contact with the 
 magnet pole. 
 
 We are now in a position to see why a magnet attracts a piece 
 of iron, and the cause' for the effect of the iron bar on the magnetic 
 needle. When a steel magnet pole is brought near to a piece of iron, 
 the iron is magnetized by induction. The positive and negative 
 poles induced in the iron are of equal magnitude. One of the induced 
 poles is attracted and the other is repelled by the steel magnet pole, 
 but that which is attracted is nearest the original magnet pole and 
 the force of attraction is therefore greater than the force of repulsion. 
 The effect of a bar of iron on a magnetic needle is caused in the same 
 way by the magnetism induced in the bar by the poles of the 
 needle. 
 
 The magnetism induced in a bar of iron may induce magnetism 
 in another piece, and this in another piece, and so on, but the mag- 
 
 29 
 
netism in each successive piece is weaker than in the preceding 
 piece. Thus a magnet may be made to support a string of several 
 nails end to end (Fig. 24). 
 
 For every pole induced in a piece of iron or steel another pole of 
 equal strength is produced. For instance, if the north pole of a mag- 
 net be touched to one end of a bar of iron a south pole is induced in 
 that end, and an equal north pole in the other end. If the two ends 
 of the iron bar be touched by the north poles of two equal magnets, 
 south poles are induced in both ends of the bar. In this case an 
 examination of the bar with a magnetic needle shows that a north 
 pole, which is equivalent to two poles, is produced near the center 
 of the bar. Again, if a magnet be broken, it will be found that 
 each piece has two equal and opposite poles. We are, therefore, 
 justified in saying that for every magnet pole that exists, there exists 
 in the same magnetic body an equal and opposite pole. This is quite 
 similar to the existence along with every electric charge of an equal 
 and opposite charge (Lesson i, page 3). Magnetic force will act 
 through a vacuum and through all materials except those in which 
 magnetism may be induced. 
 
 The actual force exerted between two magnets depends upon 
 the strength of their poles and their distance apart. If it were pos- 
 sible to have two separate magnet poles of small size as compared 
 with their distance apart, the force exerted between them would be 
 equal to the product of the strength of the poles divided by the 
 square of their distance apart. This is similar to the law of the 
 force exerted between two small isolated bodies holding electric 
 charges. 
 
 The condition required for this law of force to be fulfilled can 
 only be gained by using poles of two very long, thin magnets. The 
 force between two magnets, which may usually be measured, does 
 not follow this law directly, because the poles are of considerable 
 magnitude as compared with their distance apart. Every small por- 
 tion of the pole of one magnet exerts a force on every small portion 
 of the pole of the other magnet according to the law, and when all 
 these small forces are added together the law is apparently changed v 
 though it is based on the fundamental one. 
 
 Material in which magnetism may be induced, and which is 
 therefore attracted by a magnet, is called magnetic material. Iron 
 in its various forms is the , most strongly magnetic material known. 
 There are only a few other materials that are known to be magnetic. 
 Of these the metals nickel and cobalt are the commonest. All mate- 
 rials which are not quite strongly magnetic are usually spoken of as 
 nonmagnetic^ since they are nearly neutral as regards magnetism. 
 Magnetic materials are sometimes za\\& paramagnetic, and nonma- 
 gnetic materials are sometimes called diamagnetic. 
 
 As was stated earlier in this lesson, if a magnet be broken each 
 piece will be a complete magnet, however small the pieces may be. 
 This points to the fact, which is now generally believed to be true, 
 
 30 
 
that the final particles, or molecules, of magnetic material are little 
 magnets, each having its own north and south poles. When mag- 
 netic material is unmagnetized it is supposed that the molecules are 
 arranged in a hap-hazard manner, or in groups, so that they neutralize 
 each other's magnetic effects. When the material is subjected to the 
 influence of magnetic force the molecular magnets are all attracted 
 around, so that their poles point more or less in the same direction. 
 In Fig. 25 the small blocks may be taken to roughly represent 
 magnetic molecules which are very greatly magnified. The dark 
 ends represent their south poles and the light ends their north poles. 
 When they are arranged with their like poles all pointing in the 
 same direction as in the figure, it is readily seen that the poles in 
 the interior of the material neutralize each other's effect and mag- 
 netism shows at the ends. 
 
 It is found that some materials are more readily magnetized than 
 others. Thus, soft iron is very readily magnetized, but loses almost 
 all of its magnetism if it is slightly jarred when the external magnet- 
 izing force is withdrawn. Hard steel is usually quite hard to mag- 
 netize, but it retains its magnetism quite strongly. Generally 
 speaking, the harder the steel, the more difficult it is to magnetize, 
 and the more strongly it retains its magnetism. We are forced, then, 
 to the belief that there is some force that prevents the molecular 
 magnets from being turned away from any position which they hap- 
 pen to be in. This hindering force is called coercive force. The 
 coercive force of soft iron is quite small and of hard steel very great. 
 The effect of the coercive force is counteracted to some extent by 
 anything which is likely to make the molecules vibrate, such as 
 rough handling, heating, etc. A magnet which is dropped on the 
 floor a few times is likely to lose much of its magnetism. Heating 
 to a red heat will completely demagnetise a magnet. 
 
 If a magnet is magnetized as strongly as possible it is said to be 
 saturated. When a magnet is saturated it will generally grow weaker 
 for a certain time after magnetization, till it finally becomes constant 
 in strength. The magnet may be artificially aged, as it is called, 
 and thus brought to a fairly constant strength, by immersing i* in 
 steam for a considerable time. 
 
 There are certain similarities which may readily be seen between 
 the action of magnets and of charged bodies, but there are also 
 many marked differences, so that a close relationship is not evident. 
 There is a remarkably close relationship between magnetism and 
 current electricity, which will be taken up in the next lesson. 
 
 Before leaving this lesson it is necessary to examine the question 
 of what causes a magnetic needle to set itself north and south. The 
 mutual action of magnets which has been explained, leads at once to 
 the conclusion that the earth is a great magnet. The reason for the 
 earth's magnetism is not known, but its magnetic strength and the 
 
 31 
 
location of its poles have been carefully determined. One magnetic 
 pole is near the true north pole of the earth. This is the one 
 towards which the north pole of a magnetic needle points. The loca- 
 tion of the pole varies slightly from time to time, but the actual 
 variation of the magnetic needle from the true north, or its declina- 
 tion, and the rate at which the declination changes at any part of the 
 earth, may be determined and marked on a map. Such maps may 
 be used for correcting the indications of a compass. 
 
 Any space in which there is magnetism and consequently 
 magnetic force is called a magnetic field, or afield of magnetic force. 
 
 The magnitude or intensity of the magnetic force at any point is 
 called the strength of the field at that point. 
 
 The theoretical method of measuring the strength of a field is by 
 determining the force which a magnet pole experiences when placed 
 in the field. A magnet pole which exerts a push equal in magnitude 
 to the force called a dyne upon an exactly equal pole, when the two 
 are one centimeter (metric measure) apart, is called a magnet pole of 
 unit strength, or a unit pole. The strength of a magnetic field is given 
 by the number of dynes of force which it exerts upon a magnet pole 
 of unit strength. 
 
 If an independent north pole could be placed in front of the 
 north pole of a magnet it would be repelled by the latter pole and 
 attracted by the south pole of the magnet. This would cause the 
 independent pole to move away from the magnet's north pole and 
 towards its south pole, but as it moved it would continually change 
 its relative distance from the two poles, and the relative magnitude 
 
 c 
 c 
 
 FIG. 26. FIG. 28. 
 
 of the force exerted upon it by the two poles would vary. The 
 direction of the motion of the independent pole would depend upon 
 the relative direction and magnitude of the forces which the two 
 poles of the magnet exerted on it at every point. The actual path 
 would be a curved line very much like the line AB in Fig. 
 26, An independent south pole would move in an opposite direction, 
 of course, but over a similar path. 
 
 As already explained (Lesson 5, page 30) it is impossible to have 
 an independent magnet pole, but for this experiment the companion 
 pole may be sufficiently far removed to satisfactorily show the action. 
 A shallow glass dish containing a little water may be placed over a 
 magnet. By properly sticking a magnetized sewing needle in a cork 
 
 32 
 
FIG. 27. 
 
 it may be floated upon the water in a vertical position with one of 
 its poles close to the bottom of the dish. Then the upper pole will 
 be so much farther away from the magnet than the lower one that 
 the latter will be affected by the force due to the magnet almost like 
 an independent pole. If the lower pole of the needle is a north pole 
 it will tend to move through the water, when placed in front of the 
 north pole of the magnet, in a curved line away from the north pole 
 and towards the south pole. If the lower pole of the needle is a 
 south pole it will tend to move from the south pole towards the north 
 pole. This is exactly as already explained for an independent mag- 
 net pole. The experiment here outlined and which may be so 
 readily tried, is more striking when the magnet is a strong electro- 
 magnet such as will be explained later, because the force acting on 
 the floating needle to make it move is then greater. 
 
 The direction of the force at different points of the magnetic 
 field which is around a magnet may be shown by another simple 
 experiment. A sheet of paper may be laid over the magnet and iron 
 filings sifted over it. Now if the paper be slightly tapped the filings 
 will arrange themselves in curved lines like those shown in Fig. 27, 
 all of which converge towards the two poles. If the figure were 
 sufficiently large it would be approximately shown that every line 
 which starts out from one pole finds its way round to the other pole. 
 The lines of iron filings may be easily fixed in position if the paper is 
 paraffined before using it, by simply passing the flame of a Bunsen gas 
 burner over it. This softens the paraffine and the bits of iron stick 
 fast. 
 
 The magnetic field exists all around a magnet exactly as shown 
 by these experiments in one plane. This may be shown by hanging 
 a short magnetized sewing needle on a light thread and bringing it 
 near a magnet. It will take a position at every point so that its direc- 
 tion is tangent to the direction which a line of iron filings would 
 take at the same point. The reason for the needle taking this posi- 
 tion is because its north pole tends to go one way and its south pole 
 
 33 
 
the other, so that the needle turns around until the pull on the two 
 poles is in a direct line through the length of the needle. The iron 
 filings used in the experiment described above are nothing more than 
 little magnets caused by induction, and they take up their position 
 for the same reason that the needle does. 
 
 It must be remembered that in all cases of attraction or repulsion 
 between two bodies the force exerted is mutual, and either body will 
 be moved if not too firmly fixed. This is true whatever be 
 the cause of the force, as for instance, electrification, magnetism, 
 gravity, muscular force, or any other cause. The fact that the 
 . action is mutual may be proved by placing a bit of iron on a cork 
 floating in water and presenting a small magnet to it. The iron will 
 be attracted by the magnet and the cork will be moved through the 
 water by the force of the attraction. Now if the magnet be placed 
 upon the cork and the iron be brought near it, the attraction between 
 the magnet and the iron will again move the cork. Finally, if the 
 iron and the magnet be placed on separate corks, the corks will move 
 towards each other. This shows that \.\\z force is mutual, and it is 
 also possible to show that the pull is always equal on the iron and 
 the magnet. 
 
 A convenient way of looking upon a magnetic field is to con- 
 sider it a space which is more or less filled with magnetic lines of 
 force. The strength of field may be represented by the number of 
 lines of force to the square centimeter (metric measure). Then if 
 the strength of field be such, for instance, that a unit pole when 
 placed in it experiences a force of ten dynes, we may consider the 
 field as having ten lines of force per square centimeter. These lines 
 of force no more actually exist than do definite stream lines, or lines 
 of flow, exist ;^i water which is flowing around in a tub, but the idea 
 based on this assumed existence is a very useful and practical one. 
 
 It is useful to define the positive direction, or down stream 
 direction as it were, along lines of force. As a matter of convenience 
 the direction along the line of force outside of a magnet from the 
 north pole to the south pole, or the direction in which an independent 
 north pole would tend to move, is called the positive direction. It is 
 also useful to consider the lines of force as continuing through the 
 material of a magnet from the south pole to the north pole, so that 
 they make complete curves, as shown in Fig. 28. 
 
 From what we have learned of the mutual action of magnets we 
 can now see that when a magnet is placed in a magnetic field it 
 apparently tends to set itself in such a direction that its own lines of 
 force where they are within its body are parallel with the lines of 
 force of the external field. The eifect is exactly as though lines of 
 forceMend to turn themselves so as to be parallel with each other and 
 in the same direction. 
 
 Copyright 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON V. 
 
 Points for Review. 1. What is the magnetism called which remains in a piece 
 of steel after it has been subjected to the influence of a magnet? 
 
 2. What is the magnetism called which appears in a piece of iron when it is placed 
 under the influence of a magnet, but which disappears when the influence is withdrawn? 
 
 3. What is the law of attraction and repulsion of magnet poles? 
 
 4. What are the two poles of a magnet called, and why? 
 
 5. Why does a magnet always attract a piece of iron? 
 
 6. Upon what does the magnitude of the force exerted between two magnets depend? 
 
 7. What is the force called which causes the magnetism of steel to become 
 permanent? 
 
 8. How may a magnet be "aged"? 
 
 9. Why does the earth cause a magnetic needle to set itself north and south? 
 
 10. Is the magnetism of the earth constant in direction? 
 
 11. What is a magnetic field ? 
 
 12. What is the strength of a magnetic field ? 
 
 13. What will a magnet do if it is placed in a magnetic field ? 
 
 14. If a piece of iron is brought near a magnet, and is therefore attracted by the 
 magnet, how is the magnet affected ? 
 
 15. What is the positive, or down stream, direction along the lines of force ? 
 
 16. How do lines of force act toward one another ? 
 
 LESSON VI. 
 
 THE MAGNETIC EFFECTS OF ELECTRIC CURRENTS, 
 AND MAGNETIC CIRCUITS. 
 
 The real connection which exists between magnetism and currents 
 of electricity was not made generally known until Oersted, a Danish 
 scientist, published the fact in 1819 that a magnetic needle is dis- 
 turbed by the presence of an electric current in its neighborhood. 
 This fact had really been discovered earlier, but it did not become 
 generally known. It had also been known that under some conditions 
 lightning discharges had magnetized steel needles, but the conditions 
 had not been successfully reproduced by experimenters. The publi- 
 cation of a series of experiments by Oersted therefore led a number 
 of eminent scientists to turn their attention during the early part of 
 this century to a determination, as complete as possible, of the exact 
 relation existing between electricity and magnetism. 
 
 If a magnetic needle be placed above or below a wire which 
 carries an electric current, the needle will turn on its pivot so as to 
 set itself as nearly as possible at right angles to the wire. This may 
 be readily tried by connecting a short piece of copper wire to one or 
 two cells of gravity battery and holding the wire above the needle 
 
 35 
 
FIG. 29. 
 
 (Fig. 29) while the current flows through it. The effect on the 
 needle may be made most evident by making and breaking the 
 electric circuit, which will cause the needle to swing back and forth. 
 The current in the wire has the greatest effect in causing the needle 
 to deflect from the north and south position when the wire also lies 
 in a north and south direction that is, when the wire is parallel 
 with the needle. 
 
 When not disturbed by other magnetic effects the needle stands 
 north and south on account of the force due to the earth's magnetism. 
 When the electric current is placed so as to flow near the magnetic 
 needle, the needle is affected by the force of a magnetic field ivhich 
 is set up by the current, which tends to make the needle set itself at 
 right angles to the wire carrying the current. The needle takes an 
 intermediate position where the effect of the two magnetic forces 
 balance. Its position therefore depends upon the magnitude of the 
 force due to the earth's magnetism and the direction and magnitude 
 of the force due to the magnetism set up by the current. 
 
 Magnetism set up by an electric current is called electromagnet- 
 ism. The direction of the magnetic force due to electromagnetism 
 is always at right angles to the direction of the current which pro- 
 duces the magnetism, and the lines offeree in the magnetic field due 
 to the current must therefore be circles surrounding the wire which 
 carries the current. The reason why a magnetic field is set up by 
 an electric current is entirely unknown; merely the experimental 
 fact and its applications are known, 
 
 The strength of the magnetic field at any point due to an electric 
 current near by, depends directly upon the strength of the current 
 and upon the average distance of the current from the point 
 
 The magnetic field which surrounds a wire when a current 
 flows in it may be shown in a way similar to that used to show 
 the field around a magnet. A stout copper wire may be passed 
 vertically through a hole in a horizontal sheet of stiff paper. If iron 
 filings be sprinkled upon the paper they will arrange themselves in 
 circles around tlje wire when a current is passed through it. If a 
 small magnetic needle or compass be placed on the paper with its 
 center over a line of filings, the needle will tend to stand at a tan- 
 

 FIG. 30. 
 
 to the line (Fig. 30). An independent pole would tend to move 
 continuously around the wire along one of the lines. 
 
 The direction in which the magnetic needle points in this case 
 depends upon which side of the wire it stands, and upon the direction 
 in which the current flows in the wire. In Fig. 30 it is evident 
 from the position of the magnetic needles, the black ends of which 
 represent north poles, that the positive direction along the lines of 
 force is there left-handed or against the direction in which the hands 
 of a clock move. If the direction of the current were reversed, the 
 magnetic needles would also reverse their direction, showing that the 
 positive direction of the lines of force has a fixed relation to the 
 direction of the current. 
 
 There are various ways of remembering this relation. One is 
 to consider an ordinary right-handed screw which is being screwed 
 into or out of a nut (Fig. 31). If an electric current be considered 
 as flowing through the screw in the direction which the screw moves 
 through the nut, then the positive direction of the lines of force 
 is shown by the direction in which the screw turns. 
 
 Another way of remembering this relation is according to a rule 
 proposed by Ampere, after whom the unit of electric current was 
 named. Suppose a man lying in the wire with his head down the 
 electric stream (swimming with the electric current); then if he faces 
 a magnetic needle placed near the wire, the north pole of the needle 
 will tend to turn towards his left hand. 
 
 This relation between the direction of the current flow and the 
 deflection of a magnetic needle gives a ready method for determin- 
 ing the direction of the current in a wire, the only instrument which 
 is required being a small compass. The compass may be placed 
 
 37 
 
FIG. 32. FIG. 34. 
 
 under the wire and the direction towards which its north pole turns 
 noted. Then an application of " Ampere's rule" gives the direction 
 of the current. 
 
 Since we have seen that a force acting between two bodies 
 always affects them both, we may expect that a wire which carries a 
 current will tend to move when brought near a fixed magnet. This 
 may be readilv shown by suspending a very flexible conducting wire 
 near a fixed magnet (Fig. 32). When a current is passed through 
 the wire it will wind itself around the magnet. If the current be 
 reversed the wire will unwind and then wind around the magnet 
 again, but in the opposite direction. 
 
 The motions of the wire and the magnet are due to the appar- 
 ent tendency of magnetic lines of force to move out of a position 
 where they are not parallel and into a position where they are parallel 
 and in the same direction (Lesson 5, page 34) 
 
 By applying Ampere's rule we see that if a wire carrying a 
 current be passed above a magnetic needle and then turned back and 
 passed below the needle, both the top and bottom branches tend to 
 
 38 
 
deflect the needle in the same direction, so that the effect of the cur- 
 rent on the needle is increased. By coiling the .wire about the posi- 
 tion of the needle each additional turn will cause an additional force 
 to deflect the needle. In this way the magnetic effect of a current 
 may be greatly multiplied. It has already been said that the mag- 
 netic force at a point due to a current near it depends upon the 
 strength of the current (Lesson 6, page 36). We now see that when 
 a current is coiled around a point the force depends upon the 
 strength of the current multiplied by the number of turns in the coil. 
 The product of the current by the turns is usually called current- 
 turns or ampere-turns. 
 
 When a wire carrying a current is coiled into a ring or helix, 
 the lines of force which surround each turn seem to join together so 
 that they belong to the coil or winding as a whole (Fig. 33). Such 
 coils are often called solenoids. Such coils, when a current is passed 
 through them, exhibit all the magnetic effects which are shown by 
 steel magnets. They attract and repel magnets and other solenoids, 
 and attract pieces of iron. If suspended so that they are free to 
 swing, they turn into a north and south position exactly like 
 magnets. 
 
 This magnetic effect of coils or solenoids, led Ampere to suppose 
 that all magnetism is caused by electric currents. He therefore sug- 
 gested that the molecules of magnetic materials, and possibly of all 
 materials, have little electric currents flowing around them which 
 make them into magnets. This is called "Ampere's theory" of 
 magnetism. If it is correct it gives a ready explanation of why 
 magnetism is found in various materials, but it still leaves unex- 
 plained the reason for the electric current causing magnetism. 
 Ampere's theory, and other theories of magnetism advanced by var- 
 ious other scientists, have been before the scientific world for many 
 years, but their correctness has not yet been either proved or dis- 
 proved. We are therefore forced to content ourselves with the 
 knowledge that the molecules of magnetic materials, at least, are 
 magnetic (Lesson 5, page 31). 
 
 If a bar of hard steel be placed in a solenoid through which a 
 current is passing, it becomes strongly magnetized, and remains per- 
 manently magnetized when the current is stopped or the steel is with- 
 drawn from the solenoid. This effect is exactly the same as would 
 be obtained by touching the steel with a permanent magnet, but the 
 magnetic effect of a solenoid with many turns of wire may be made 
 much greater than any permanent magnet and the steel may there- 
 fore be more readily saturated by the solenoid. 
 
 If a bar of soft iron be placed in the solenoid it becomes even 
 more strongly magnetized than the steel, when the current is turned 
 on. When the current is turned off, the iron loses nearly all of* its 
 magnetism. If the bar is very soft Swedish iron its coercive force is 
 
 39 
 
so small that the least tap shakes practically all the magnetism out 
 of it. Harder and less pure iron retains a little of the magnetism, 
 the amount depending upon the quality of the iron. The magnetism 
 which is retained by iron after it has been magnetized is called resid- 
 ual magnetism. 
 
 The property of soft iron by which it becomes strongly magnet- 
 ized when placed within a solenoid carrying an electric current, and 
 then loses its magnetism upon breaking the current, was discovered 
 by William Sturgeon in 1825. An arrangement consisting of a soft 
 iron core which is surrounded by a solenoid or winding is called an 
 electromagnet. Electromagnets are of the greatest value in the 
 electrical industries because they can be built of practically any desired 
 size and form, and of enormous strength. The magnets of com- 
 mercial dynamos and electric motors are always electromagnets. 
 Figure 34 shows two forms of horseshoe electromagnets. 
 
 At the time of the discovery of the electromagnet nothing was 
 known of its great commercial future; but it was welcomed with the 
 highest scientific interest. At that day the laws of electric circuits 
 were unknown, the common insulated wire of today was not made, 
 and the manufacture of an electromagnet was a matter of much 
 labor. Moreover, the only sources of current were, at first, plain 
 zinc-copper cells, and later, Grove, Daniell, or similar types of galvanic 
 cells. Many electromagnets were soon made, however, and their 
 effects were carefully studied by enthusiastic scientists, in spite of the 
 difficulties to be overcome. By the year 1845, n ly 5 years ago, the 
 investigators had succeeded in overcoming their lack of experimental 
 facilities and had mapped out the laws of magnetic circuits very much 
 as we know them at the present time. Thus was laid the foundation 
 of the profession of electrical engineering. 
 
 From what has preceded we may see that a solenoid in which a 
 current flows and which contains a soft iron core is a stronger magnet 
 than a similar solenoid containing a hard steel core, and it is a very 
 much stronger magnet than a similar solenoid containing no core. 
 Remembering that according to our ideas of lines offeree, the strength 
 of the magnetism of the solenoid and core depends on the number of 
 lines of force which pass through the solenoid ; then since so many 
 more lines of force pass through a solenoid when a steel bar is placed 
 in it than passed through it when the space within the solenoid was 
 simply occupied by air, we may conclude that lines of force are more 
 readily set up in steel than in air. Since a soft iron core causes more 
 lines of force to pass through a solenoid than does a steel core, we 
 may also conclude that lines of force are even more readily set up in 
 soft iron. 
 
 The relative ease with which magnetic lines of force may be set 
 up in a body is called its permeability. As a matter of convenience 
 it is usual to say that the permeability of air is unity (i). As com- 
 
 4,0 
 
pared with this, the permeability of soft iron may be enormously great. 
 In some cases it becomes many thousand times as great as that of air. 
 
 The permeability of all materials, except a few highly magnetic 
 ones, is very nearly unity. The proper division between materials 
 that are called paramagnetic and those that are called diamagnetic 
 (Lesson 5, page 30) depends upon whether their permeability is greater 
 than unity or is slightly less. 
 
 We may divide materials into good conductors of magnetic lines 
 of force or good magnetic conductors, and poor magnetic conductors. 
 There are no materials which we may look upon as being really 
 magnetic insulators, as we may look upon some materials as being 
 practical insulators of electricity. The permeability of a material 
 may be called its specific magnetic conductivity, or the magnetic con- 
 ducting power of a block of the material which is one centimeter long 
 and has an area of one square centimeter. The actual magnetic 
 conducting power or conductivity of a piece of material decreases 
 with the length and increases with the, cross-section of the piece. 
 This may be likened to electrical conducting power or conductivity 
 (Lesson 2, page 13). The opposite or reciprocal of magnetic conduct- 
 ing power may be called magnetic -resistance or reluctance, and any 
 path through which lines of force pass may be called a magnetic cir- 
 cuit. These terms will be seen to be entirely similar to the terms 
 applied in the case of electric currents. 
 
 By similarity with electric circuits we may say that it takes some 
 force to set up lines of force in a magnetic field or magnetic circuit. 
 We may call this magnetomotive force or magnetic pressure, terms 
 which are similar to electromotive force and electrical pressure. 
 The number of lines of force in any magnetic circuit is equal to 
 the magnetic pressure divided by the reluctance of the circuit. It 
 can be shown mathematically that the magnetic pressure in a com- 
 plete magnetic circuit is equal to the number of ampere-turns mul- 
 tiplied by a constant which is practically equal to i%. In order that 
 the .strongest possible magnetism shall be produced in any magnetic 
 circuit it is necessary to have the circuit made up as far as possible of 
 material having the highest permeability that is, soft iron and to 
 arrange as many ampere-turns as possible to set up the magnetism. 
 
 The apparent similarities of electric and magnetic circuits, and 
 their really fundamental differences, will be taken up in some of the 
 later lessons. 
 
 Copyrighted, 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON VI. 
 
 Points for Review \. How is a magnetic needle affected when brought near an 
 electric current? Why? 
 
 2. What is the magnetism called which is set up by an electric current? 
 
 3. How may the relation between the direction of lines of force which surround 
 a current, and the direction in which the current itself flows, be remembered? 
 
 4. How may the direction in which a current flows be determined through the 
 indications of a compass needle? 
 
 5. How is a current affected by the presence of a magnet? 
 
 6. How does coiling a wire around a magnetic needle affect the magnetic force 
 which the current in the wire exerts on the needle? 
 
 7. What dees the phrase "ampere-turns" mean? 
 
 8. In what respects are solenoids carrying a current similar to steel magnets? 
 
 9. What is Ampere's theory of magnetism? 
 
 10. What affect does a solenoid carrying a current have on a bar of steel which is 
 placed inside of it? What effect does it have on a bar of iron? 
 
 11. What is residual magnetism? 
 
 12. What is an electromagnet? 
 
 13. What is meant by the magnetic permeability of a material? What is its prac- 
 tical value for most materials taken to be? How does its value for most materials com- 
 pare with its value for a few magnetic materials? 
 
 14. What is meant by the magnetic reluctance of a magnetic circuit? What is meant 
 by magnetomotive force or magnetic pressure? 
 
 15. What arrangements must be made in order that the strongest possible magnet- 
 ism may be set up in a magnetic circuit? 
 
 LESSON VII. 
 
 OHM'S LAW OF THE FLOW OF ELECTRICITY. 
 
 When water is forced through a pipe under pressure from a 
 pump or other source of pressure, the stream of water which flows is 
 proportional to the pressure divided by the frictional resistance which 
 the pipe presents to the flow of the water. In the same way, when 
 a current of electricity flows through a wire under the pressure from 
 a battery or other source of electricity, the current which flows 
 in the circuit is equal to the pressure divided by the resistance of the 
 circuit. This relation between electric current, pressure, and resis- 
 tance is called Ohm's Law, after the name of the German scientist 
 who first formally announced it. The relation representing Ohm's 
 Law is often written C = |, where C, E and R stand for current, 
 pressure and resistance. This is a very good form in which to com- 
 mit the relation to memory. The expression as written may be read 
 C equals E divided by R. From the relation as written above it is 
 evident, also, that E equals C times R, and R equals E divided by 
 
 42 
 
C. Consequently if any two out of the three fundamental electrical 
 quantities which exist in a circuit are given, the third can at once 
 be calculated. Thus, if a 16 candle-power incandescent lamp is 
 known to take y 2 an ampere when connected to a circuit which fur- 
 nishes current at a pressure of no volts, the resistance of the lamp 
 when in operation may be calculated at once to be no divided by ^, 
 which gives the resistance as 220 ohms. 
 
 In this example we have assumed that the source of electricity 
 has sufficient capacity to keep up the full pressure at the lamp termi- 
 nals when current is flowing through the lamp. Sometimes this is 
 not the case on account of the resistance to be found in the source 
 itself, or the internal resistance of the source. A similar condition 
 is frequently met when a pump is attached to a large hose. When 
 the hose nozzle is closed the pump will give a large pressure, but 
 when the nozzle is opened the pressure falls because the pump does 
 not have sufficient capacity to keep up the supply. 
 
 When it is desired to determine the current that will flow 
 through a circuit due to a pressure from a source of current that has 
 an appreciable internal resistance, it is necessary to add up the re- 
 sistances of all parts of the circuit before making the calculation. 
 For instance, if two cells of battery each giving a pressure of i.i 
 volts, and each having an internal resistance of 3 ohms, be con- 
 nected to an external circuit of 2.8 ohms resistance, then the total 
 resistance in the circuit is 8.8 ohms and the pressure which acts to 
 pass current through the circuit is 2. 2 volts. The current flowing 
 under these circumstances is ^ ampere (C ER or ^2.2/8.8). 
 
 The resistance to the flow of water through a pipe is a surface 
 or "skin" effect, and depends upon the velocity with which the 
 water flows, the number and form of the bends in the pipe, the form 
 of its cross section, and its length. The true electrical resistance of 
 a conductor is quite different from this, since it simply depends upon 
 the nature of the metal from which the conductor is made, the area 
 of its cross section, its length, and its temperature. 
 
 The greater the cross section of a conductor the greater is its 
 electrical conducting power, and therefore the less its resistance; 
 and the longer the wire the less is its conducting power, and there- 
 fore the greater is its resistance. The cross sections of the ordinary 
 cylindrical wires are proportional to the squares of their diameters, 
 and consequently the conducting powers of similar wires are directly 
 proportional to the squares of their diameters. This makes the 
 resistances of similar wires to be inversely as the squares of their 
 diameters. For instance, if a certain copper wire has a resistance of 
 one ohm, the resistance of a copper wire of the same length but of 
 twice the diameter is only one-fourth of an ohm, since the square of 
 two is four. 
 
 The adopted definition of the value of the ohm is based upon 
 this property of electrical resistance depending simply upon the 
 
 43 
 
nature of the metal composing the conductor, its temperature, its 
 length and the inverse of its cross section. The approved definitions 
 of all the electrical units were adopted at the Electrical Congress 
 held in Chicago in August, 1893. The definition of the unit of 
 resistance makes one ohm equal to the resistance of a column of 
 pure mercury which is 106.3 centimeters long and has a uniform 
 cross section which contains 14.4521 grammes of mercury, the 
 temperature being that of melting ice. This gives the column the 
 uniform cross section of one square millimeter (metric system). The 
 ohm as thus defined is called the International Ohm to distinguish it 
 from units based on definitions adopted at previous electrical con- 
 gresses, and which differ slightly from the International Ohm and 
 from one another, exactly as different kinds of quart measures differ 
 from one another, as is told in books on arithmetic. It is generally 
 believed that the definitions given by the Chicago Electrical Congress 
 will be universally accepted and will never be changed. The units 
 by which electricity is measured will then be the same in all coun- 
 tries. This is true of no other units which are used in common 
 measurements. 
 
 Before the Chicago Electrical Congress was held, the funda- 
 mental definition of the ampere had usually been based upon the 
 electromagnetic effect of currents, but at that Congress a definition 
 was adopted which is based on the electrochemical effect of currents. 
 The International Ampere as thus defined is the steady current which 
 deposits silver at the rate of .001118 grammes per second from a 
 solution of silver nitrate in water, the solution being of a fixed 
 strength to make sure of the action being regular. 
 
 In order that the fixed relation represented by Ohm's L,aw(C |) 
 shall hold with these definitions, the definition of the International 
 Volt by the Chicago Congress is the pressure which causes a current 
 of one ampere to flow through a resistance of one ohm. 
 
 Since a column of mercury is an inconvenient device to handle, 
 standard resistances made of mercury are not used in ordinary meas- 
 urements' of electrical resistance, but coils of German silver wire, or 
 other wires of high resistance, are used. These coils are carefully 
 adjusted in resistance to a desirable number of ohms and they can 
 then be used in the measurement of the resistance of any conductor 
 according to methods which will be explained in later lessons. Mer- 
 cury resistances are used only in well-equipped scientific laboratories 
 to determine the^real resistances of the common wire resistance coils. 
 
 The measurement of electrical currents is also more frequently 
 carried out in practical tests by means of instruments depending 
 upon the magnetic effect of the currents, than according to the means 
 indicated in the definition of the ampere. Methods of measurement 
 based on the electrochemical effect of currents are very valuable for 
 determining whether the indications of electromagnetic instruments 
 are correct. 
 
FIG. 35. 
 
 On page 4 of Lesson i is given a table which shows the compara- 
 tive order of the conducting powers of various materials. It is seen 
 that metals stand at the head of the list, and their conducting power 
 is so much better than that of other materials that we ordinarily 
 speak of them alone as the conductors of electricity. Amongst the 
 pure metals themselves there is considerable difference in conducting 
 power, while mixing impurities in metals or mixing metals to- 
 gether generally decreases their conducting power. The following 
 table gives a number of the better known metals and common alloys 
 in the approximate order of their conducting powers. The figures 
 at the right hand of the names of the metals show the average con- 
 ducting power of pure metals and of alloys of fixed composition, in 
 percentages of the conducting power of pure silver. Pure silver is 
 the best conductor known, but the table shows that it is very closely 
 approached by pure copper. 
 Silver . . . 100. Aluminum . . 54. Wrought Iron . . 16. Lead .... 8. 
 
 Copper... 97. Zinc. 28. Nickel 12. Mercury. 1.6 
 
 Gold 75. Platinum ... 17. Tin 12. Cast Iron 3. 
 
 Platinum Silver made of 2 parts platinum and i part silver 6.4 
 
 German Silver made of 5^ parts copper, 2 parts zinc, 2j4 
 
 parts nickel 3.5 
 
 German Silver made of 6 parts copper, 2^ parts zinc, i^ 
 
 parts nickel , 5. 
 
 German Silver made of 5 parts copper, 3 ^ parts zinc, i ^ 
 
 parts nickel 7.5 
 
 The quality of a metal and the way in which it has been 
 handled in the course of manufacture affects the conducting power 
 to a considerable degree. Pure copper that comes from the ore of 
 the Lake Superior copper mines has a little higher conductivity than 
 that coming from the Arizona mines. Annealed metals (that is, 
 metals which have been softened and toughened by slow cooling from 
 a high temperature) generally have a slightly greater conductivity 
 than hardened metals, and wrought metals than cast metals. 
 
If two wires be connected in parallel (that is, so that a current 
 divides between them as shown in Fig. 35) the current flowing in each 
 is equal to the pressure between their common terminals divided by 
 their individual resistances. For instance, if the two wires have 
 resistances of 4 and 6 ohms respectively and the pressure between 
 their terminals (the points A and B, Fig. 35) is 12 volts, the cur- 
 rent flowing through the first wire is 12/4=3 amperes and that through 
 the second is 12/6=2 amperes. 
 
 We have already seen that the current due to a fixed pressure 
 which flows through any resistance is inversely proportional to the 
 resistance. Accordingly the currents flowing through the two 
 wires of the example should be in the proportion of j{ and ^. This 
 is true, since 3 is ^- of 12 and 2 is ^ of 12. 
 
 The total current flowing through the circuit containing the two 
 wires in parallel is evidently 2 plus 3, or 5 amperes. Since the pres- 
 sure causing these 5 amperes to flow through the wires is 12 volts, 
 the resistance of the circuit between A and B, or the joint resistance 
 of the two wires in parallel, must be 12/5 or 2.4 ohms. This may be 
 conveniently calculated directly from the conductivities, which, it 
 will be remembered, are reciprocal or inverse to the resistances (lyes- 
 son 2, page 13). The conductivity of the first wire is therefore % 
 and that of the second wire is %. The joint capacity of two or more 
 pipes which deliver water between two tanks is equal to the capaci- 
 ties of all the separate pipes added together. In the same way the 
 joint conducting power of electric circuits which are connected in 
 parallel, or divided circuits, as they are often called, is equal to the 
 conducting powers of the parts added together. The joint-conduct- 
 ing power or conductivity in the example is therefore *^ P^ us Yt> or 
 512. The resistance of the divided circuit is the inverse of this, 
 which is equal to 12/5 or 2.4, as previously calculated. 
 
 This shows that simply adding together the resistances of the 
 individual parts of a circuit will not always give the total resistance 
 of the circuit. In fact, such an addition gives the total resistance 
 only when all the individual resistances belong to parts of the circuit 
 which are connected in series (Lesson 7, page 43). When part of the 
 total circuit is made up of conductors in parallel it is necessary to 
 first calculate the joint resistance of that part and then add that to 
 the resistance of the remainder of the circuit. It is easily seen that 
 the joint resistance of conductors in parallel is equal to the resistance 
 of a single conductor with which they might be replaced without chang- 
 ing the total resistance of the circuit. 
 
 The total resistance of a circuit made up of parts connected in 
 series is equal to the sum of the individual resistances of all ihe parts. 
 The total resistance of a circuit made up of parts connected in par- 
 allel is equal to the reciprocal of the total conductivity of the circuit, 
 and the total conductivity is equal to the sum of the individual conduc- 
 tivities of the parts. 
 
FIG. 36. . 
 
 Circuits are sometimes spoken of as simple circuits when the 
 parts are all in series, and compound or derived circuits, when the 
 parts are in parallel. Parallel connection is sometimes called connec- 
 tion in multiple or multiple arc. 
 
 In Fig. 36 is shown a circuit which contains a part composed of 
 two conductors in parallel. Suppose that the resistances in ohms of 
 the different parts are as marked, then the total resistance of the cir- 
 cuit is 12 ohms. If the pressure developed by each of the twc cells, 
 which are represented by the usual sign =, is 1.2 volts, the current 
 flowing through the circuit is 2.4/12 = .2 amperes. 
 
 A little consideration of what precedes will show that when two 
 wires of equal resistance are connected in parallel, their joint resist- 
 ance is just half as great as the resistance of either wire. If three 
 wires of equal resistance are connected in parallel, their joint resist- 
 ance is one-third as great as the resistance of one of the conductors, 
 and so on. If the wires of equal resistance were connected in series 
 instead of in parallel, the total resistances would be two, three, and 
 so on, times as great as a single wire. 
 
 A simple rule for calculating the joint resistance of two wires 
 which are connected in parallel is to multiply together the indi- 
 vidual resistances of the wires and divide this product by the sum of 
 the individual resistances. This conies directly from the laws of the 
 electric current and the resistances of divided circuits, but it is gen- 
 erally simpler, as already said, to consider the conductivities when 
 calculating joint resistances of parallel circuits. 
 
 When a wire is connected in parallel with another it is often 
 called a shunt, because it switches off or shimts a part of the current 
 from the other wire. The wire to which a shunt is attached is said 
 to be shunted. Special shunts put up in boxes are frequently used 
 to protect electrical instruments which are required for electrical 
 
 47 
 
measurements, by shunting a known part of the current around the 
 instruments when they might be injured if the total current passed 
 through them. 
 
 Since Ohm's Law shows that the electrical pressure between 
 two points in a circuit is equal to the current flowing in the circuit 
 multiplied by the resistance of the part of the circuit between the 
 points (Lesson 7, page 42), we may say that the pressure along a wire 
 falls in proportion to the resistance passed over. Thus, suppose the 
 terminals of a copper wire of uniform cross section and 10 feet long, 
 be connected to the poles of an electric battery furnishing a pressure 
 of two volts. Now since equal lengths of the uniform wire may be 
 considered as having equal resistances and all parts of the wire carry 
 the same current, the electrical pressure measured between the mid- 
 dle of the wire and one end must be equal to the pressure measured 
 between the middle and the other end, and this must also be equal 
 to one volt or one-half the total pressure measured between the ends 
 of the wire. In the same way the pressure measured across any por- 
 tion of the wire bears the same proportion to the two volts' total pres- 
 sure, as the length of the portion bears to the whole length of the 
 wire. 
 
 If one end of the wire while still connected to the battery is 
 connected to earth (by connecting to a water or gas pipe) it may be 
 considered as being at zero pressure; then the other end is at an 
 actual pressure of two volts. (The difference of pressure between 
 the two ends of the wire was considered before without taking into 
 account their actual pressures. The same thing is often done in con- 
 sidering the flow of water or gas through a pipe.) The middle of 
 the wire is at a pressure of one volt, while 2^/2 feet or one-quarter the 
 length of the wire from the upper end the pressure is i ^ volts, and 
 7 ^ feet or three-quarters of the length of the wire from the upper 
 end the pressure is y 2 volt. 
 
 If the wire were not of a uniform cross section, or were com- 
 posed in different parts of different metals, then the resistance of 
 equal lengths would no longer be the same. The pressures meas- 
 ured across the portions of the wire would no longer be proportional 
 to the length of the portions, but would be proportional to their 
 resistances, as before. 
 
 The general rule may therefore be written as a result of Ohm's Law; 
 the electrical pressure along a conductor through which a given current 
 flows, falls directly as the resistance passed over. The same rule holds 
 in the case of gas or water flowing through a pipe. Suppose it requires 
 ten pounds pressure to cause 500 gallons of water to flow per minute 
 through a certain straight pipe 200 feet long. If the pipe be cut in 
 half, 5 pounds pressure is sufficient to pass the same amount of water 
 through either half. If pressure gauges are attached with proper 
 precautions to the pipe at intervals of 20 feet, each gauge will show 
 
 48 
 
a pressure of one pound less than the preceding one, when taken in 
 the direction of the current. This shows that the pressure falls 
 directly as the resistance passed over, as in the case of the electric 
 current. 
 
 Reference has already been made to the effect of temperature on 
 the resistance of metals. THe resistance of most metals increases as 
 the temperature rises, but in the case of a few metals, the most im- 
 portant of which are some alloys and carbon, the resistance falls as 
 the temperature increases. The fall of the resistance of carbon as 
 the temperature rises, is sufficiently great to reduce the working 
 resistance, or hot resistance, of an incandescent lamp filament to only 
 about one-half the resistance which it has when at the usual atmos- 
 pheric temperature. The resistance of liquids and of most insulating 
 materials, as far as they are measurable, decreases as the temperature 
 rises. 
 
 The resistance of most pure metals seems to change at nearly 
 the same rate, namely: about .4 of i per cent per degree of the centi- 
 grade thermometer scale or .22 of i per cent per degree of the 
 Fahrenheit thermometer scale. (One centigrade degree is equal to | 
 of a degree of the Fahrenheit scale.) This is a fairly accurate value 
 of the temperature coefficient of ordinary copper. A change of .4 of 
 one per cent per centigrade degree means a change of one per cent 
 in resistance up or down for every 2^ degrees centigrade when the 
 temperature varies up or down. This is also nearly equivalent to 
 one per cent for every five degrees of the Fahrenheit or common ther- 
 mometer scale. 
 
 The temperature coefficient of alloys depends very much 
 upon the composition of the mixture. In general, German 
 silver may be taken to have a temperature coefficient about one- 
 tenth as great as that of copper. The temperature coefficients of 
 the alloys, whose comparative conductivities are given in the first 
 part of this lesson, are compared below with that of copper: 
 
 CENT. FAHR. 
 
 Copper .40 .22 
 
 Platinum Silver .030 .017 
 
 German Silver .033 .018 
 
 German Silver .036 .020 
 
 German Silver .040 .022 
 
 The two columns of figures in this table show the approximate 
 temperature coefficient of the metals expressed as the percentage 
 change of resistance per degree centigrade and Fahrenheit. 
 
 Copyrighted, 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON VII. 
 
 Points for Review. 1. What is Ohm's law? 
 
 2. Suppose a pressure of ten volts is maintained between the terminals of a wire 
 the resistance of which is four ohms, how much current flows? 
 
 3. If a current of five amperes flows through a wire which has a resistance of 12 
 ohms, what is the pressure between the terminals of the wire? 
 
 4. If a battery of five gravity cells, each of which gives a pressure of 1.08 volts and 
 has an internal resistance of four ohms, be connected in series with an external resist- 
 ance of seven ohms, what current flows through the circuit? 
 
 5. If two cells which respectively give pressures of 1.8 volts and 1.08 volts are con- 
 nected to a circuit in opposition (that is, with their poles connected so that they tend to 
 send currents in opposite directions), and a current of .4 amperes flows, how much cur- 
 rent will flow if the cells are connected to the same circuit properly in series? 
 
 6. If two copper wires of equal length have resistances of four and nine ohms 
 respectively, and the diameter of the first is one-eighth inch, what is the diameter of 
 the other? 
 
 7. If the resistance of -a coil of wire is found to be 105 ohms, and a piece of the 
 same wire which is ten feet long has a resistance of 1.5 ohms, how many feet of wire are 
 contained in the coil? 
 
 8. Why is the volt defined as the pressure which is required to pass a current of 
 one ampere through a resistance of one ohm? 
 
 9. Why are mercury resistances not used in every day measurement of resistances? 
 
 10. How does the conductivity of copper compare with that of other metals? 
 
 11. Why is wire made from Lake Superior copper preferred for electrical purposes 
 by users of copper wire? 
 
 12. Suppose an electric battery is connected to two external circuits in parallel, one 
 of them having a resistance of 20 ohms and the other a resistance of 40 ohms, what pro- 
 portion of the total current will flow through each circuit? 
 
 13. Suppose the battery of the third example gives a pressure of 20 volts and has an 
 internal resistance of 6% ohms, how much current flows through the battery, and how 
 much through each of the external circuits? 
 
 14. What is the total resistance of a series circuit made up of parts having the fol- 
 lowing resistances: 1st part, 4 ohms; 2nd part, 2 ohms; 3rd part, 2/4 ohms? 
 
 15. What is the resistance of a circuit made up of the same parts in parallel? 
 
 16. Suppose three wires, each having a resistance of 4 ohms, are first connected in 
 series and then connected in parallel, what is their joint resistance in each case? 
 
 17. If a uniform wire 20 feet long measures 1 ohm, what is the fall of pressure per 
 foot when one ampere flows through it? What is the fall of pressure per foot when two 
 amperes flow through it, the resistance being assumed to remain constant? 
 
 18. V/hat effect does temperature have on the resistance of most metals? What 
 effect does it have on the resistance of carbon, liquids, and most insulators? 
 
 19. What is the approximate temperature co-efficient of copper? 
 
 20. How many Fahrenheit degrees change of temperature is required to change the 
 resistance of a copper wire one per cent? 
 
 50 
 
LESSON VIII. 
 
 HEATING EFFECTS OF ELECTRIC CURRENTS. 
 MISCELLANEOUS EFFECTS OF ELECTRIC CURRENTS. 
 
 When one coulomb of electricity is passed through a wire under 
 the pressure of one volt, a certain amount of work is done, exactly 
 as a certain amount of work is done when a gallon of water is 
 pumped through a pipe under a pressure of one pound. In the case of 
 the water the work done is measured \n foot-pounds^ which means that 
 a force equivalent to one pound has been moved through a distance 
 of one foot. In order to determine the foot-pounds of work done in 
 pumping water, the pressure under which the water is pumped must 
 be converted into its equivalent feet of head and the quantity of 
 water must be given in pounds. A pressure of one pound is equiva- 
 lent to the head of a column of water 2 ^ feet high, and the weight 
 of a gallon of water is about 8^ pounds. Consequently if one gallon 
 of water be passed through a pipe under a pressure of one pound, 
 about 19^2 foot-pounds of work is done. (2^3x8^ = about 19^2.) 
 In the same way when one coulomb of electricity is passed 
 through a wire under a pressure of one volt, the amount of work 
 done is called one joule, after the name of Joule, a great English 
 scientist and engineer. 
 
 As a general thing we do not care to pump a single gallon of 
 water through a pipe, but we wish to pump a given number of gal- 
 lons per minute. In this case for each gallon passed per minute 
 through the pipe under a pressure of one pound, about 19^ foot- 
 pounds of work must be done every minute. Suppose it is desired 
 to pump 120 gallons (1,000 pounds) of water per minute through a 
 pipe under a head of 33 feet, the work required to do this is 33,000 
 foot-pounds per minute. The rate at which work is done, that is, the 
 amount done in a given time, is called poiver. In dealing with 
 mechanical power it is divided into units called horse-power. A 
 horse-power is equal to 33,000 foot-pounds of ivork done per minute, so 
 that in the last example exactly one horse-power is required to move 
 the water. 
 
 The horse-power of a water fall is calculated in a way which is 
 similar to the preceding examples. Suppose a stream discharges 
 480 gallons or 4,000 pounds of water per minute over a fall 25 feet 
 high, the power of the water is 100,000 foot-pounds per minute or a 
 little over three horse-power. The horse-power of a steam engine is 
 also calculated in a similar manner. For instance, in an engine 
 which is supplied with steam which exerts an average pressure on 
 the piston of 40 pounds per square inch along the whole stroke and 
 the piston of which has a surface of TOO square inches, the total pres- 
 sure exerted by the steam on the piston is 4,000 pounds. If the 
 
 51 
 
stroke of the engine is one foot, the piston moves two feet per revolu- 
 tion, and consequently the steam exerts 8,000 foot-pounds of work 
 in each revolution. If the engine runs at 250 revolutions per minute 
 the work done by the steam is 2,000,000 foot-pounds per minute or just 
 a little more than 60 horse-power. This is called the indicated horse- 
 power of the engine. Most of it is available for driving machinery, 
 but a portion is used in overcoming the friction ot the engine itself. 
 
 We have seen that when a coulomb of electricity is sent 
 through a wire under a pressure af one volt, an amount of work is 
 done which is called a joule; also that a current of one ampere is a 
 current which conveys one coulomb per second. (Lesson 2, page 
 12.) Consequently when a current of one ampere is passed through 
 a wire under a pressure of one volt, the amount of work done is equal 
 to one joule per second. This represents a certain amount of power 
 which is called a watt, after James Watt, a great English engineer 
 and the inventor of the modern steam engine. The power repre- 
 sented by one watt is equal to one seven hundred and forty-sixth 
 part of a horse-power, or there are 74.6 watts in a horse-power. In 
 speaking of the power of electrical machinery, it has become usual 
 to use the electrical term ivatt, and for a larger and frequently more 
 convenient unit the kilowatt is used. This is equal to 1,000 watts, 
 or about i % horse-power. 
 
 When an electric current flows through a circuit, the power 
 used in the circuit is equal to the current multiplied by the total 
 pressure causing the current to flow. That is, the power in watts is 
 equal to the current in amperes multiplied by the pressure in volts, 
 or P=C E. Part of this power may be used in causing electro- 
 chemical action, by charging a storage battery for instance, or it 
 may be used in driving machinery through the medium of an 
 electric motor, but some of the power is always used in overcoming 
 the resistance of the wires which convey the current. This is some- 
 what similar to the use of some of the indicated power of an engine 
 in overcoming the friction of the engine itself. 
 
 When mechanical power is used in overcoming friction or other 
 forms of resistance, it is not lost but is converted into an equivalent 
 amount of heat. A general law may be stated that energy (that is, 
 the capability of doing work) is never destroyed, but it may be trans- 
 formed from one form to another. This is called the Law of the Con- 
 servation of Energy. When power is transformed from one form to 
 another, there is always some loss of the amount of useful 
 power. The apparently lost power has not been destroyed, but has 
 been converted into heat. For instance, when the mechanical power 
 conveyed by a running belt is changed by means of a dynamo of sat- 
 isfactory size into electrical power, about ten per cent of the availa- 
 ble power is lost. That is, the electrical power delivered by the 
 dynamo is about ten per cent less than the mechanical power which 
 
is given to the dynamo. This difference has not been destroyed, 
 but has been converted into heat in overcoming the friction of the 
 dynamo bearings, the resistance of the wire windings of the dynamo, 
 and in other ways. A dynamo which is in operation is always found 
 to be warmer than the surrounding air, which shows that some of 
 the power delivered to it is changed into heat that goes to warm the 
 machine. The usefulness of this amount of power is therefore lost, 
 but the energy is not destroyed. 
 
 The power which is used in overcoming the electrical resistance 
 of a wire when a current is passed through it is converted into heat 
 which warms the wire. The heat produced is proportional to the 
 number of watts required to overcome the resistance of the wire, and 
 this is equal to the difference of pressure at the terminals of the wire 
 multiplied by the current flowing in it (P = C E), provided all 
 the power expended in that part of the circuit is used in heating the 
 wire. According to Ohm's Law, pressure is equal to current times 
 resistance, or E = C R. Consequently C times E is equal to C 
 times OR, or C squared times R. Hence the power required to over- 
 come the resistance of a wire is equal to the square of the current 
 multiplied by the resistance, or P = C E = C 2 R. By again substi- 
 tuting according to Ohm's Law, it may also be shown that the 
 power lost in a wire is equal to the pressure squared divided by the 
 resistance, or P = C E = C 2 R = E 2 /R. 
 
 Since the portion of the available electrical power of a circuit 
 which is lost in heating the conductors is equal to the current 
 squared times the resistance of the conductors, it is often spoken of 
 as the C squared R loss. 
 
 According to the Law of the Conservation of Energy, which 
 was experimentally proved by Joule, for every unit of work trans- 
 formed into heat there is an equivalent amount of heat produced. 
 Consequently if we have two wires, the first of which has double the 
 resistance of the second, and equal currents are passed through them, 
 the power lost and the heat produced in the first will be twice as 
 great as in the second. 
 
 It is possible to measure an electric current by the heat pro- 
 duced when it is passed through a known resistance. This is 
 usually done in an instrument called a calorimeter (Fig. 37), which 
 is a vessel containing water or some other liquid in which the 
 resistance is immersed. The vessel is usually double walled or 
 arranged in some other way so that it will not lose heat rapidly by 
 radiation into the air, A thermometer is immersed in the liquid to 
 determine its rise of temperature due to the heat given it from the 
 wire. The amount of heat which is required to raise the tempera- 
 ture of a gramme of water one degree of the centigrade scale is called 
 a calorie. The number of calories given to the water in the calori- 
 meter by the wire, is determined from the amount of water and its 
 
 53 
 
rise in temperature, proper corrections being made for the effect of 
 the vessel. The experiments of Joule and of Rowland, an American 
 scientist, have shown that the work represented by one joule is 
 equivalent to the heat represented by practically .24 of a calorie. 
 Consequently, the total number of calories of heat produced in one 
 second by the current passing through the wire in the calorimeter is 
 equal to .24 C 2 R. The total heat produced in any time is also equal 
 to .24 C 2 R multiplied by the number of seconds in the time. This 
 may be written in the form H =.24 C 2 R T. By determining the 
 total heat produced in the calorimeter in a fixed time, when the 
 current is passed through a known resistance, the value of the cur- 
 rent may be determined. The square of the current is equal to the 
 calories divided by .24 times the resistance, multiplied by the time 
 in seconds. 
 
 FIG. 37. 
 
 It may be seen from what precedes that one ampere flowing 
 through a resistance of one ohm expends continuously a power of one 
 watt and produces . 24 calories of heat every second. 
 
 The expansion or lengthening of a wire when it is heated by a 
 current passing through it may also be used to measure the current, 
 as will be fully explained later. 
 
 The actual rise of temperature on the part of a wire when a cur- 
 rent passes through it, depends upon several things in addition to the 
 
amount of heat produced in it. A long, thick wire and a short, thin 
 wire of the same material, and having the same resistance, will come 
 to very different temperatures when equal currents are passed through 
 them. If there is sufficient difference in their diameters, the thin 
 wire may become red hot on account of the passage of a current which 
 is only sufficient to make the thick wire appreciably warm. 
 
 When a current passes through a wire a certain amount of heat 
 is produced during every second which the current flows. For a short 
 time after the current is started the wire rises in temperature, and 
 finally reaches a certain fixed temperature. When the temperature 
 becomes fixed it is evident upon a little thought that as much heat 
 must leave the wire by radiation to surrounding objects, convection 
 by air currents, or conduction to objects touching the wire, as is pro- 
 duced by the flow of the current. If more heat is given to the wire 
 than is carried off by these means, its temperature must rise, and if 
 on account of a decrease in the current the amount of heat given to 
 the wire is less for a time than the amount given off, the temperature 
 must fall until the two are equal again. The capability of a wire to 
 get rid of heat by radiation and convection depends upon the color 
 and condition of its surface, and also roughly upon the extent of the 
 surface. The amount of heat which leaves any surface in a second 
 also depends upon the number of degrees by which its temperature 
 is higher than that of the air and surrounding objects. The amount 
 of heat which is required to bring a wire to a given temperature also 
 depends upon the capacity of the material for holding heat, or its 
 specific heat as it is called. Consequently the actual temperature to 
 which any wire will rise when carrying a certain current, can be 
 exactly determined only by trying the experiment. 
 
 The fact that the ability of a wire to emit heat is directly depend- 
 ent upon the extent of 'its surface, causes a wire with an insulating 
 covering to remain cooler in the open air than a similar wire without 
 the covering, but carrying an equal current. This seems at first 
 sight exactly opposed to the facts as seen in covered boiler pipes. 
 There is no contradiction, however, because the thickness of the 
 insulation is entirely comparable with the diameter of the wire, and 
 the outside surface of the insulation is therefore so much greater than 
 that of the wire, that the additional surface more than makes up for 
 the difficulty which the heat experiences in getting through the 
 .insulation, and the heat finds it easier to leave the insulated wire. 
 This effect is most decidedly shown when the outer surface of the 
 insulator is black. When steam pipes are covered for the purpose 
 of retaining their heat, the thickness of the covering is thin com- 
 pared with the diameter of the pipes, so that the outside surface of 
 the covered pipes is not much greater than the surface of the pipes 
 when bare. Consequently the effect of the thickness of the cover- 
 ing which is placed in the path of the heat as it leaves the pipe is 
 
 55 
 
greater than the effect of the increased surface, and the heat finds it 
 more difficult to leave a covered pipe. When wires are closed up in 
 mouldings or under plaster, as is often the case with the electric 
 light wires in buildings, they become very much hotter than when 
 exposed in such a way that they may be cooled by air currents. 
 
 Electric currents cause various effects besides those of electro- 
 chemistry, electromagnetism and electric heating. These effects are 
 of various kinds, but of small commercial importance, and in most 
 cases seem to be due to some action of the current upon the mole- 
 cules of the material through which it flows. Some of the effects are 
 undoubtedly due to electrochemical action, though they have often 
 been attributed to some unknown action of the current. The only 
 one of these effects which is of sufficient importance outside of the 
 field of purely speculative science to require attention, is the physio- 
 logical action of the current. Galvani accidentally discovered this 
 action through some experiments performed upon frogs. His dis- 
 covery has been followed up by many scientists down to the present 
 day, and a vast array of facts has been determined relating to the 
 effects of currents on living organisms. The researches of these 
 scientists have shown that protoplasm, which is the fundamental 
 basis of all living bodies, has the power of contracting when an elec- 
 tric current passes through it. Moreover, a living animal nerve is 
 always excited to action by the passage through it of an electric 
 current from an external source. If the terminals of a battery cell 
 be touched to the tongue, a peculiar taste may be noticed. This taste 
 may also be caused by laying a copper and a silver coin upon the 
 tongue with their edges touching. In this case a current is set up 
 through the metals, the saliva of the mouth serving as the fluid. If 
 the terminals of a battery cell be touched to the temples, or so that 
 the current flows from the forehead to the hand, flashes of light are 
 frequently noticeable, due to the excitation of the nerves of the eye 
 by the current. In the same way the nerves of smell and hearing 
 may be excited. 
 
 When a sufficiently powerful electric current is passed through 
 the ordinary nerves, a feeling of tickling, pricking, or pain may be 
 observed. If the current is sufficiently strong, it may cause a very 
 painful muscular contraction, and if excessive, the current may cause 
 death. The muscular effect due to a strong current is ordinarily 
 called a shock. The severity of shock depends upon the electrical 
 pressure which causes the current to flow through the body, but it 
 also depends largely upon the physiological condition of the person 
 who receives the shock. 
 
It has been found that electric currents naturally exist in the 
 living muscles and nerves of animals, and that muscular exertion 
 seems to cause them. These currents disappear with the death of 
 the animal, which probably shows that the electric currents have 
 some function in the action of the nervous system. 
 
 The physiological action of electric currents gives a good basis 
 for their use in the treatment of some diseases, and they have been 
 used with marked success in some cases. This question will be fully 
 treated in special lessons, and it is sufficient to add here that electric 
 treatment should never be applied except under the immediate direc- 
 tion of a competently trained physician. The indiscriminate use of 
 electrical treatments of any kind is likely to do more harm than good. 
 
 Copyrighted, 1895, 
 
 ITT 
 JRNIA- 
 
The National School of Electricity. 
 
 .REVIEW OF LESSON VIII. 
 
 Points for Review. 1. What is a foot-pound? What is a joule? 
 
 2. What is a horse-power? What is a watt? What is a kilowatt? 
 
 3. How many watts are in a horse-power? 
 
 4. What is the reason that the bearings of machinery become warm? 
 
 5. When a current of 10 amperes flows through a resistance of 2 ohms, how much 
 power is used? How much heat is produced per second? 
 
 6. If a current of 10 amperes is caused to flow through a circuit under a pressure 
 of 100 volts, how much power is used? If part of the power is used in electrochemical 
 operations but the total resistance of the circuit is 5 ohms, what proportion of the power 
 is used in the C 2 R loss? 
 
 7. Why does a black covered insulated wire remain cooler when carrying a certain 
 current than a bare wire of equal size carrying the same current and exposed to the same 
 conditions? 
 
 8. What is the effect of a current of electricity when passed through the nerves of 
 animals? What is the muscular effect ot a strong current? 
 
 9. Is it safe to use patent electro-medical devices, or receive electrical treatment 
 from untrained hands? 
 
 IX. 
 
 GALVANOMETERS AND VOLTAMETERS. 
 
 Instruments for detecting and measuring electric currents, the 
 indications of which are dependent upon the deflection of a magnetic 
 needle caused by the magnetic effect of the current flowing in a coil 
 which surrounds the needle, are called galvanometers. These instru- 
 ments are made in a great variety of forms and are widely used for 
 measurements in laboratories and shops. 
 
 In most forms of galvanometers the magnetic needle is placed at 
 the center of a coil of wire. This coil may have a great number of 
 turns of fine wire, in which case the galvanometer is sensitive, that 
 is, the needle is appreciably deflected by a very small current; or 
 the coil may have but few turns of thick wire, in which case the gal- 
 vanometer is intended for use with comparatively large currents. In 
 many cases the coil of the galvanometer is placed so that it stands in an 
 exact north and south position (that is, in the magnetic meridian) like 
 
the needle. The magnetic force due to the coil, which is at right angles 
 to its wire (Lesson 6, page 36), is then at right angles to the magnetic 
 force of the earth and also to the length of the needle. When the 
 coil is in this position a current in the coil exerts its greatest force 
 to deflect the needle. 
 
 When a galvanometer is connected in a circuit the presence of a 
 current is shown by the deflection of the needle. The direction of 
 the current is shown by the side towards which the north pole of the 
 needle moves (Lesson 6, page 37). The strength of the current is 
 indicated by the amount of the needle's deflection, since the position 
 which the needle takes depends upon the relative magnitude of the 
 magnetic forces due to the current and the earth (Lesson 6, page 35). 
 The earth's magnetism can be considered to be approximately con- 
 stant at any fixed point. 
 
 When the diameter of the galvanometer coil is much greater 
 thau the length of the needle, the tangents of the angles through 
 which the needle is deflected by various currents are proportional to 
 the currents. Such a galvanometer is called a tangent galvanometer. 
 (Fig. 38.) Other galvanometers in which the coil is moved so as to 
 bring the needle back to zero (Fig. 39), are called sine galvanometers, 
 because the sine of the angle through which the coil is moved is proper- * 
 tional to the current causing the deflection. In some rough galvan- 
 ometers a pointer is attached to the needle, and the deflection is read 
 off on a divided circle over which the pointer moves. The circle is 
 usually divided uniformly in degrees. For exact measurements such 
 a method of reading deflections is not sufficiently accurate and re- 
 flecting galvanometers are used. (Fig. 40.) A small mirror is attached 
 to the magnet in these, and the deflections are read off by means of a 
 small telescope which shows the reflection in the mirror of a stationary 
 scale. When the needle, with its mirror, moves, the reflection of 
 the scale as seen in the telescope appears also to move and the deflection 
 of the needle is thus determined. Instead of using a telescope and 
 scale as is usually done in America, a lamp and scale (Fig. 41) may 
 be used, as is usually done in England. In this case abeam of light 
 from a lamp which is placed behind a slit in front of the gal- 
 vanometer is reflected by the mirror upon a scale, where it shows as 
 a spot of light. When the needle with its mirror is deflected, the 
 spot of light moves along the, scale, thus showing the magnitude ot 
 the deflection. This is a very convenient arrangement to use when 
 testing must be done in dark rooms or vaults, but it cannot be used 
 in a light place. 
 
 The support of the needle is sometimes in the form of a finely 
 wrought pivot, and the needle is sometimes set with an agate or 
 ruby center so that it may move easily. The friction of the finest 
 pivot, however, is so great that it destroys the sensitiveness of a fine 
 
 59 
 
FIG. 39. FIG. 42. FIG. 43. 
 
 galvanometer, so that in all fine galvanometers the needles are sus- 
 pended by means of & fibre which is usually made of unspun cocoon 
 silk. The fineness of this fibre depends upon the weight of the 
 needle with its mirror, and it is sometimes so fine that it can 
 scarcely be seen. The length of the suspension varies from a small 
 fraction of an inch to many inches. 
 
 It is often convenient to make the needle of a galvanometer 
 independent of the direction of the earth's magnetism, or to vary the 
 strength of the directive force, that is, the force which holds the 
 needle in the magnetic meridian. For this purpose galvanometers 
 are generally arranged with one or more directive magnets or 
 controlling magnets. One is shown as a curved bar placed on a 
 stem above the galvanometers of Fig. 40. By varying the position 
 
 60 
 
FIG. 41. 
 
 of the magnet with respect to the needle, the needle may be controlled 
 as desired, and the galvanometer may be set in any desired position. 
 
 In order that a galvanometer may be made very sensitive, it is 
 desirable to make the controlling force very weak in some cases 
 much weaker than that due to the earth's magnetism. Conse- 
 quently the effect of the earth's magnetism must be overcome. For 
 this purpose what are known as astatic needles are used. These con- 
 sist of a pair of needles of practically equal size and magnetic strength 
 which are fastened to a light thin wire, one above the other so that 
 their north poles point in exactly opposite directions. It is usual to 
 arrange a coil of wire for each needle, so that the galvanometers 
 have two coils. In some very sensitive galvanometers there are 
 eight needles arranged astatically, and eight coils. 
 
 The forms in which galvanometer needles are made are quite 
 various. Some needles are in the form of a partially split bell, one 
 side being the north pole and the other being the south pole. Other 
 needles are made of flat discs or rings, which are so magnetized that 
 a portion of the edge serves as the north pole and an opposite por- 
 tion as the south pole. The commonest form of needle is one made 
 up of several little magnets, made from a watch spring, laid side by 
 side with their poles all the same way. These are usually fastened 
 to the back of the galvanometer mirror or to a little disc of 
 aluminum. 
 
 There is a very convenient form of galvanometer in which the 
 coil is suspended so as to move in the magnetic field of a strong 
 Ahorse shoe magnet. (Fig- 42). In this instrument the relations of 
 
 /c 
 
coil and magnet are practically the reverse of those in the common 
 galvanometers. This is called a d' 1 Arsonval galvanometer, after a 
 French scientist who put it in useful form. The suspension of the 
 coil of a d'Arsonval galvanometer must be arranged so that the 
 current may get into and out of the coil. The coil is therefore often 
 supported between stretched phosphor-bronze wires which are con- 
 nected to it at the top and bottom and which serve as leads for the 
 current. Sometimes the coil is suspended on a silver wire by means 
 of which the current can enter the coil, and a wire at the bottom of 
 the coil dips into a bottle of mercury so that the current can get out. 
 
 One reason that a d'Arsonval galvanometer is convenient tor 
 general use is because it v&dttid beat, that is, when the coil is deflected 
 it goes at once to its position without a tedious period of swinging 
 back and forth. Ordinary galvanometers may be made more or less 
 dead beat by surrounding the needle with a ball of copper, or by 
 attaching to the suspensions wings of mica or aluminum which are 
 enclosed in a small chamber. 
 
 In order that a galvanometer may be used to actually measure 
 currents in amperes, the constant of the galvanometer must be 
 known, or the galvanometer must be calibrated or standardized. 
 
 When the deflections of a galvanometer bear some fixed relation 
 to the currents causing the deflections, it is said to have a constant 
 For instance, in the case of a tangent galvanometer the current 
 .which causes a certain deflection of the needle is given in amperes 
 by multiplying the tangent of the angle of deflection by the constant 
 of the galvanometer. The constant of a tangent galvanometer may 
 be directly calculated when the coil is circular, and its diameter and 
 number of turns and the strength or the earth's magnetism are 
 known. The constant may also be determined by passing a current 
 of known strength through the galvanometer and observing the 
 deflection. 
 
 When the deflections of the galvanometer are not known to bear 
 a fixed relation to the currents causing them, the galvanometer 
 must be experimentally calibrated. That is, currents of various 
 known strengths must be passed through the galvanometer and the 
 deflections observed. These observations may be set down in a table 
 so as to be used in future work with the galvanometer, or the obser- 
 vations may be plotted in a curve on cross ruled paper. Such a 
 calibration curve is often convenient since the value of a current 
 corresponding to any deflection may be at once determined from it. 
 
 When galvanometers are used simply for the detection . of 
 currents or for comparing the relative magnitude of currents as is 
 frequently the case, a calibration is unnecessary. 
 
 An instrument for measuring currents by means of their electro- 
 chemical action, which is often used in calibrating galvanometers, 
 is called a voltameter. We have alreadv seen that chemical action 
 
goes on in a battery cell when a current is passed in either direction 
 through the cell, and that the amount of the action is proportional 
 to the number of coulombs of electricity passed through the cell 
 (Lesson 4, page 24). The chemical action in a voltameter is 
 similar to that which takes place in a voltaic cell, but both plates 
 are of the same material and there is therefore no tendency to set up 
 a current due to the direct action of the cell. 
 
 An electric current seems to flow through some liquids in a 
 different way from that in which it flows through solid conductors. 
 In fact, liquids may be divided into three classes on the ground of 
 their action when subjected to the effect of an electric pressure: i. 
 Those that appear as insulators of a high grade, such as paraffine oil, 
 turpentine, etc. 2. Those which conduct like solids, without appar- 
 ent chemical action, such as mercury, metals in a melted condition, 
 etc. 3. Those in which chemical decomposition occurs when a 
 current flows through them, such as solutions of acids, or metallic 
 compounds, and some melted solid compounds. 
 
 Liquids of the latter class are called electrolytes, and the process 
 of their decomposition by electrochemical action is called electrolysis. 
 A cell in which electrolysis is carried on is generally called an 
 electrolytic cell, or when the electrochemical action is used to de- 
 termine the strength of the current flowing through the cell, it is 
 called a voltameter as already stated. The plates of an electrolytic 
 cell are called electrodes. The positive electrode (the one at which 
 the current enters) is often called the anode, and the negative elec- 
 trode, the cathode. The products of the electrolysis are often 
 called ions. 
 
 The earliest form of voltameter is one in which sulphuric acid 
 greatly diluted by water, is electrolyzed. This is called a water 
 voltameter. A form of water voltameter is shown in Fig. 43. When 
 this is to be used, diluted acid is poured into the funnel at the back, 
 and rises to the top of the two arms in front, if the stop cocks at 
 their tops are open. After the tubes are filled the cocks are closed, 
 and the current is passed between the platinum electrodes, EK. 
 The electrochemical action set up by the current causes oxygen to go 
 to the positive pole or anode, and hydrogen to go to the negative pole 
 or cathode. The gases rise in the tubes above their respective elec- 
 trodes, and displace the water. While the direct action of the current 
 is to cause a decomposition of sulphuric acid, additional chemical ac- 
 tion occurs which makes the total action equivalent to the decompo- 
 sition of water. Water is composed of two parts by bulk of hydrogen to 
 one part of oxygen, and consequently the tube over the cathode collects 
 twice as much gas as that over the anode. If a steady current is passed 
 through such a voltameter for a given number of seconds the strength 
 of the current can be determined from the amount of the gases col- 
 lected per second. For, the number of coulombs of electricity passed 
 
 63 
 
through the voltameter is determined from the amount of the gases 
 collected and their electrochemical equivalent (Lesson 4, page 
 24). The number of coulombs passed through the circuit per 
 second is equal to the current in amperes. 
 
 A water voltameter is not a very convenient or satisfactory in- 
 strument, and voltameters in which the electrolytes are solutions of 
 the salts of metals are preferred for real measurements. When such 
 a solution is electrolysed between plates of the metal contained in the 
 solution, the solution is decomposed; the metal from the solution goes 
 ivith the current to the cathode where it is deposited and the acid 
 part of the compound goes to the anode, which it attacks and forms a 
 new portion of the compound. The cathode should therefore be ex- 
 pected to gain exactly as much metal from the deposit as the anode 
 loses by the attack of the acid. This would be true if no chemical 
 action occurred except that directly caused by the current. It is a 
 fact that the character of a deposited metal often varies with the cur- 
 rent strength by means of which it is deposited, or the strength of 
 the solution used as the electrolyte. Copper is sometimes deposited 
 in the form of a black powder instead of a smooth, bright layer of 
 metal. Silver is often deposited in crystals which build across the 
 electrolyte between the electrodes. Tin forms a "tree" of tin crys- 
 tals when deposited from a tin chloride solution, the branches of 
 which spread out from the electrode through the solution. The 
 greatest care must be used to get satisfactory measurements by means 
 of a voltameter. The loss of the anode is seldom as reliable a meas- 
 ure of the current as the gain of the cathode, because bits of metal 
 are liable to be loosened up on the former and fall off, and the anode 
 also often suffers from oxidation. 
 
 When a silver voltameter is used for the measurement of a cur- 
 rent, as is assumed in the definition of the ampere (Lesson 7, page 44), 
 the electrolyte is a solution of the nitrate of silver of fixed strength. 
 The cathode is usually a platinum bowl upon which the silver is 
 deposited, and the anode is a wire or plate of pure silver which is 
 wrapped in filter paper to keep bits of silver from dropping onto the 
 cathode. Before a measurement of current is to be made, the cathode 
 is very accurately weighed, the solution is then poured in and the 
 anode put in place. The current is turned on and continued for a 
 desirable number of seconds. It is then stopped, the cathode is care- 
 fully washed and dried, and finally again weighed with great care. 
 From its gain in weight the value of the current is determined. 
 
 On account of the expense of the silver consumed and the care 
 required in using a silver voltameter, it is not satisfactory for meas- 
 uring currents exceeding about one ampere. For larger currents, a 
 voltameter having copper plates and a solution of copper sulphate for 
 electrolyte is generally used. The meter used by Edison companies 
 to determine the Quantity of electricity delivered per month to cus- 
 
tomers, usually consists of a voltameter with amalgamated zinc plates 
 and an electrolyte of zinc sulphate. 
 
 The weight in grammes of different metals deposited by one 
 ampere in one second (that is, their electrochemical equivalent, Les- 
 son 4, page 24,) is given below. 
 
 ELECTROCHEMICAL 
 EQUIVALENT. 
 
 Hydrogen oooo 1 04 
 
 Silver , 001118 
 
 Copper 000328 
 
 Zinc 337 
 
 Copyrighted, 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON IX. 
 
 Points for review. 1. What is a galvanometer? 
 
 2. What is the fundamental difference in the construction of galvanometers for use 
 With very small currents and with large currents? 
 
 3. How is the presence and direction of a current in a circuit shown by a galvan- 
 ometer? 
 
 4. How is the strength of a current indicated by a galvanometer? 
 
 5. What is a tangent galvanometer? What is a reflecting galvanometer? 
 
 6. How are the needles of fine galvanometers suspended? 
 
 7. What is a d'Arsonval galvanometer? 
 
 8. What is meant by the term "dead beat?" What is meant by the term "calibrate?" 
 
 9. What is a voltameter? 
 
 10. What is meant by the terms electrolyte, electrolysis, electrode? 
 
 11. What occurs when a solution of a metallic salt is electrolyzed? 
 
 12. What kinds of voltameters are most commonly used for the calibration of 
 instruments? 
 
 X. 
 
 MEASUREMENT OF ELECTRICAL RESISTANCE. 
 
 All useful methods of measuring electrical resistance depend 
 directly upon the indications of Ohm's Law. The simplest method 
 of measuring a resistance is by what is called substitution. The 
 resistance to be measured is connected in series with a galvanometer 
 and a constant battery and the deflection of the galvanometer is 
 noted. Then the unknown resistance is removed from the circuit 
 and a variable resistance box or rheostat (Fig. 44) is substituted for it. 
 The resistance of the resistance box is then adjusted until the galvan- 
 ometer deflection is the same as before. Then the resistance in- 
 serted in the circuit by means of the box is equal to the unknown 
 resistance, because in the two cases the same current, as shown by 
 the galvanometer, flows through the circuit, and the total electrical 
 pressure acting in the circuit is the same in each case; consequently, 
 according to Ohm's Law, the resistance of the total circuit must be 
 the same in the two cases. It is necessary that no changes be made 
 in the circuit besides the substitution of the variable known resist- 
 ance for the unknown one. 
 
 Resistance boxes are generally boxes containing spools of silk- 
 covered wire, each of known resistance, which may be used in elec- 
 trical measurements. German silver or some similar alloy having a 
 comparatively low conductivity and a small temperature co-efficient 
 (Lesson 7, page 44, and Lesson 7, page 49,) is generally used in 
 making the spools or coils for resistance boxes. In making the coils, 
 
FIG. 44. 
 
 FIG. 45. 
 
 the proper length of wire for each is taken and doubled at the middle, 
 and is then wound double upon a spool. The object of doubling the 
 wire is to avoid the effects due to self-inductance, which will be 
 explained later. After the spools are wound, they are dipped in 
 paraffine and then placed inside the box and fastened to the under 
 side of the top of the box by brass bolts (Fig. 45), which also fasten 
 brass blocks to the upper side of the top. (Fig. 44.) The individ- 
 ual ends of each coil are connected to adjoining brass blocks so that 
 all the coils are in series when the blocks are not connected. This 
 is shown in Fig. 46, where the ends a and b of one of the resistance 
 coils are fastened to the brass blocks E and H, while the ends c and 
 d of the next coil are fastened to the blocks H and M. The brass 
 blocks are so arranged that they may be connected together by plugs 
 which fit in tapering holes as shown in the figure. 
 
 If such a resistance box be connected in a circuit when all the 
 plugs are removed, the current flows through all the resistance coils 
 in series. If one of the plugs be inserted in a hole, the correspond- 
 ing resistance coil is short-circuited that is, a negligibly small 
 resistance (that of the plug) is connected in parallel with it, and no 
 appreciable current flows through the coil. Since the resistance of 
 the plug is practically negligible the resistance of the circuit is 
 reduced by the amount of the resistance of the corresponding coil 
 when a plug is inserted. The resistance of a box may therefore be 
 varied at will by simply inserting or removing plugs. 
 
 Resistance boxes generally have a series of coils of different 
 resistances, usually given in tenths, units, tens, hundreds, etc., of ohms. 
 The final adjustment of the resistance of the coils of a fine resistance 
 box is a matter requiring rreat care, and is effected by soldering more 
 or less of the doubled ends of the wire together after the spool is 
 mounted in its box. In order that the adjustment may be made in 
 this way it is necessary that the resistance of the coil when wound 
 
 67 
 
on the spool be a little greater than the desired final value. When 
 adjusting coils great care must be taken to avoid errors due to the 
 temperature of the coils changing, since the wires are likely to be- 
 come heated by the soldering. 
 
 The measurements for determining the exact value of the coils 
 are made by what is called a Wheatstone bridge, after Wheatstone, 
 an English scientist and inventor. This consists of an arrangement 
 of resistance coils which are used with a battery and galvanometer as 
 shown diagramatically in Fig. 47. In the figure, A, B and C 
 represent resistance boxes with coils of known resistance ; D is the 
 resistance to be measured; L and G are a battery and a galvanometer; 
 K! and K 2 are keys placed in the circuits with the battery and galvan- 
 ometer, by means of which the circuits may be made and broken; 
 M, N, P, and Q are points where the various bridge circuits are con- 
 nected together. 
 
 From an application of the law of the fall of potential along a 
 resistance as deduced from Ohm's Law (Lesson 7, page 49), it is easy 
 to see how the resistance of the coil D is determined by this device. 
 Suppose the battery key, Kj, be depressed, then current will flow 
 from the battery through the key to the point P. Here it divides, 
 and part goes to Q by way of M and the other part by way of N. 
 From Q the current returns to the battery. The points P and Q are 
 at a certain difference of electrical pressure which depends upon the 
 battery, and which we will call B, and the fall of pressure from P 
 to Q by way of either M or N is equal to E. The fall of pressure 
 between P and M is (according to the law that the fall of pressure is 
 proportional to the resistance passed over) B^ where d and c repre- 
 sent the resistances of the branches of the bridge D and C respect- 
 ively. In the same way the fall of pressure between P and N is equal 
 to B^. If the fall of pressure between P and M is greater than that 
 between P and N, the point M is at a lower pressure than N, and if 
 
FIG. 48. 
 
 the galvanometer key be depressed a current will flow from N to M 
 through, the galvanometer, deflecting the needle. Now if the resist- 
 ance b be increased until the fall of pressure between P and N is the 
 greater, a current will flow from M to N when the galvanometer key 
 is depressed and the needle will be deflected in the opposite direction. 
 Finally if the resistance of b be so adjusted, by arranging the plugs, 
 that the fall of pressure between P and M and between P and N is 
 the same, the pressures at the points M and N are equal and no cur- 
 rent will flow through the galvanometer when the key is depressed, 
 and the needle will not be deflected. The bridge is then said to be 
 balanced. In this case E^ = B^, or, what is the same thing, \ = ~- 
 From this proportion we get d=b^; that is, the unknown resistance 
 of D is equal to the resistance of B, multiplied by the resistance of 
 C divided by that of A. Put in the form of a proportion this may 
 be written, a is to b as c is to d\ or, a is to c as b is to d. The solution 
 of either of these proportions gives the results given above. If a and 
 c are equal, and the bridge is balanced, b must be equal to d, so that 
 the resistance of the unknown branch or arm of the bridge is given 
 at once by the resistance of the coils in circuit at B. In the figure, 
 the resistance of C is ten times as great as that of A and therefore the 
 resistance of D is ten times that of B and is 150 ohms. The arms A 
 and C are generally called the ratio arms of the bridge and B the 
 rheostat. 
 
 The Wheatstone bridges that are commonly used are not made 
 up from three separate resistance boxes as indicated in Fig. 47. 
 
FIG. 49. 
 
 The common forms of Wheatstone bridge contain all the resistance 
 coils in one box, and the coils are connected up in such a way that 
 they form a bridge. Binding posts, generally marked B, G, and R or 
 X, are arranged for the connection to the bridge at the proper points 
 of the battery, galvanometer and the unknown resistance which is to 
 be measured. Fig. 48 shows such a bridge made up in a box so as to 
 be portable. At the front are seen the battery and galvanometer 
 keys. This form of bridge is often called the postoffice pattern, be- 
 cause its arrangement is similar to the bridge used by the British de- 
 partment of postal telegraphs. Fig. 49 shows another way in 
 which the resistance coils are often arranged to make a very accu- 
 rate and convenient bridge for use in laboratories where it may be 
 permanently fixed. 
 
 Measurements of resistance may be made with a fine bridge to a 
 remarkable degree of accuracy. In fact, the ease and accuracy to be 
 attained in bridge measurements are only rivalled in weighing with 
 fine balances. It is not unusual to have the resistance coils of a fine 
 bridge adjusted to an error of less than 1-50 of one per cent of their 
 desired value as represented by a standard coil, or within two parts 
 out of ten thousand at a fixed temperature. In adjusting the coils of a 
 resistance box so closely, or in accurately measuring a resistance by a 
 bridge, careful account must be taken of the temperature. If the re- 
 sistance coils of a bridge are exactly correct at one temperature they 
 are not correct at any other temperature. (Lesson 7, page 49!) 
 
 It is frequently convenient to have a portable bridge which is 
 entirely self contained that is, the box of which contains the gal- 
 vanometer and battery as well as the resistance coils. In this case 
 all that is necessary to make a measurement of resistance is to connect 
 the unknown resistance to the proper binding posts, press the keys, 
 and adjust the plugs till the galvanometer gives no deflection. Such 
 bridges are generally called testing sets. One is shown in Fig. 50. 
 
 In making resistance measurements with a bridge the battery 
 key should be depressed before the galvanometer key, or irregular 
 and incorrect indications will often be given on account of the self- 
 inductance of the unknown resistance. This is particularly true 
 when the unknown resistance is the windings of an electromagnet 
 
 70 
 
FIG. 50. 
 
 FIG. 51. 
 
 or any of the windings of a dynamo. Great care should always be 
 exercised not to injure the galvanometer or the fine wire coils by 
 passing too great a current through them. 
 
 For very accurate comparisons of two resistances, as when the 
 value of a standard resistance coil is to be determined in terms of a 
 mercury column or another coil, the Wheatstone bridge is made up 
 in another form. (Fig. 51.) Here we have two arms of the 
 bridge, A and B, made up of a uniform wire of high resistance and 
 small temperature co-efficient. The other two arms contain the two 
 coils. The galvanometer terminal corresponding to M (Fig. 47) 
 is made up by means of a binding post, bat the other terminal is 
 
 71 
 
arranged so that contact may be made at any point along the bridge 
 ivire. When the galvanometer contact is placed at the point on the 
 bridge wire which gives a balance, the resistances of the parts of the 
 bridge wire on each side of the galvanometer contact are to each 
 other as the two resistance coils, according to the bridge formula 
 already developed. When the bridge wire is calibrated, that is, when 
 the resistance per centimeter of length at every point of the wire is 
 determined, the ratio of the resistance of the two coils is given by the 
 ratio of the resistances of the two parts of the wire. When the bridge 
 wire is very uniform and the measurement is not required to be very 
 exact, the resistances of the two parts of the wire may be taken to be 
 proportional to their lengths. Bridge wires are usually made of an 
 alloy containing platinum and silver, or platinum and iridium. Bridges 
 of this form are usually called divided wire or meter bridges. 
 
 Measurements of very great resistances, such as the insulation 
 reistance of a well-insulated wire between its conductor and ground, 
 often require a higher power than may be conveniently reached by a 
 bridge. In this case a fine reflecting galvanometer and a large test- 
 ing battery are used. The testing battery usually consists of silver 
 chloride cells put up in sets of 50 or 100 in boxes so as to be porta- 
 ble. The galvanometer and battery are connected in series with 
 some known large resistance, and the deflection of the galvanometer 
 is read. Then the known resistance is removed from the circuit and 
 that which it is desired to measure is inserted in its place. The 
 deflection of the galvanometer is again read and from the two deflec- 
 tions the unknown resistance may be calculated. The known or 
 standard resistance'^ usually from 25,000 to 1,000,000 ohms in resist- 
 ance, i, 000,000 ohms is called a megohm, the prefix "meg" coming 
 from a Greek word meaning great. The insulation resistances of 
 wires and cables that are measured thus are frequently as great as 
 thousands of megohms, so that it is necessary to use a very fine gal- 
 vanometer to get a readable deflection through them, and the gal- 
 vanometer must be shunted (Lesson 7, page 48) when the deflec- 
 tion is taken with the standard resistance in circuit. Galvanometers 
 usually have corresponding shunt boxes come with them which have 
 three coils marked respectively i, gV, -99-9- When the shunt box is 
 connected in parallel with the galvanometer, either of these shunts 
 may be placed in the circuit by means of a plug, or the shunt circuit 
 may be broken. When the shunts are plugged into circurt, T V, TOD> 
 or TW^ P ar t of the whole current flows respectively through the gal- 
 vanometer. Fig. 52 shows a common form of shunt box. 
 
 As an example, suppose it is desired to measure the insulation of 
 an electric light cable one-half mile long, a fine galvanometer, a test- 
 ing battery of 200 cells, and a standard resistance of one-half megohm 
 being available. When connected up and shunted by the -g-J-g- shunt, 
 
the galvanometer gives a deflection of one hundred. Then its con- 
 stant, or the resistance of the circuit in megohms which would be 
 indicated by a deflection of i when the galvanometer is not shunted, 
 is loox 1000 x^ = 5OOOO, 1000 being the multiplying power of the 
 shunt and y 2 the value of the standard-resistance in megohms. Now 
 when the standard resistance is removed from the circuit, and in its 
 place one end of the connecting wire is attached to the conductor ot 
 the cable and the other end to the ground, suppose the reading of the 
 galvanometer without a shunt is 50. The insulation resistance of the 
 cable is - 5 -A A = 1000 megohms. Then the insulation resistance of a 
 similar cable for a length of one mile is 500 megohms, since the 
 paths for the current to leak out of the two half miles are in parallel. 
 Other methods of measuring high resistances and special methods 
 of measuring very low resistances are sometimes used but they need 
 not receive attention here. 
 
 Copyrighted, 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON X. 
 
 Points for Review: 1. How is resistance measured by substitution? 
 
 2. How are resistance boxes generally made? 
 
 3. Why is German silver usually used in resistance boxes? 
 
 4. How are the coils of resistance boxes adjusted? 
 
 5. What is a Wheatstone bridge? 
 
 6.. What is the process of measuring a resistance by a Wheatstone bridge? 
 
 7. Suppose a resistance is to be measured by bridge, and after the bridge is balanced 
 the rheostat resistance reads 15.6 ohms, while the ratio arms are 100 (A) to 10 (C), what 
 is the value of the unknown resistance? 
 
 8. In another case suppose the rheostat reads 2,600 ohms and the ratio arms are 10 
 (A) to 1,000 (C), what is the value of the unknown resistance? 
 
 9. Why should the battery key be depressed before the galvanometer key when 
 making bridge measurements? 
 
 10. How may very high resistances be measured? 
 
 11. Suppose a reflecting galvanometer shunted with the ^9 shunt gives a deflec- 
 tion of 80 when using a certain battery, the standard resistance being 25,000 ohms; what 
 is the galvanometer constant? 
 
 12. Suppose the deflection is 120 with the \ shunt when a certain resistance is 
 substituted for the standard as above, other things being unchanged, what is the value of 
 the resistance? 
 
 XI. 
 
 EVERY DAY MEASUREMENTS OF ELECTRIC 
 CURRENTS AND PRESSURES. 
 
 We have already seen that electric currents may be measured 
 by taking advantage of three different and independent effects of the 
 current. These are: i, the electrochemical effect; 2, the magnetic 
 effect; 3, the heating effect. By taking advantage of the first effect 
 we measure currents by voltameters (Lesson 9, page 62); as a result 
 of the second effect we measure currents by means of galvanometers 
 (Lesson 9, page 58); from the third effect we may measure currents 
 by means of the expansion of a wire which is heated by the passage 
 of the current through it (Lesson 8, page 54). 
 
 Voltameters, as already said, are principally used for calibrating 
 galvanometers or for similar purposes, as they are not sufficiently 
 convenient for general use. The liquid must be kept fairly pure and 
 of the proper density. Conveniences must be available for cleaning, 
 drying, and accurately weighing the cathodes. In order that a satis- 
 factory measurement of the current may be made, the period during 
 which it flows through a voltameter must be considerable. 
 
 74 
 
For one purpose only have they been found particularly useful 
 in everyday measurements; that is, as a meter such as many of the 
 Edison Illuminating Companies use (Lesson 9, page 64) . Volta- 
 meters were used for this purpose in the early days of electric light- 
 ing with incandescent lamps and have continued in use until now. 
 Even for that purpose their everyday use is not being extended, as 
 good mechanical meters that are more reliable are now to be had. 
 
 Nearly all our common instruments for measuring currents de- 
 pend upon the magnetic effect of the current for their indications, 
 and are really modified galvanometers with pointers to show the 
 deflection. 
 
 Galvanometers or other instruments intended especially for con- 
 venient use in every day measurements of currents, are generally 
 called amperemeters or ammeters, because they measure amperes. 
 Amperemeters are made in various forms, all more or less portable. 
 Almost every manufacturer of dynamos, or other electrical machinery, 
 manufactures amperemeters which may be used in service with 
 their machines. Amperemeters are used universally where electric- 
 ity is used, and they are made to measure currents consisting of only 
 a few thousandths of an ampere, or milliamperes (milli comes from a 
 Latin word meaning thousand), up to the enormous currents gener- 
 ated by some of the larger electric lighting plants consisting of 
 thousands of amperes. In large electric lighting plants or works 
 many amperemeters may be seen mounted on the wall or on a board 
 among switches for controlling the current. These are used to show 
 the dynamo attendants how much current is being generated by the 
 plant at any moment, and what proportion is furnished by each 
 dynamo. Amperemeters are used in laboratories to determine the 
 current used in experiments, and to determine the amount of current 
 used in the operation of electric lamps, electric motors, or other 
 electric devices. Physicians use amperemeters to measure the cur- 
 rents used in the electrical treatment of their patients. For the latter 
 purpose the currents are usually measured in milliamperes. The 
 currents used in telegraphy are also usually measured in milliamperes, 
 and the currents used in operating telephones are usually measured 
 in microamperes, or millionths of amperes (micro coming from a 
 Greek word meaning small). Amperemeters that are specially made 
 to measure thousandths of amperes, or milliamperes, are called 
 milliamperemeters. Externally, milliamperemeters look like ordi- 
 nary amperemeters, to which they bear the same relation that a very 
 sensitive galvanometer bears to a similar but less sensitive instru- 
 ment. 
 
 The mechanical details entering into the construction of mag- 
 netic amperemeters differ very widely. They may be roughly 
 divided into three classes: (i) those having permanently magnetized 
 parts which are moved by magnetic force set up by a current in the 
 
1i 
 EDISON 
 
 SYSTEM, 
 
 AMPERE METER 
 
 FIG. 53 
 
 FIG. 56. 
 
 _ _ ^ FIG. 57. 
 
 FIG. 54. 
 
 coils of the instrument; (2) those having soft iron parts which are 
 moved by the magnetic attraction set up by a current in the coils of 
 the instrument; (3) those having no iron in their construction, but 
 having two coils, one of which is moved by magnetic force exerted 
 between them when a current flows in both. The moving parts of 
 amperemeters are -usually mounted on pivots made so that the 
 friction is small. If the magnetic force caused by a current in the 
 coils had nothing except the friction to overcome, every current 
 would pull the pointer clear across the scale to the stop. It is desir- 
 able to construct the instrument so that the movement of the pointer 
 is proportional to the current in the coil, so a proper force must be 
 arranged to hold the pointer back. This may be done by properly 
 counter weighting the moving parts so that the magnetic force must 
 raise them against the force of gravity, or by arranging a proper 
 spring to oppose the magnetic force. Fig. 53 shows an instrument 
 in which a curved iron wire is drawn into a coil of wire where the 
 
 76 
 
 
FIG 55. 
 
 current flows through the coil. The weight of the moving parts of 
 the instrument serves to keep the pointer at zero when no current 
 flows. When a current flows it exerts an attraction on the iron wire 
 core, which overcomes the effect of the weight of the moving parts, the 
 iron core is attracted into the coil a certain distance, and the pointer 
 moves proportionally. This instrument evidently belongs to the sec- 
 ond class. Instruments of the second class may be cheaply made. 
 They are therefore commonly made by dynamo builders for use with 
 their dynamos in electric light plants'. Fig. 54 shows another 
 form of amperemeter of the same class. 
 
 Instruments having soft iron in their moving parts cannot be 
 made extremely accurate because the iron does not always respond 
 equally to the same magnetic changes on account of its coercive 
 force (L,esson 5, page 31); consequently instruments of the second 
 class can only be used where great accuracy is not required. It is 
 sufficient for the amperemeters used in electric plants to be correct 
 within five per cent, and instruments of the second class serve 
 jvery well. For testing which requires greater accuracy instruments 
 belonging to the first and third classes must be used. These can be 
 made so that their readings do not vary more than one-half of one 
 per cent from true values when they are used with proper care. 
 
 Fig. 55 shows a Weston amperemeter, which is practically a 
 d'Arsonval galvanometer with the moving coil mounted on pivots 
 and arranged with a pointer to play over a scale, and the whole ar- 
 ranged in a very convenient portable form. This instrument may 
 be looked upon as the most satisfactory representative of the first 
 class, to which it bears the same relation that a d'Arsonval galvano- 
 meter bears to a galvanometer with a movable magnetic needle. 
 
Weston amperemeters are used a great deal where accurate portable 
 current measuring instruments are required, because they are accu- 
 rate, convenient, and well made. 
 
 Magnetic instruments belonging to the third class are really not 
 galvanometers, but are called electrodynamometers, because their in- 
 dications are caused by the magnetic pull of the current in the fixed 
 and movable coils upon itself. Fig. 56 shows the ordinary form of 
 electrodynamometer when arranged for use as an amperemeter. This 
 is often called the Siemens electrodynamometer. In it, one coil is fast- 
 ened to the frame of the instrument, and the other, which stands at 
 right angles to the first, is suspended by a heavy silk fibre so that it 
 is free to move. The end, of the wire composing the movable coil 
 dips into little cups containing mercury which are connected with a 
 circuit so that the current can enter and leave the coil. The mov- 
 able coil is attached to a spring, the other end of which is connected 
 to a thumbscrew by means of which the spring may be twisted. When 
 a current flows in the coil, the magnetic force tends to turn the 
 movable coil around so as to place it parallel with the fixed coil. 
 (Lesson 6, page 38.) This force is balanced by twisting the spring 
 by means of the thumbscrew. The amount of twist as shown by a 
 pointer attached to the screw is proportional to the force exerted by the 
 coils on each other. This force is proportional to the square of the 
 current flowing in the coil, since the magnetism set up by each coil 
 is proportional to the current and they act on each other mutually. 
 
 Other instruments for measuring currents by their direct mag- 
 netic action, as in the Siemens electrodynamometer, have been 
 designed, but they have not been made sufficiently portable to bring 
 them into much use. The most important of these are the current 
 balances of Sir William Thomson, now Lord Kelvin. In these the 
 fixed and movable coils are parallel and horizontal. The force with 
 which the coils tend to move toward each other when a current flows 
 in them is directly balanced and weighed by means of a slider mov- 
 ing on a scale beam. In order to avoid any effect from the earth's 
 magnetism, coils are placed at both ends of the balance arm and are 
 electrically connected so that the magnetic force of the two sets of 
 coils tends to tip the beam in the same direction. 
 
 Instruments utilizing the heating effect of the current may be 
 called hot wire instruments. If the wire be carefully enclosed so that 
 its temperature is not affected by air currents, it will rise to a definite 
 number of degrees in temperature for every current that is passed 
 through it, and the rise is proportional to the square of the current. 
 (Lesson 8, page 53.) The length of the wire increases practically in 
 direct proportion to its rise in temperature when it is heated, and the 
 length again decreases when the wire is cooled. Consequently, when 
 currents of different strength flow through a wire it will take up a 
 corresponding length with each current, and measuring its length 
 therefore measures the square of the current. A simple form of 
 
amperemeter depending on this action is shown in Fig. 57. A long 
 thin wire is clasped at one end in a stationary binding post and the 
 other end is wrapped around and fastened to a small wheel of metal. 
 This wheel is supported in steel pivots, one of which is connected to 
 another binding post. Tlie wire is kept under a constant strain by 
 means of a spring the end of which is also fastened to the periphery 
 of the wheel, so that when the wire is heated and lengthens, the 
 wheel is turned by the contraction of the spring, and when the wire 
 is again cooled and contracts it pulls the wheel back to its old posi- 
 tion. The wheel carries a pointer the position of which may be read 
 on the graduated scale when any current flows in the wire. 
 
 Many amperemeters have scales that are uniformly graduated 
 and the readings of which can only be converted into amperes by 
 consulting a calibration curve or a table giving the values of differ- 
 ent readings in amperes. In other instruments multiplying the read- 
 ings by a fixed constant which has been experimentally determined, 
 converts them into amperes. In still other instruments, which are 
 said to be direct reading, the scales are so divided and marked that 
 the divisions read directly in amperes. It is needless to say that 
 direct reading instruments are the most convenient for use. 
 
 Currents which rapidly alternate in direction, as do the currents 
 of many electric light plants, cannot be measured by magnetic 
 instruments having permanent magnets, since the tendency of such 
 currents is to first deflect the moving parts in one direction and then 
 in the other, and the pointer stands still or nearly so. Such currents 
 can be measured by magnetic instruments of the second class because 
 the soft iron core is a/ways attracted by a coil in which a current flows 
 without regard to the direction of the current. The iron cores in 
 instruments designed to measure alternating currents must be made 
 up from fine iron wires so that currents shall not be set up in them 
 by the reversals of the magnetism, as will be explained later. 
 
 Klectrodynamometers and other instruments depending for their 
 indications upon the mutual attractions of two coils, may be used 
 to measure alternating currents because the current reverses in the 
 two coils at the same instant, and the magnetic attraction between 
 the coils is therefore always in the same direction. The heating 
 effect of currents is always independent of their direction, so that 
 hot wire instruments may be used to measure alternating currents. 
 
 When very large currents are to be measured, it is often incon- 
 venient and expensive to build an amperemeter of sufficient capacity 
 for the purpose. In this case an amperemeter of small capacity may 
 be shunted by a copper or german silver wire or rod, and the 
 shunted instrument may then be calibrated and used to measure the 
 large current. This arrangement is becoming quite common in the 
 largest electric light works where very great currents are to be 
 measured. Nearly all Weston amperemeters consist of a milliam- 
 peremeter arranged with a proper shunt inside the case so that the 
 desired range is obtained. 
 
 79 
 
FIG. 58. 
 
 The commonest method of measuring- an electric pressure is to 
 measure the current which it causes to pass through a known high 
 resistance. The resistance may be connected permanently in the 
 circuit of a sensitive amperemeter, such as a milliamperemeter, and 
 the instrument may be calibrated so that its indications may be 
 readily converted into volts. 
 
 Instruments that are used for every-day measurements of 
 electric pressures are called voltmeters, because they measure volts. 
 By properly dividing the scale upon which the indications are made, 
 voltmeters may be made direct reading. Fig. 58 shows a Weston 
 direct reading voltmeter, in which the working parts are similar to 
 those of the amperemeter shown in Fig. 55; but in the voltmeter, 
 a high resistance spool of fine wire is placed in series with the 
 d'Arsonval galvanometer coils, instead of a low resistance s^*"it 
 being placed in parallel with it, as is done in the amperemeter. A 
 voltmeter is shown in Fig. 59 which is made upon the same 
 principle as the amperemeter shown in Fig. 53, but the coil is 
 wound with many turns of fine wire, making a high resistance, 
 instead of being made with a few turns of coarse wire. This form 
 of voltmeter has the same fault as the amperemeter of the same class, 
 that of being not very accurate, and it therefore is not as satisfactory 
 for use in many places as more accurate instruments made with very 
 little or no iron in their working parts. In electric light plants 
 where current is produced for use in incandescent lamps, it is very 
 important that the pressure be kept as closely as possible to the 
 exact pressure with which the lamps were designed to be used. 
 Consequently, in such places the most accurate and reliable volt- 
 meters or pressure indicators, as they are sometimes called, are 
 needed. 
 
 Voltmeters of this kind are usually made with a very high 
 resistance so that only a small current flows through them and they 
 
 80 
 
I 
 
 /O, O0O &MW3 TO 
 
 /yyyVAAAAAAAAAAAAAAAAAA * > 
 /OO 
 
 O-J 
 
 FIG. 63. 
 
 
 FIG. 61. 
 
 SI 
 
 XJNIVE 
 
may therefore be used without an appreciable change of the current 
 in a circuit. 
 
 Fig. 60 shows a hot wire voltmeter which is called after its 
 inventor, Cardew. This was at one time largely used to measure 
 alternating electric pressures, and is still used quite generally for the 
 same purpose in England. The indications of this instrument are 
 dependent upon the expansion of a very fine platinum-silver wire 
 (YO^-O inch in diameter) through which the current passes. This wire 
 is from 8 to 12 feet long and of such high resistance per foot that 
 its resistdnce alone is sufficient for use up to a pressure of 1 20 volts, 
 but another resistance coil is put in series with the instrument when 
 it is used to measure higher pressures. 
 
 Another entirely distinct method of measuring electric pres- 
 sures is by means of electrometers. On page 8 of L,esson i, it was 
 said that electrometers are instruments for determining the amount 
 of electricity on a charged body by measuring its attraction for an- 
 other charged body. It w^also explained on page 13 of Lesson 2 that 
 electricity at rest at a high pressure constitutes a positive charge, 
 and electricity at rest at a low,pressure constitutes a negative charge. 
 It is a fact that the terms positive and negative charge must be taken 
 as relative terms exactly as are the terms high and low pressure. An 
 electrometer is an instrument by means of which the attraction 
 between two charges may be measured. One form of electrometer is 
 shown in Fig. 61. In this there is a needle made of aluminum and 
 a sort of pillbox cut into qiiadrants (quarters). If the opposite quar- 
 ters be connected together as shown and one pair of quarters be con- 
 nected to the needle, and a charge of one sign be communicated to 
 the needle and its connected pair of quadrants, and a charge of the 
 opposite sign to the other pair of quadrants, the needle will tend to 
 be deflected by the attraction and repulsion of the charges. The 
 force with which the needle tends to turn may be measured by a 
 torsion head as in an electrodynamometer, or by suspending the 
 needle so that a certain portion of its weight must be lifted as it 
 turns. If the two poles of a battery, for instance, be connected to 
 the two terminals of the electrometer, one terminal is brought to a 
 high pressure and the other to a low pressure on account of the action 
 of the battery, and they therefore hold corresponding positive and 
 negative charges. The deflection of the needle indicates the pressure 
 developed by the battery. This pressure may be directly read off in 
 volts if the instrument has been properly calibrated. In the same 
 way if the two ends of a resistance through which a current is flow- 
 ing, such as an electric lamp, be connected to the electrometer, one 
 terminal is brought to a high and the other to a low pressure and the 
 deflection of the needle shows the difference of pressure between the 
 ends of the resistance. Electrometers made for use in everyday 
 measurements of electric pressure are usually called electrostatic 
 
 82 
 
voltmeters and are used to some extent, particularly for measuring 
 alternating electric pressures. They can be used for the latter pur- 
 pose, since the polarity of the two pairs of quadrants and oi the 
 needle change at the same instant and consequently the needle is 
 deflected continously in the same direction. Fig. 62 shows an elec- 
 trostatic voltmeter made for measuring pressures of several thousand 
 volts. 
 
 Still another method of measuring an electric pressure is to 
 compare it with a standard pressure. If between the points whose 
 difference of pressure it is desired to measure, a known large resist- 
 ance be connected, a small current will flow through the resistance, 
 and the pressure will fall along the path of the current in proportion 
 to the resistance passed over. Now suppose the terminals of a bat- 
 tery cell be connected in series with a galvanometer to certain points 
 on the resistance (Fig. 63) in such a way that the pressure of the cell 
 is in opposition to the difference of pressure between the poLits. If 
 the latter pressure be greater than that of the cell, a current will flow 
 through the cell and galvanometer, and the galvanometer needle 
 will be deflected. The same thing will occur if the pressure of the 
 cell is the greater, but the current will be reversed. Finally, if the 
 portion of the resistance which is between the terminal connections 
 of the cell be so adjusted that no current flows through the galvano 
 meter, the fall of pressure through that part of the resistance exactly 
 equals x the pressure produced by the cell. The total pressure to be 
 measured is then equal to the pressure developed by the cell multi- 
 plied by the ratio of the total resistance to the balancing resistance. 
 In the figure the pressure of the cell is marked 1.2 volts, the total re- 
 sistance is 10,000 ohms and the balancing resistance is 100 ohms. 
 Assuming a balance, the total pressure must be 1.2x10,000 100=120 
 volts. 
 
 A special arrangement for measuring pressures by comparison is 
 often called a potentiometer, and the cells used for the comparison are 
 called standard cells. It is evident that standard cells must develop 
 a very uniform pressure under all conditions of their use. The best 
 standard cell is that called Clark* s cell, after its inventor. This was 
 recommended by the Chicago Electrical Congress to be used as a 
 comparative standard of pressures, and its pressure was given in 
 accordance with experimental tests to be 1.434 volts at 15 Cen- 
 tigrade when set up according to fixed instructions. Professor Car- 
 hart has endeavored to make a standard cell with exactly one volt 
 pressure. Voltmeters have been made upon the principle of a poten- 
 tiometer. 
 
 Electric currents may be indirectly measured by means of a volt- 
 meter, and a known resistance placed in the circuit through which 
 the current flows. In this case the voltmeter is used to measure the 
 difference of pressure between the ends of the resistance, and the 
 current may be at once calculated from Ohm's law. 
 
 Copyrighted. 1894 
 
 83 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XL, 
 
 Points for Review. 1. What are the three effects by which electric currents may 
 directly measured? 
 
 2. What are amperemeters? 
 
 3. What is a milliampere? What is a microampere? 
 
 4. What are the three classes'of magnetic amperemeters? 
 
 5. What is the general arrangement of Weston amperemeters? 
 
 6. What are electrodynamometers? 
 
 7. What are hot wire instruments? 
 
 8. How are the scales of amperemeters divided? 
 
 9. By what instruments can alternating currents be measured? Why? 
 
 10. How may very large currents be conveniently measured? 
 
 11. What is the commonest method of measuring electric pressures? 
 
 12. What are voltmeters? 
 
 13. What is the principle of the Cardew voltmeter? 
 
 14. How may electrometers be used to measure electric pressures? 
 
 15. What are electrostatic voltmeters? 
 
 16. How may standard cells be used in measuring electric pressures? 
 
 17. How may a current be measured by using a voltmeter and a standard resistance? 
 
 XII. 
 
 EVERY-DAY MEASUREMENTS OF ELECTRIC POWER. 
 CONDENSERS AND THE MEASUREMENT OF THEIR 
 
 CAPACITY. 
 
 The electric power which is used in any part of a circuit, may 
 be determined by measuring by an amperemeter the curient flowing, 
 and by a voltmeter the pressure, or voltage as it is often called, at 
 the terminals of the portion of the circuit. These being multiplied 
 together, give the power in watts (Lesson 8, page 52). Instruments 
 are made in which the double measurement and multiplication is all 
 made together, so that their indications are directly proportional to 
 power. Such instruments are called wattmeters because they meas- 
 ure watts. The simplest form of wattmeter is an electrodynamo- 
 meter in which one coil is wound with many turns of fine wire 
 exactly as though it were to be used as a voltmeter coil, and the 
 other coil is wound as though it were to used in an amperemeter. 
 For convenience we will call them \h^ pressure coi/and the current coil. 
 The action of such a wattmeter is best explained by an illustration. 
 Suppose it is desired to measure the power used by an electric motor, 
 then the current coil of the wattmeter is connected in series with 
 the motor, and the pressure coil is connected across the termi- 
 
 84- 
 
nals of the motor. The magnetic ef- 
 fect of the current coil is therefore pro- 
 portional to the current which flows 
 through the motor, and that of the 
 pressure coil is proportional to the 
 pressure at which the current is sup- 
 plied to the motor. The indications of 
 an electrodynamometer are propor- 
 tional to the product of the magnetic 
 effects of the two coils (Lesson n, page 
 76). Consequently, in this case the in- 
 dications are proportional to current 
 times pressure or watts, instead of cur- 
 rent times current, as in the Siemens 
 electrodynamometer (Lesson u, page FIG. t>4. 
 
 Wattmeters may be calibrated by comparing their readings, 
 when connected to a circuit, with the indications of standard volt- 
 meters and amperemeters. By proper construction and adjustment 
 of their scales they may be made direct reading. 
 
 It is also possible to make electrostatic wattmeters, and watt- 
 meters based upon other principles. 
 
 Recording wattmeters may be used to show the amount of power 
 used each month by the customers of electric plants. The com- 
 monest form of wattmeter used for this purpose is that shown in 
 Fig. 64, known as the Thomson wattmeter, after its inventor. This 
 consists of a little electric motor without any iron in its workin'g 
 parts, which is arranged with its revolving part or armature as a 
 pressure coil, and its magnetizing coil as a current coil. The mag- 
 netic pull which tends to make the armature rotate is proportional 
 to the product of the two magnetizing effects, and this is proportional 
 to the watts in the circuit, exactly as in an electrodynamometer. If the 
 speed of such an armature is made to be proportional to the magnetic 
 pull it is easily seen that every revolution of the armature means a cer- 
 tain number of watts used for a fixed length of time. Such instruments 
 usually have attached to the spindle of the armature a set of dials 
 like those of a gas meter which record the revolutions and are so 
 marked that the consumption of electric power may be recorded in 
 watt- hours. Watt-hours are the product of the number of watts by 
 the number of hours during which the power is used. If no txter- 
 nal retarding force were applied to the armature of such an instru- 
 ment it would run away as soon as placed in service, and in order that 
 its speed may be proportional to the watts the retarding force must 
 be proportional to the speed. This Is very ingeniously arranged in 
 the Thomson recording wattmeter by placing at the bottom of the 
 spindle a flat disc of copper on either side of which are placed the 
 poles of magnets. The rotation of the disc between the magnet 
 
 86 
 
poles generates electric currents in it which are attracted by the 
 magnets and retard the motion of the disc. 
 
 Other meters for use in determining the amount of power con- 
 sumed by customers, which are externally similar to the Thomson, 
 only read ampere-hours. An ampere hour is equal to 3,600 ampere- 
 seconds, but one ampere-second, or one ampere flowing for one 
 second, means the transfer of one coulomb of electricity through the 
 circuit. Consequently the readings of meters which record in ampere- 
 hours are directly comparable with the indications of the Edison elec- 
 trolytic meter which has been mentioned before (L,esson 9, page 64). 
 Meters which read in ampere-hours are sometimes called coulomb- 
 meters. The reading of ampere hours has no relation to the power con- 
 sumed in a circuit unless the pressure in a circuit is known, but in the 
 cases, where such meters are used the pressure is intended to be kept 
 at a constant known value so that the watt-hours used by each customer 
 may be easily determined, when desired, by multiplying the ampere- 
 hour reading of his meter by the pressure in the circuit. 
 
 If the bottom of a cylindrical can, filled with water, be connected 
 by means of a tube to the bottom of a similar can of different diame- 
 ter standing on the same level, the water will flow into the second 
 can until it stands at the same height in both. The quantity of 
 water in each vessel when the flow has ceased is proportional to the 
 capacity of the vessel* During the flow the water falls in one can 
 and rises in the other. In the same way if a conductor, such as a 
 brass ball carrying an electric charge be touched by an uncharged con- 
 ductor, part of the charge flows to the second conductor. During the 
 flow the electric pressure of one conductor falls and the pressure of the 
 other rises. After the flow has ceased the electrical pressure of the two 
 conductors is equal (compare Lesson 2, page 13). The quantity of 
 electricity on the two conductors is not equal unless the conductors 
 are exactly similar, but the quantity on each will depend upon its 
 capacity to hold electricity, or its electrical capacity. The electrical 
 capacity of a conductor depends upon its size, shape, and surround- 
 ings. It is measured by the number of coulombs of electricity required 
 to raise the electrical pressure of the conductor one volt, exactly as the 
 capacity of a cylindrical can is measured by the number of gallons of 
 water required to fill it to the depth, or head, of one foot. 
 
 When the pressure of a conductor is raised one volt by the 
 charge of one coulomb, the conductor is said to have a capacity of 
 one farad, after Faraday, the distinguished English scientist. 
 
 The electrical pressure of a conductor carrying a charge of elec- 
 tricity is ordinarily reckoned as the difference between it and the 
 average electrical pressure of the earth's surface, which is called zero. 
 This is similar to the reference of levels or heights to the sea level as 
 a zero point from which to start. The electrical pressure of a charged 
 conductor cannot be measured by an ordinary voltmeter since the 
 
charge would be at once dissipated by the current which would flow 
 through the voltmeter when connected between the conductor and the 
 earth. The pressure may, however, be measured by a sufficiently sen- 
 sitive electrometer or electrostatic voltmeter. For instance, in the case 
 of a quadrant electrometer which was briefly described in the preceding 
 lesson (Lesson n, page 82), the needle and its pair of quadrants may 
 be connected to earth and the other pair of quadrants to the charged 
 body. Then if the instrument is sufficiently sensitive the needle will 
 be deflected an amount which is proportional to the difference 
 between the earth's electrical pressure and that of the charged body. 
 
 The presence of charges of an opposite sign near a charged con- 
 ductor has a remarkable influence on the conductor's capacity. For 
 instance, if pieces of tin-foil are pasted upon the two sides of a sheet 
 of mica and the two tin-foil coatings are given opposite charges, the 
 charges act inductively on each other and consequently increase their 
 capacities. Such an arrangement is called a condenser. The tin-foil 
 sheets are called the coatings or plates of the condenser and the insu- 
 lating material is called the dielectric. The coatings of a condenser 
 may be made of any conducting material, and the dielectric of any 
 insulating material. 
 
 The combined capacity of the coatings is the capacity of the con- 
 denser. A condenser has a capacity of one farad when a charge of 
 one coulomb of electricity raises the difference of electrical pressure, 
 or potential, of the plates by one volt. To charge a condenser with 
 a certain quantity of electricity means that a positive charge of the 
 given quantity is placed upon one plate and an equal negative charge 
 on the other. 
 
 A condenser may be charged in either of two ways: ist, by con- 
 necting one plate to earth and placing the charge on the other plate, 
 when the required opposite charge will collect on the grounded plate 
 by induction; and, by connecting the two plates of the condenser to 
 the two terminals of an electric battery, or other source of electricity, 
 when the charge is communicated by the action of the battery. 
 
 Every electrical conductor, as we have seen, has capacity, and 
 when an insulated wire is laid in the earth or is strung over- 
 head it becomes one plate of a condenser. The other plate of the 
 condenser is the earth, and the dielectric is the insulating covering 
 of the wire, or the air which is between it and the earth. The capa- 
 city of a wire has a great deal of effect on its usefulness in telephone 
 service. Every hundredth of a microfarad per mile of conductor 
 reduces very considerably the distance through which the telephone 
 will work satisfactorily. The capacity of ocean cables is also a matter 
 of much importance, and capacity effects are of importance in teleg- 
 raphy and in the transmission of power by alternating currents of 
 electricity. 
 
 The capacity of a condenser depends directly upon the area of 
 
 87 
 
its plates, their closeness together, and the specific inductive capacity 
 of the dielectric. Different insulating materials have very different 
 values as dielectrics. The inductive action seems to be stronger 
 through some materials than through others, and it is less active 
 through air than through any solids or liquids. Consequently a 
 condenser which has air for a dielectric has less capacity than one of 
 exactly equal size with a solid dielectric. The ratio of the capacities of 
 two such condensers is called the specific inductive capacity of the solid 
 dielectric. The annexed table gives the approximate specific induc- 
 tive capacities of various materials. That of air is taken as unity as 
 a matter of reference, because the inductive effect is less through it 
 than through any common substance. 
 
 SPECIFIC INDUCTIVE SPECIFIC INDUCTIVE 
 
 MATERIAL,. CAPACITY. MATERIAL. CAPACITY. 
 
 Air i. Gutta-percha 2.5 
 
 Petroleum 2.1 Shellac 2.9 
 
 Turpentine 2.2 Sulphur ... 3.7 
 
 Rubber 2.3 Mica -,. 6.6 
 
 Paraffine 2.3 Glass 5.0 to 10.0 
 
 The table shows the importance of carefully selecting the insu- 
 lation for telephone cables in order that their capacities may be the 
 least possible. In fact, the insulation directly surrounding the indi- 
 vidual wires of such cables is often made from crinkled paper so 
 that air makes up a considerable part of the material between the 
 wires. While glass is one of the best of insulators, it is one of the 
 poorest materials to use for the continuous insulation of the wires in 
 telephone cables on account of its great specific inductive capacity. 
 
 Insulated wires and cables placed underground always have a 
 much greater capacity than wires of the same size and length placed 
 overhead. This is largely because the dielectric of the underground 
 wires is so much thinner than that of the overhead wires, and par- 
 tially because the inductive capacity of solid dielectrics is greater 
 than that of air. The capacity of an overhead wire strung at a 
 height of thirty feet above the ground is only about one twentieth of 
 that of a similar wire well insulated with a rubber compound and 
 placed underground, and only about one tenth of that of a similar 
 wire insulated with cotton and paraffine and placed in a cable under- 
 ground. 
 
 It is very important to make measurements of the capacity of 
 conductors to be used in telephony and telegraphy. This may be 
 done in various ways, but the method that is generally used is to 
 directly compare the capacity of the wire with that of a standard 
 condenser by means of a ballistic galvanometer. Standard condens- 
 ers are made of various capacities and put up in boxes so that they 
 may be readily used for various purposes. Since a capacity as large 
 as a farad is very seldom met in the electrical industries, standard 
 
 88 
 
FIG. 65. 
 
 condensers are usually made equal to microfarads (one millionth of a 
 farad) or fraction of microfarads, and the microfarad has become the 
 common unit in which capacities are measured. Fig. 65 shows an ad- 
 justable condenser which is made with five divisions of . i microfarad 
 each. The five divisions may be put in parallel so that the total 
 capacity is y 2 microfarad. Since the capacity of a condenser is directly 
 proportional to the area of the plates, connecting condensers in parallel 
 gives a total or combined capacity which is equal to the sum of the in- 
 dividual capacities. Again, since the capacity depends inversely 
 upon the thickness of the dielectric, connecting condensers of equal 
 capacity in series, gives a combined capacity equal to the capacity oj 
 one condenser divided by the number in series, because connecting 
 condensers in series has the effect of adding together the thickness 
 of the dielectrics in the different condensers. Where condensers of 
 different capacities are connected together in series, the combined 
 capacity is equal to the reciprocal of the sum of the reciprocals of the 
 individual capacities. ( * , K = J ; k x -j- x / k 2 + 1 , k 3 . ) (Compare combi ned 
 resistances, Lesson 7, page 46). Condensers connected in series are 
 sometimes said to be connected in cascade. 
 
 The plates of standard condensers are usually made of .tinfoil, 
 and the dielectric of mica, paraffined paper, or oiled paper. 
 
 A ballistic galvanometer is simply a sensitive galvanometer 
 which is not dead beat. In this case, if a certain quantity of elec- 
 tricity be passed through the coils of the instrument in a very short 
 interval of time, its magnetic effect on the needle is very much like 
 that of a blow, while the magnetic effect of a steady current on the 
 
needle is like that of a steady push. The needle of a galvanometer 
 where such a transient current or discharge passes through it, swings 
 off through an angle which is proportional to the quantity of elec- 
 tricity in the! discharge, provided the angle of swing or throw is not 
 too great. To measure the capacity of a cable, a standard condenser 
 is selected of a capacity nearly equal to that of the cable. The con- 
 denser is charged by a few cells of battery, and by means of a key 
 its connections are then changed so that it discharges through a 
 galvanometer. The throw of the galvanometer needle is observed. 
 The same battery is now connected with one terminal to the cable 
 conductor and its other terminal to the cable sheathing or to the 
 earth. In this way the cable is charged. The cable and earth con- 
 nections are then transferred to the galvanometer by means of the 
 key, and the cable is discharged through the galvanometer. The 
 throw of the needle is again observed. The two throws are propor- 
 tional to the quantities of electricity in the charges of the condenser 
 and the cable. Since these were charged by the same battery and 
 therefore to the same pressure, the quantities of electricities are pro- 
 portional to the respective capacities. Therefore the capacities are 
 proportional to the throws. 
 
 The object of taking a condenser of a capacity nearly equal to 
 that of the cable is to make the throws nearly alike and thus avoid 
 instrumental errors. When a proper condenser cannot be obtained, 
 a shunt may be used, but this is also likely to introduce errors when 
 used with discharges. The insulation of the instruments and their 
 connections must be as perfect as possible in capacity tests, as is also 
 necessary in insulation tests (Lesson 10, page 72). 
 
 As an example, suppose the discharge of a y 2 microfarad con- 
 denser when charged by five cells gives a galvanometer throw of 200 
 divisions; and when a cable two miles long is charged by the same 
 cellsj arM discharged through the galvanometer, the throw is 180. 
 Then the capacity of the cable is iff X^=-45 microfarads, and the 
 capacity of the cable per mile is .45 / 2=. 225 microfarads. 
 
 A Ley den jar (Fig. 66) is a condenser made out. of a glass jar 
 which is coated with tin foil both outside and inside. 
 
 Copyrighted, 1894, 
 
 90 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XII. 
 
 Points for Review. 1. What are wattmeters? 
 
 2. How may an electrodynamometer be used as a wattmeter? 
 
 3. For what purpose are recording wattmeters and coulomb-meters used? 
 
 4. What is a watt-hour? An ampere-hour? 
 
 5. What is electrical capacity? 
 
 6. What is a farad? Why is a microfarad commonly used as the unit for measur- 
 ing capacity? 
 
 7. What is a condenser? What is the capacity of a condenser? 
 
 8. How may a condenser be charged? 
 
 9. Upon what does the capacity of a condenser depend? 
 
 10. What is the specific inductive capacity of a substance? 
 
 11. Why must the insulating material for telephone cables be carefully selected to 
 avoid excessive capacity? 
 
 12. Why is glass a poor material to use for continuously insulating telephone cables? 
 
 13. Why do underground wires have a greater capacity than similar wires strung 
 overhead? 
 
 14. If three condensers of ^ microfarad be connected in parallel, what is their com- 
 bined capacity? 
 
 15. How may capacities be compared by using a ballistic galvanometer? 
 
 16. Suppose that a l /$ microfarad condenser causes a throw of 80 divisions when 
 charged with three cells and discharged through a galvanometer, while the throw caused 
 by a five-mile wire, when charged by the same battery, is 120 divisions; what is the 
 capacity of the wire per mile? 
 
 17. What is a leyden jar? 
 
 LESSON XIII. 
 
 ELECTROLYTIC DEPOSITION OF METALS. 
 
 The electrochemical operations which result in depositing 
 metals from a solution of their metallic salts are very wide-spread 
 in the industries and are of great usefulness. The magnitude of the 
 works involved in most of the operations does not approach that of 
 works built for the purpose of furnishing electricity for light and 
 power; nor do the ordinary electrolytic operations appeal to the ordi- 
 nary observer as do the applications of electricity to transmitting 
 messages, driving street cars, or furnishing light or power. Never- 
 theless we owe to electrochemical operations many of the common, 
 est necessities and comforts of life. The commercial applications of 
 electrolysis cover a wide and useful range from nickel and silver 
 Elating to electrotyping for the use of the printer; and from methods 
 of bronzing and gilding to methods of smelting certain ores and 
 
 91 
 
refining metals. Lesson 13 will be given to the consideration 
 of the commonest and most useful of these applications. Nearly 
 all of the processes depend upon the laws of chemical action 
 which have already been described in the lessons on electric 
 batteries and voltameters (Lessons 3, 4 and 12), but frequently 
 the solutions used are quite complex, so that the chemical action 
 which occurs is complicated and not always fully understood. 
 
 A working knowledge of the processes of electrodeposition of 
 metals has been possessed only since 1800, and, indeed, many of the 
 more important processes of plating, electrotyping, etc., have been 
 discovered since 1840 or 1845, while some of the important opera- 
 tions of electrometallurgy, such as the electrolytic recovery of 
 aluminum 'and the commercial refining of copper by electrolysis, 
 have not been employed until within a very few years. The next 
 few years seem destined to see electrolysis and electrometallurgical 
 processes (processes of treating metals in which electricity is used) 
 put into extended use in the recovery of various metals from their 
 ores, and in some hitherto little explored fields, such as the purifying 
 of drinking water and sterilising of sewage. 
 
 Electroplating is the process of covering articles of metal with a 
 thin layer of another metal by means of electrolysis. The covering 
 usually consists of nickel, silver, or gold, and the base or covered 
 metal is ordinarily of some composition such as white metal, Britan- 
 nia metal, german silver, or brass. The details of the process are 
 quite different for the different metals used in plating. We will first 
 take up silver plating, as silver is the most important metal in plat- 
 ing processes. The commonest salts of silver are chloride of silver, 
 nitrate of silver, cyanide of silver, acetate of silver, sulphide of silver, 
 and oxide of silver. A salt of a metal is a chemical combination 
 formed by the action of an acid on the metal. Thus, sulphide of 
 silver is a combination of sulphur and silver, and nitrate of silver is 
 formed by the chemical action of nitric acid upon silver. Nitric acid 
 is a chemical combination of hydrogen with oxygen and nitrogen, 
 the oxygen and nitrogen in this case forming what is called an acid 
 radical. The radical of nitric acid has a greater chemical attraction 
 or affinity for silver than for hydrogen. Consequently when silver is 
 immersed in nitric acid the silver is attacked and dissolved, during 
 which process it combines with the acid radical and forms nitrate of 
 silver, while the hydrogen of the acid is given off. The salts of silver 
 which are used in electroplating are usually made from the nitrate. 
 The nitrate of silver is produced by adding pure silver in small quan- 
 tities at a time, to a warm mixture of one measure of distilled water 
 to four measures of the strongest pure nitric acid. The action of the 
 acid upon the silver is very intense and causes much heat to be given 
 off (compare the action of sulphuric acid upon zinc, Lesson 3, page 15) 
 and if the mixture be too hot or too much silver be added, the liquid 
 
 92 
 
may boil over. In this case the mixture may be cooled by adding a 
 little cold distilled water. When the mixture will dissolve no more 
 silver the solution may be put in a covered jar and set in a dark place 
 until it is required for use. 
 
 For use with a silver voltameter a properly diluted solution of 
 nitrate of silver is used (Lesson 9, page 64), but the deposit from a 
 nitrate solution does not make a satisfactory plating. The best silver 
 plating solution is one containing cyanide of silver. Cyanide of silver 
 is the salt formed by the combination of silver with prussic acid. 
 A solution of cyanide of silver is formed by slowly adding to 
 the silver nitrate solution made substantially as already described 
 a weak solution of cyanide of potash or white prussiate of 
 potash. The cyanide of potash used should be dissolved in about ten 
 times its own weight of distilled water. The addition of the potash 
 solution to the nitrate of silver solution should be continued as long 
 as a white precipitate forms, but no longer, or some of the silver is 
 lost. The precipitate which forms is cyanide of silver. This should 
 be allowed to settle, after which the clear liquid may be carefully 
 poured or drawn off. The precipitate is then washed a number of 
 times by pouring distilled water over it and stirring, allowing the 
 precipitate to settle and pouring off the liquid. Cyanide of silver 
 does not dissolve in water but readily dissolves in a solution of cyanide 
 of potash in water, aud silver plating solutions are usually made by 
 so dissolving the silver cyanide. Cyanide solutions are extremely 
 poisonous and therefore must be handled carefully, and on account of 
 the value of the silver which they contain must be handled without 
 waste. 
 
 The vats in which silver plating operations are carried out are 
 usually made of wood, though they are sometimes made of sheet-iron 
 lined with wood. They are of various dimensions, but generally are 
 from two to three feet wide, five to six feet long, and about thirty 
 inches deep. When the solution is made up and put in the vat for 
 service it usually does not require changing for a number of years. 
 It sometimes requires filtering, and the addition of water to supply 
 that lost by evaporation, or the addition of cyanide salts to supply 
 losses which have come about by electrolysis. The exact proportions 
 of the solutions used for silver plating in different factories vary con- 
 siderably, but they are nearly always substantially as already described. 
 The general arrangement of a plating vat is shown in Fig. 67, 
 where the flat plates inside the vat are sheets of silver which are con- 
 nected to the positive pole of the source of current, and form the 
 anodes of the electrolytic cell (Lesson 9, page 64), whilethe spoons, 
 forks, and the other articles to be plated form the cathode. The 
 supports for the anodes and cathodes are usually made of brass or 
 copper tubes laid across the top of the vat. The articles to be plated 
 are ordinarily supported on looped pieces of insulated copper wire 
 
 93 
 
FIG. 67. 
 
 FIG. 68. 
 
 (Fig. 68). The insulation of these supports where they are immersed 
 in the liquid is important in order to avoid an unnecessary and ex- 
 pensive deposit of silver upon them. The silver deposit made on the 
 cathodes occurs as a result of electrolysis, and an equal amount of 
 silver goes into the liquid from the anode when all is working well 
 (compare Lesson 9, page 64). 
 
 The quality of the deposit which is made in electroplating is of 
 the first importance. The three points to be looked after most care- 
 fully are the strength of the current as compared with the magni- 
 tude of the surface to be plated, the composition, density, and tem- 
 perature of the plating solution, and the condition of the articles to 
 be plated when put into the solution. The current for plating was 
 formerly furnished by batteries but it is now ordinarily furnished 
 from small dynamos which produce a low pressure properly 
 adapted for its purpose. The pressure may also be adjusted to a con- 
 siderable extent by means of a resistance box connected in circuit 
 
with the magnetizing coils oi the dynamo. The current and pres- 
 sure supplied by the dynamo may be measured by means of an 
 amperemeter and a voltmeter. One dynamo of sufficient size maybe 
 used to supply current to several plating vats. The vats may be 
 connected either in series or in parallel, depending upon the pressure 
 developed by the dynamo. When the current is of the proper amount 
 the covering which is deposited upon the plated articles is hard, white, 
 adheres closely, and is deposited with reasonable rapidity. When 
 the current is too small the deposit usually is of good quality but the 
 plating progresses too slowly. When the current is too great the 
 plating is likely to become gray or black and rough, while gas is 
 sometimes given off at the cathode. A discoloration of the silver 
 deposit may also occur from impurities in the liquid. Such discolor- 
 ation may often be removed by proper after treatment of the plated 
 articles, but to this attention cannot be given here. 
 
 The form of the articles to be plated often has much to do with 
 r.ie quality of the plating. Thus bulky articles with a given sur- 
 face often do not plate as rapidly as thinner articles with exactly the 
 same amount of surface to be covered. Edges and points often 
 gather a granular or rough deposit while the flat parts of the same 
 articles take a satisfactory, hard deposit. Such difficulties can be 
 overcome only by making a proper mutual adjustment of the dis- 
 tances between anodes and cathodes, the quality of the liquid, and 
 the current per unit surface of the articles. When articles which have 
 great irregularities of surface are to be plated, the distance between 
 anodes and cathodes must be greater than that which is satisfactory 
 when the articles have a uniform surface, otherwise the more promi- 
 nent points of the articles will receive a heavy deposit while the 
 hollows may receive little or no deposit. It is important that all 
 plated articles be given a uniform deposit of proper thickness upon 
 the surfaces which it is desired to cover. The thickness of silver plat- 
 ing ordinarily varies from the thinnest possible coating to the thickness 
 of thin writing paper, depending upon the quality of the product. 
 
 There is a method of plating by simply dipping the articles 
 in a proper silver solution which is used to silver small articles, 
 such as hooks and eyes, on which the coating is too thin to be 
 really measured. In this case the plating is not due to electro- 
 lytic action, but simply to chemical action between the silver solu- 
 tion and the metal composing the articles to be covered. This is 
 called plating by simple immersion. 
 
 In preparing articles for silver plating, the greatest care must be 
 taken to make them absolutely clean and bright, or the plating will 
 not take a permanent hold, but will peel off. It is first necessary to 
 prepare the articles for the kind of coating they are intended to 
 receive; if the plating is intended to be polished, the articles must 
 be polished, all deep scratches must be removed, etc. This may be 
 
 ^<e* 
 f * 
 
 iXTNI' 
 
 K^ 
 
done by filing, scouring, polishing, etc. After this preparation the 
 cleaning is begun by dipping in a warm solution of caustic potash 
 or soda which cleans off all grease. This solution is made by dis- 
 olving commercial lye in water, and it may be used continuously 
 until its caustic properties are used up. After dipping in lye the 
 articles are washed in. water and are then sometimes dipped in dilute 
 acid to give them a proper surface. They are next washed with 
 great care and then placed in the depositing vat. It is quite common 
 to cover articles to be silver plated with a very thin coating of mer- 
 cury. The object of this is to avoid oxidation of the articles which 
 causes the plating to peel. Coating with mercury is called quicking 
 and it may be effected by dipping the articles into a dilute solution 
 <*f nitrate of mercury, or the solution of some other mercury salt. 
 
 FIG. 69. 
 
 During the operations of dipping, the articles should be sup- 
 ported upon wires or in wire baskets. They should not be touched 
 with the fingers since the points so touched are made greasy and the 
 deposit will not take. 
 
 After the plating is completed in the bath the articles must be 
 put through a series of operations to give the plated surface the 
 proper finish. This is largely done by polishing on rapidly revolv- 
 ing" wheels made of brass wires, leather, and canvas. The processes 
 are called scratching, buffing, and polishing. The same tools are 
 used for polishing the articles before plating. In the case of some 
 articles the polishing is done by means of hand burnishers, which are 
 smooth tools made of steel, agate, or similar hard materials. Some 
 rorms of burnishers are shown in Fig. 69. 
 
 Plating with gold is carried on in very much the same way as 
 plating with silver. The commonest solution is of cyanide of gold 
 made up in a manner quite similar to that used in making up the 
 cyanide of silver solution. The solution is generally used when hot 
 and great care to have all details exactly right is necessary to get a 
 deposit of satisfactory color. It is particularly important that all the 
 materials used in making the solution shall be pure. Gilding the 
 inside of silver cups, sugar bowls, and cream pitchers is commonly 
 done by filling the article to be gilded with the hot solution, hang- 
 
 96 
 
FIG. 70. 
 
 FIG. 71. 
 
 FIG. 73. 
 
 
 FIG. 72. 
 
 FIG. 74. 
 
 ing a gold anode in the shape of a cylinder in the center of the solu- 
 tion, and finally connecting up a battery so that the article to be 
 gilded is the cathode (Fig. 70). The extreme cleaning of articles for 
 gold plating is usually not as important as in silver plating, since 
 the hot solution helps in the cleaning. 
 
 Nickel plating is probably the most generally used of all the 
 different styles of plating. The base upon which nickel is plated is 
 usually brass, copper, iron, or steel. The soft white metals which 
 are often silver plated are seldom nickel plated. The hardness of 
 nickel and its durable polish, give to nickel plating great advantages 
 for use in the finish of sanitary appliances, car fittings and decora- 
 tions, small nuts, bolts, screws, chains, etc., used in small machin- 
 ery, bicycles, stove fronts, metal lamps, and many similar appliances. 
 The solution which is used in nickel plating is made from a com- 
 bined sulphate of nickel and sulphate of ammonia. In a dry state, 
 this is ordinarily known as nickel salts or the double sulphate of nickel 
 and ammonia. This double salt may be purchased in the market. In 
 order to make a nickeling solution the pure salt, which comes in green 
 crystals, is obtained, and is dissolved in hot water at the rate of 
 about three quarters of a pound to a gallon of water. The vat used 
 to hold a nickeling solution is usually of wood lined with lead. The 
 joints in the lead lining are burned together, not soldered. In the 
 preparation of articles for nickel plating, they must be very carefully 
 polished and cleaned by scouring, dipping in a hot lye solution, and 
 pickling in acid. The fatter is very important since the acid takes 
 off from the artic^s the thin covering of oxide which is likely to 
 stick to iron, copper and brass and which the nickel solution has 
 
 97 
 
no tendency to remove. If the oxide covering is not removed the 
 nickel plate will come off, or strip as it is called, while the articles 
 are being finally burnished. The final proceSvSes of nickel plating 
 are polishing and burnishing. Articles to be plated with nickel are 
 hung in the liquid or bath very much as already described under sil- 
 ver plating. When a number of small articles are to be nickel plated 
 they are often suspended in a string from the same wire. Fig. 71 
 shows the manner of suspending screws in the nickel bath, and Figs. 
 72 and 73 show the manner of suspending bicycle spokes and chains. 
 Fig. 74 shows the form of a vat generally used in nickel plating. 
 Nickel vats are generally larger than those used in silver plating. 
 It is particularly important that no organic (non-metallic) impurities 
 be allowed in a nickel bath, as these ruin the quality of a nickel 
 deposit. 
 
 Electrotyping is a process of reproducing type and wood cuts, by 
 means of an electroplating of copper, which is used in nearly all 
 large printing establishments. In electrotyping, an impression or 
 mould is made of the type which is set up as for printing. This 
 mould is usually made in wax or soft paper pulp b^pressing it hard 
 upon the type. After the surface of the mould is properly trimmed 
 up, it is coated with fine plumbago or some similar conductor which 
 is carefully brushed over it, so that an electrolytic shell of copper 
 may be deposited upon it. The plumbagoing, as it is called, is 
 necessary because the mould itself is a non-conductor, and the 
 current which is necessary to make an electrolytic coating cannot be 
 sent through it. Plumbago is powdered graphite, and is a fairly 
 good conductor (L,esson i, page 4), so that a thin coating brushed 
 over the surface of the mould enables it to conduct the current. 
 Thus prepared, the mould is hung in an electrolytic bath consisting 
 of a solution of sulphate of copper (blue vitriol) to which a small per- 
 centage of sulphuric acid is added. The anode of the electrolytic 
 cell is a plate of copper, and the cathode is the mould. TLo thick- 
 ness of the shell of copper which is deposited on the mould varies 
 from that of the sheet of paper upon which this lesson-is printed to 
 several times that thickness, depending upon how much printing is 
 to be done from the electrotyped plates. When the copper deposit is 
 of proper thickness the mould is removed from the bath, and the 
 copper shell is separated from the mould. The shell is then trimmed 
 and finally "backed up" by a filling of type metal wnich is melted 
 and poured upon the back of the shell. Electrotyped plates have a 
 great advantage over type in having a permanent form and in wear- 
 ing much better. As soon as the mould for electrotyping is taken off 
 from set up type, the type may be distributed and u^ed again. 
 
 Copper plating is also used sometimes to give a bronze finish to 
 iron lamp posts, gas fixtures, etc., and it is used to make a foundation 
 coating upon iron articles which it is desired to sliver plate. 
 
 98 
 
Plating with other metals than those referred to above, such as 
 iron, tin, and zinc, is sometimes carried out for special purposes. For 
 this purpose, special solutions and peculiar care in handling the articles 
 must be used. It is even possible to electrolytically deposit brass, 
 german silver, or other alloys. The latter requires extreme care, 
 however, in the management of the solutions and the regulation 
 of the electric pressure and current supplied to the vats. 
 
 A very useful application of electrometallurgy is the refining of 
 copper. In this operation the crude copper which comes from ordinary 
 smelting works with from two to five per cent of impurities, is re- 
 fined by electrolysis so that only very minute amounts of impurities 
 remain. We have already seen (Lesson 7, page 45) the effect of im- 
 purities in reducing the conductivity of copper and other metals. 
 When the electrolytic method of refining copper is properly carried 
 out, it leaves such a small amount of impurities that the electrolytic 
 copper has almost as great conductivity as pure copper. Copper 
 wires to be used in electric lighting and in the manufacture of elec- 
 tric machines are therefore generally made of electrolytic copper. It 
 is usual for sucr wires to have more than 96 per cent of the conduct- 
 ivity of pure copper. The small amount of impurities which do 
 remain in electrolytically refined copper is largely composed of silver 
 and iron. In electrolytic refining the crude copper is cast into heavy 
 plates which are used as anodes in depositing vats, the solution in 
 which is copper sulphate with a little sulphuric acid. The cathodes 
 at first are sheets of pure copper, but they grow by deposition into 
 thick plates of copper which may be worked into bars and drawn 
 into wires as desired. The action in the depositing vats is quite 
 similar to that which goes on in a copper voltameter. In copper re- 
 fining works enormous dynamos are used, and a great number of 
 tanks or vats each containing a number of anodes and cathodes ar- 
 ranged alternately and connected in parallel are provided. The vats are 
 ordinarrly connected in series; or sets of a number of vats connected in 
 series are connected in parallel. The pressure required to pass the cur- 
 rent through each vat is quite small and consequently a number of vats 
 may be connected in series without causing the total pressure to ex- 
 ceed 100 volts. It is desirable that the pressure required at each vat 
 be as little as possible in order to avoid the deposition of impurities 
 on the cathodes and also to save power. The power used in each 
 vat is equal to the difference of pressure between the anodes and 
 cathodes multiplied by the current flowing through the vat. It is 
 desirable to have as great a current flow as will give a fairly smooth 
 deposit, in order that the time required in depositing each pound of 
 copper may be as small as possible. Any reduction made in the cur- 
 rent without changing the pressure simply reduces in a proportional 
 rate the amount of copper deposited so that the power required to 
 deposit a pound of copper is not materially changed. If the pressure 
 
 QQ 
 
FIG. 75. 
 
 required to pass the current through the vats be reduced without 
 changing the current it at once reduces the power required to de- 
 posit a pound of copper and a saving in the cost of manufacture is 
 effected. In order to reduce the pressure, the anodes and cathodes 
 are set as closely together as possible without interfering with the 
 circulation of the solution. 
 
 During the process of refining copper by this means, the impuri- 
 ties of the crude copper are mostly dissolved in the solution or are 
 thrown to the bottom of the vats as mud or sludge. Copper that is 
 to be electrolytically refined, usually contains some silver and a little 
 gold. During the refining process these form salts which form part 
 of the mud and the precious metals are recovered by the ordinary 
 method of smelting when the mud is removed from the vats from 
 time to time. Iron and lead are also contained in the crude copper, 
 as are small quantities of other metals. The lead goes to the 
 bottom of the tank like the gold and silver, and the iron dissolves in 
 the solution but is not deposited on the cathode except in very small 
 amounts unless the pressure at the vat is too high. Electrolytic 
 refining of crude copper from ores which contain silver is particularly 
 useful because it is the cheapest method of separating the silver from 
 the copper. Great electrolytic refineries have, therefore, been erected 
 at the Montana copper mines, the ores of which usually contain a 
 valuable portion of silver. 
 
 Electrolysis has been applied to the refining of other metals but 
 without great success. It has also been used in the recovery of pre- 
 cious metals from their ores, but without success, although it may yet 
 prove its value in working certain kinds of ores. 
 
 The electric arc, such as is seen in arc lamps but much larger, 
 has been successfully used in working ores, especially in producing 
 the metal aluminum. This application is usually called electric 
 
 100 
 
smelting. The action which occurs in electric smelting is partially 
 due to chemical action set up by the intense heat cf the electric arc 
 which melts the ores, and partially due to the electrolysis caused by 
 the current passing through the ores in their melted state. It is by 
 electric smelting from corundum and similar material that the Cowles 
 Company of Lockport, N. Y.,make the aluminum which is found in 
 their well-known aluminum bronze. It is also by the electric smelt- 
 ing process that the Pittsburg Reduction Company make aluminum. 
 Fig. 75 shows a set of electric furnaces which are used in England 
 for the production of aluminum bronze. The current for these is 
 furnished by a dynamo of several hundred horse-power. 
 
 Copyrighted, 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XIII. 
 
 Points for Review. 1. What is electroplating? 
 
 2. What metals are most commonly used in plating? 
 
 3. What is a salt of a metal? 
 
 4. What is nitrate of silver? What is cyanide of silver? What is cyanide of 
 potassium? 
 
 5. How may nitrate of silver be made? 
 
 6. Why is a solution of nitrate of silver not used for silver plating? What is the 
 best solution to use for silv.er plating? 
 
 '7. How may cyanide of silver be made? 
 
 8. What kinds of vats are usually used in silver plating? 
 
 9. Why must the anodes in a plating vat be of the same metal as the plate which 
 is deposited? 
 
 10. Upon what does the quality of the electrolytic deposit of a metal depend? 
 
 11. What is the effect on a silver deposit when the current is too great and when 
 it is too small? 
 
 12. What is the object of cleaning articles before placing them in the plating bath? 
 How is the cleaning done?. 
 
 13. How are the insides of silver articles gilded? 
 
 14. Upon what base metals is nickel usually plated? 
 
 15. Why is nickel used instead of silver for finishing articles which will receive 
 severe usage? 
 
 16. What solution is used for nickel plating? 
 
 17. What effect on nickel plating is caused by the presence of organic impurities in 
 the bath? 
 
 18. For what purpose is electrotyping used? 
 
 19. Why are the ordinary electrotype molds plumbagoed? 
 
 20. How is an electrotype finished after the copper shell has been deposited? 
 
 21. Upon what does the thickness of the copper shell of an electrotype depend? 
 
 22. What are the advantages of electrolytic refining of copper? 
 
 23. What solution is used in electrolytic copper refining? In what form does the 
 copper come to the electrolytic refining? 
 
 24. Why is electrolytic copper preferred for wire which is used in the electrical 
 industries? 
 
 XIV. 
 
 THE ELECTRIC TELEGRAPH. 
 
 The electric telegraph, as it is commonly used, is to a large 
 degree a growth from the discoveries of Prof. Joseph Henry of 
 Princeton College. Henry's discoveries were directly applied to 
 telegraphy by S. F. B. Morse, an American artist and inventor, 
 whose fame rests principally upon his telegraphic inventions. 
 Before the inventions of Morse, many means of communicating 
 over considerable distances had been tried. In some of these 
 electricity played a part, but the essential feature of the tele- 
 graphs of the present day, the electromagnet, was not applied 
 
FIG. 76. 
 
 POSITION AND MOVEMENT. 
 FIG. 77. 
 
 with success until Morse's inventions. Indeed, Morse's inventions 
 came very soon after the discovery of the essential property of the 
 electromagnet, that of losing its magnetism upon the interruption of 
 the magnetizing current. (Lesson 6, page 39). The Morse system of 
 telegraphy has four elements connected in series: ist, a battery or 
 other source of an electric current; 2nd, a key by means of which the 
 electric circuit may be made and broken to produce signals; 3rd, a 
 line of wire running from the point at which the signals are pro- 
 duced to the point where the signals are to be received; 4th, an 
 electromagnetic sounder or register by means of which the signals 
 may be distinguished or recorded. 
 
 The battery ordinarily used in telegraphy is the common gravity 
 form described in lesson 4, but galvanic batteries have been widely 
 replaced in telegraph service by small dynamos during the past few 
 years. 
 
 The ordinary form of telegraph key is shown in Fig. 76. This 
 consists of a lever, which is pivoted so that it may be moved through 
 a small vertical range by pressing the fingers upon the button at its 
 end (the left hand of the figure). The spring shown at the center of 
 the figure tends to keep the lever at the upper end of its stroke, so 
 that an operator, in making signals with the key, need only depress 
 the lever and it will return to its normal position upon removing the 
 pressure. The operator's fingers are therefore placed on the button 
 in the way shown in Fig. 77. The left-hand leg of the key is con- 
 nected to a contact point which is seen directly above the leg and 
 
 103 
 
FIG. 78. 
 
 which is insulated from the frame. The lever carries a correspond- 
 ing contact point directly above the insulated one. The upper 
 contact point is in electrical contact with the right hand leg 
 through the metal of the lever and frame. When the key is con- 
 nected into a circuit by cutting the circuit wire and attaching the 
 two ends to the two legs of the key, the circuit may be made and 
 broken at the will of the operator by depressing or raising the lever. 
 Fig. 78 shows a key without legs but with binding posts for the con- 
 nection of the wires. As ordinarily arranged, a telegraph circuit is 
 broken only at the time of making signals, consequently a switch is 
 placed on the key so that the circuit can be closed when the key is 
 not in use. The handle of the switch is sLown in the figure just to 
 the left of the contacts. With this arrangement of the circuit it 
 is possible to place a number of stations in series on one line (Fig. 
 79) and since the circuit is normally complete that is, it is always 
 complete when not in use the operator at any station may signal 
 any other at any time, provided no other operator is using the line. 
 
 The current used in telegraphy is quite small it does not often 
 exceed fifty milliamperes and therefore it is possible to satisfactorily 
 use the earth for one side of the circuit. A telegraph line therefore 
 ordinarily consists of a wire supported on wooden poles and running 
 
 FIG. 79. 
 
 from station to station. At its ends the wire is connected to the earth 
 by means of ground plates, as shown in Fig. 79. The wire used is 
 generally number 6 or 8 iron wire gauge wire made of the best gal- 
 vanized iron, but for some short lines steel wire is used and for some 
 of the most important lines between large cities, copper wire is used. 
 Wires as large as number 4 and as small as number 10 are sometimes 
 used. The choice of the size and kind of wire depends largely upon the 
 length and importance of the line, upon which depends the amount 
 
FIG. 80. 
 
 oi battery power which is necessary to operate the signals and the 
 care with which the line is kept in good condition. 
 
 In the earlier days of the Morse telegraph it was thought neces- 
 sary to receive the signals constituting a telegraphic message in a 
 permanent form by means of a recording register. Such a register 
 is shown in Fig. 80. This consists of a case containing a horse shoe 
 electromagnet, the windings of which are connected in series with 
 the telegraph circuit. Over the poles of the magnet is an armature 
 of soft iron which is held against a stop by the pull of the magnet 
 when the current flows through the circuit. When the current is 
 interrupted by means of a key, as in sending signals, the electro- 
 magnet loses its magnetism and the armature is no longer attracted, 
 so that a small spring which is attached to it, is able to pull it back 
 from the stop. Thus, as current impulses are sent along, the line by 
 making and breaking the circuit at a key, the pulls of the magnet 
 and of the spring alternately draw the armature forwards and back- 
 wards. The movement of the armature is recorded or registered by 
 means of a pen or a blunt point on a narrow strip of paper which is 
 automatically fed from the roll shown in the figure. This paper 
 tracing of the signals may be read by the receiving operator and 
 translated into ordinary language upon a telegraph blank, and then 
 delivered to the person for whom the message is intended. 
 
 Telegraphic signals are made up of a combination of long and 
 short current impulses, which are made by pressing the sending key 
 at proper intervals and for proper periods, and which are recorded on 
 a register as long and short dashes. Each combination of dashes 
 represents a letter of the alphabet or a certain much used word or 
 
 105 
 
phrase. The Morse Alphabet, as it is called, which is used in this 
 country, is given below. (Fig. 81). 
 
 Morse Alphabet. 
 
 A B C D E P G 
 
 HI J K L M N 
 
 OP Q R S T U 
 
 V W X Y Z & 
 
 , NUMERALS. 
 1 2 3 4 5 
 
 6789 
 
 PUNCTUATION MARKS 
 Period. Comma. Semi-colon Colon 
 
 Quotation mark. Parenthesis. Interrogation, 
 
 Paragraph. Exclamation. Dollar mark 
 
 As the Morse telegraph came into considerable use, the opera- 
 tors found that they could read the signals passing over the line by 
 listening to the clicks of the register armature as it moved back and 
 forth between its stops under the influence of the current impulses. 
 The paper roll was therefore. abandoned, as " reading by sound " was 
 quicker and more convenient than translating the message from the 
 paper tracing of the signals. To make reading by sound as easy as 
 possible the working mechanism of the register was altered into that 
 of the sounder (Fig. 82). The figure plainly shows the arrangement 
 ot the sounder. The armature has attached to it a substantial brass 
 bar. This bar is pivoted at its right hand end, as shown in the 
 figure, so that its left hand end may move up and down between ad- 
 justable stops, as shown. To the right hand end of the bar is 
 attached a spring which draws the bar against the upper stop when 
 no current is flowing in the magnet. The black cylinders shown 
 about the center of the figure, compose the magnet. This magnet 
 consists of two cores of iron about three-eighths of an inch in diame- 
 ter and one inch and a half long, wound with wire and covered with 
 black paper or a short piece of hard rubber tube. The cores are 
 screwed fast to an iron base so as to make a horse-shoe electromagnet. 
 The armature is shown at the top of the figure, above the magnet. 
 When current flows in the magnet winding, the armature is attracted 
 
 toe 
 
FIG. 83. 
 
 and the bar drawn against the lower stop. As the bar moves back 
 and forth it makes a sharp click when it strikes one of the stops. 
 The strength of the spring is adjustable by means of a screw so that 
 the sounder may be adjusted for use within a certain range of cur- 
 rents of different strength. To successfully read signals from a 
 sounder much experience is necessary, but operators become very 
 expert by long practice. It is necessary in reading to distinguish 
 between the clicks of the armature against the top and bottom stops. 
 A little consideration will show that the length of time between the 
 clicks when the armature strikes the bottom stop and when it strikes 
 the top stop distinguishes between dots and dashes, since the dots 
 and dashes represent intervals during which current is flowing 
 through the magnet. The interval of time between the top click and 
 the bottom click represents the spacing between letters and words, 
 because the spacing represents intervals during which no current 
 flows, or during which the signal key is open. 
 
 Telegraph sounders require only a fraction of an ampere to oper- 
 ate them, but to cause that fraction to flow through a long line 
 
 107 
 
KEY. 
 
 LOCAL BATTERY 
 
 FIG. 84. 
 
 which necessarily has a high resistance, requires the use of a battery 
 of a very large number of cells. This is undesirable because the 
 cells are expensive to buy and to keep up. Long telegraph lines are 
 therefore furnished with instruments which operate like sounders, 
 but which are made very sensitive by placing a great many turns of 
 fine wire on their magnets, so that they may even be satisfactorily 
 operated on eight or ten milliamperes. These instruments are called 
 relays (Fig. 83). Reading signals directly from a relay is not usually 
 attempted as the motion of its armature is so delicate that it makes 
 very little noise, but the armature and one of its stops are arranged 
 as part of a local circuit which contains a sounder and a couple of 
 gravity cells. (Fig. 84.) As the relay armature moves back and 
 forth it makes and breaks the local circuit and reproduces in it the 
 signals which pass over the main line. The sounder in the local 
 circuit gives the signals exactly as they pass over the line. Fig. 85 
 shows a set consisting of key, relay, and sounder on a common base. 
 To still further economize in long and important lines arrange- 
 ments are made to send more than one message at a time over each 
 wire. When a telegraph wire is arranged so that two messages may 
 be transmitted over the wire at once, one being sent from each end, 
 the wire is said to be duplexed. When it is so arranged that both 
 
 308 
 
taei 
 
 FIG. 85. 
 
 messages may be sent from one end, the wire is usually said to be 
 diplexed. Diplexed wires are not ordinarily used, except in com- 
 bination. When a wire is arranged so that four messages may be 
 transmitted over it at once, two being sent from each end, it is said 
 to be quadruplexed. In arranging a quadruplex, a combination is 
 practically made of a duplex and a diplex arrangement. These ar- 
 rangements for multiple telegraphy, as it is called, will be explained 
 in the next lesson. 
 
 Copyrighted, 1894, 
 
 109 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XIV. 
 
 Points for Review : 1. What are the eosential elements of an electric telegraph? 
 
 2. What is the normal condition of a telegraph circuit? 
 
 3. How are telegraphic signals made? How are they received? 
 
 4. What kind of wire is used for telegraph lines? 
 
 5. How are letters and words made out of telegraphic signals? 
 
 6. What is a sounder? What is a relay? 
 
 7. Why are relays used in telegraphing? 
 
 8. What is a "local circuit?" 
 
 9. What is the object of multiple telegraphy? 
 
 10. What is duplex telegraphy? What is diplex telegraphy? What is quadruplex 
 telegraphy? 
 
 LESSON xv. 
 
 MULTIPLE TELEGRAPHY. 
 
 The commonest arrangement for duplex telegraphy requires a 
 special relay which is connected as shown in Fig. 86. It is seen that 
 three points of connection are made to the wire which is wound on 
 the electromagnet of the relay, one point at each end of the wire and 
 a third at the middle of the wire. The wire is wound in the usual 
 way with one-half on each leg of the electromagnet. If a current be 
 passed through the wire from one end to the other, the relay will 
 act exactly as a common relay. If a current be sent into the relay 
 from the middle point, the current will divide, and the two parts will 
 pass through the windings on the two legs of the electromagnet in 
 such a way that their magnetic effects are in opposite directions. If 
 the two divisions of the current are made equal, their opposite 
 magnetic effects are equal and exactly neutralize each other, so that 
 the core and amature of the relay are not affected. Such a relay is 
 called a differential relay. The exact arrangement of the windings 
 on a differential relay may vary considerably, but the purpose and 
 effect of all arrangements are exactly the same as described. Fig. 87 
 is a diagram of the connections made at a telegraph station for 
 duplex telegraphy using a differential relay. This arrangement is 
 often called the differential duplex. The figure shows by the arrow- 
 heads that the current sent into the line at the station divides in the 
 relay belonging to that station and half of it passes to the ground 
 through the resistance R and back to the battery. The other half 
 of the current goes through the line and the relay ot the station for 
 which the signals are intended, and returns to the battery by way of 
 the earth. The two halves of the current pass through the coils of 
 the home relay in opposite directions and neutralize each other's 
 magnetic effects. A message sent from the home key therefore does 
 
 no 
 
B, main battery. K, Key. M, differential relay. S, sounder. 
 
 L 1 and L 2 , local batteries. N, D, transmitter. 
 
 X 8 and X 3 , special resistances. 
 
 not affect the home relay. That part of the current which goes into 
 the line passes through the winding of the distant relay in the usual 
 manner so as to make a signal. In this way the operator at any 
 station on a line can signal another without affecting his home relay. 
 Two messages can therefore be transmitted at the same time in 
 opposite directions between two stations without interference. This, 
 of course, requires two operators^ one to send and one to receive the 
 message at each station. The figure shows a special device called 
 a transmitter, for making the signals. This is explained on page 114. 
 In order that a differential duplex may work, it is absolutely 
 necessary that the current in the relay divide quite accurately into 
 halves. This is effected by properly adjusting the resistance of the 
 rheostat R, in the home branch of the circuit. The branch of the 
 circuit at each -station containing the resistance R, is called the arti- 
 ficial line, since it is made to represent as far as possible the condition 
 of the actual line in order to make the duplex work satisfactorily. 
 The resistance of the line is easily balanced by making the resistance 
 R so that it may be adjusted by plugs to suit the condition of the 
 line (Fig. 88.) Certain smaller resistances (X 2 and X 3 in the figure), 
 are also used in the home circuits to smooth the action of the trans- 
 mitter in making and breaking the circuit. The electrostatic capa- 
 city of the line affects the rise and fall of the current in it as the sig- 
 nals are transmitted, and in order to get the best results with the 
 differential relay the artificial line is arranged with a condenser, C, 
 connected in parallel with the resistance R, the capacity of which 
 balances that of the line. This condenser is usually made of sheets 
 of tin foil insulated by sheets of thin mica or paraffined linen paper, 
 and the capacity connected into circuit may be adjusted by means of 
 plugs (Fig. 89). When a telegraph circuit is arranged for duplex 
 working it is necessary to have a line battery at each of the stations 
 
 111 
 
n 
 
 FIG. 91 
 
 FIG. 
 
 For simple working all the battery may be placed, if desired, at a sin 
 gle point along the line. 
 
 When a line is arranged for diplex working, two keys are placed 
 at the sending station and two relays are placed at the receiving 
 station. One of these relays called the polarized relay, has a perma- 
 nently magnetized steel armature. When the armature, which lies 
 across the poles of a horseshoe electromagnet, is permanently mag- 
 netized, or polarized, it is attracted when the current flows in one 
 direction and repelled when the current flows in the opposite direc- 
 tion. Advantage may best be taken of this by placing the polarized 
 armature between the poles of the electromagnet, as in Figs. 90 and 
 91. The end of the armature will then stick against either pole 
 indifferently when no current flows, if it is not restrained by a spring. 
 When a current flows in one direction the end of the armature will 
 move up to one pole, and when the current is reversed the armature 
 will move over to the other pole, as shown in the figures. A polar- 
 ized relay made upon this principle may be operated by signals 
 which are given by reversing the current in the circuit instead of 
 making and breaking the circuit as in simple telegraphy. Fig. 92 
 shows a common form of polarized relay in which the armature is 
 kept polarized by means of a strong permanent magnet to which it is 
 attached. A key for sending signals by reversing the current is 
 called a pole changer. 
 
 112 
 
FIG. 94. 
 
 113 
 
GROUND CS3 GROUND 
 
 FIG. 95. 
 
 B 1 , small battery. B 2 , large battery, K 1 , pole changer and key. K 2 , transmitter 
 and key. R 1 , polarized relay. R a , neutral relay. 
 
 It is possible to send signals to a common or neutral relay over 
 the same line as that used with the polarized relay without interfer- 
 ing with the action of the latter. In order that the currents which 
 actuate the polarized relay shall not also work the neutral relay, the 
 latter is adjusted to respond only to a current which is greater than 
 that required to actuate the polarized relay. Hence the operation of 
 the diplex arrangement depends upon the use of currents of two 
 strengths. One of these currents is quite weak and is reversed in 
 sending signals, so that a polarized relay is used with it; the other 
 current is stronger and is increased and decreased by the sending key 
 in sending signals to the neutral relay, instead of making and break- 
 ing the circuit, since doing the latter would interfere with the signals 
 sent to the polarized relay. A key arranged to increase and decrease 
 the current in sending signals is usually called a transmitter. The 
 increase and decrease of the current is gained by alternately connect- 
 ing into circuit a large and a small battery (Fig. 95). In order that 
 common telegraph keys may be used by the operators in sending 
 messages by the diplex arrangement it is usual to work the pole 
 changer and transmitter by means of electromagnets like sounders, 
 connected individually in local circuits in series with the sending 
 keys. Figs. 93 and 94 show common forms of pole changer and 
 transmitter. Fig. 95 is a diagram of the connections at the sending 
 and receiving stations upon a diplex telegraph line. 
 
 The commonly used quadruplex is essentially a combination of 
 the differential duplex and the diplex which have just been explained. 
 A diagram of the circuits at a quadruplex station is shown in Fig. 
 96. It is to be seen in this that the polarised and neutral relays of 
 the diplex arrangemetU are used, but they are wound in differential 
 fashion. This makes it possible to send two messages from a sta- 
 tion without interference with each other (diplex), and at the same 
 time without interfering with the receiving of two messages by the 
 differential instruments at the same station. The key arrangements for 
 the quadruplex are the same as those of the diplex, so that a pole chang- 
 er and transmitter are used, though for simplicity they are not shown 
 
 114 
 
1< Jl* 
 
 FiG. 96. 
 
 D 1 , D*, dynamos. K 1 , pole changer and key. K 2 , transmitter and key. 
 
 R 1 , polarized differential relay, R 2 , neutral differential relay. 
 
 X and x, adjustable resistances. C, condenser. 
 
 in the figure. The figure shows the use of dynamos in the place of 
 batteries. The coils marked 600 and 900 are resistances which take 
 the place of the resistances X 2 and X 3 of Fig. 87. The coil marked 
 1 200 is used to reduce the current in making signals for the neutral 
 relay and fills the purpose of the division of the battery into a large 
 and a small section (page 114). The figures represent the resistances of 
 the coils in ohms. The signals for the neutral relay are made by 
 alternately cutting this large resistance into and out of the circuit 
 by the transmitter. The action can be understood by an examina- 
 tion of the figure. For satisfactory quadruplex working the artifi- 
 cial line must be kept well adjusted, or trouble is experienced from 
 blurring the signals in the differential instruments. 
 
 Another form of duplex which depends for its operation on a 
 balance similar to that of a Whcatstone bridge, is often used on 
 ocean cables. The arrangement is shown in Fig. 97. In this the 
 relay is located at M. In the arms of the triangle, R A and R B 
 are fixed resistances, and P is a variable resistance. Now, according 
 to the principle of the Wheatstone bridge, if the resistance in R A 
 is to the resistance in R B as the resistance of the line is to the 
 
 115 
 
GROUND (3 3 GROlWfr 
 
 FIG. 98. 
 
 K 1 and K 2 , pole changers with keys. R 1 and R 2 , polarized differential relays. 
 
 B 1 and B 2 , batteries. 
 
 resistance P, no current will flow through the relay M from the home 
 battery. Current from a distant battery will, however, work the 
 relay, since it will come in from the line to A, where it will divide, 
 part going around through A R B in it, path to earth, but the 
 greater part passing through the lower resistance of the relay. 
 
 A duplex may also be operated by means of differential polar- 
 ized relays (Fig. 98). This is practically the same in operation as 
 the differential duplex already explaJ ied, but pole changers are used 
 to send the signals, and polarized relay: are used to receive them. 
 
 Various plans for sending more than four messages over one 
 wire have been devised, but they have been too complicated to be 
 successful in operation and can receive no attention here. Where 
 more than four messages arc sent over a wire, the arrangement is 
 ordinarily called multiplex telegraphy. 
 
 Methods have also been devised for sending messages by 
 machines instead of by hand. Such machines are used to a con- 
 siderable extent for special work, such as sending press despatches, 
 stock quotations, etc. It is usual for machine-sent messages to be 
 received by special machine recorders, which print the messages 
 either in Morse characters or directly in the English alphabet. 
 Where machines are used in telegraphy the arrangement is ordi- 
 
narily spoken of as automatic telegraphy. 
 
 Devices have also been invented by means of which a written 
 message or sketch may be transmitted exactly as it is written or 
 drawn. These are ordinarily spoken of as devices for autographic 
 telegraphy. The most successful of these arrangements is the Gray's 
 telautograph, but none of them have yet come into common use on 
 account of their complications. 
 
 Copyrighted, 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XV. 
 
 Points for Review: 1. What is a differential relay? For what purpose is it used? 
 
 2. What is the principle of the operation of the differential duplex ? 
 
 3. What is a polarized relay ? For what purpose is it used ? 
 
 4. What is a pole changer? For what purpose is it used ? 
 
 5. What is the principle of the operation of the diplex ? 
 
 6. What is the principle of the operation of the quadruplex ? 
 
 7. What is the principle of the operation of the bridge duplex ? 
 
 8. Why is an artificial line used in duplex and quadruplex telegraphy ? 
 
 9. What is autographic telegraphy ? 
 
 XVI. 
 
 THE TELEPHONE. 
 
 Unlike telegraphy, which is the oldest commercial application 
 of electricity, the telephone is one of the later commercial applica- 
 tions. The word telegraph comes from two Greek words which 
 mean, when combined, to. write at a distance, while telephone comes 
 from two Greek words which mean to speak at a distance. The first 
 telephone that can be given the credit of commercial success, was 
 invented by Alexander Graham Bell, and was privately exhibited by 
 him at the Centennial Exposition at Philadelphia, in 1876. Dr. 
 Elisha Gray applied for patents on practically the same telephone 
 mechanism at the same time, and the simultaneous invention of the 
 instrument by the two noted men gave rise to one of the famous pat- 
 ent law suits of the age. Since that time the usefulness of this device 
 has been greatly increased by other inventions which make its service 
 more perfect. The telephone in its improved form has added wonder- 
 fully to the ease and quickness with which many kinds of business 
 may be transacted, and it may be said to have almost revolutionized 
 some of the methods of doing business. 
 
 The telephone originally exhibited by Prof. Bell consisted of 
 two instruments quite similar to the ear pieces or receivers which 
 are now used. One of these instruments was used as a receiver and 
 the other was used to talk into, or as a transmitter, and the two were 
 connected by wires (Fig. 99). The construction of these instruments 
 may be best explained by reference to Fig. 100, which is a cut of a 
 late type Bell receiver. In this figure, R represents a rubber case, 
 
 118 
 
UKE 
 
 FIG, 99. 
 
 FIG. 100. 
 
 FIG. 101. 
 
 NS, a magnet tipped with a piece of soft iron, and W, a spool of 
 very fine wire slipped over the soft iron tip of the magnet and con- 
 nected to the binding posts at the end of the rubber case. D is a 
 diaphragm or disc made of thin varnished iron. This diaphragm is 
 firmly clamped all around its edges in such a position that its center is 
 very close to the end of the magnet NS. When one of the instru- 
 ments is brought close to a speaker's mouth the waves of sound 
 caused by his speech strike the diaphragm and cause it to vibrate, or 
 move back and forth. The speaking end of the instrument is formed 
 into a mouthpiece of such a shape that it gathers in a large volume of 
 the waves of sound, and concentrates their effect upon the diaphragm. 
 Most of the magnetic lines of force (Lesson 5, page 34). belonging 
 to the magnet, pass through the coil of wire and many of them enter 
 the iron diaphragm on their path to the opposite pole. As the dia- 
 phragm vibrates from the effect of a voice, it moves back and forth in 
 front of the magnet. These vibrations are too small to be seen by 
 the eye, but they are of sufficient extent to cause the number of lines 
 of force which enter the disc to increase considerably as it approaches 
 the magnet, and decrease as it moves away from the magnet. In 
 
 119 
 
this way the distribution of the lines of force around the end of the 
 magnet is altered with each movement of the disc, and the number 
 of lines of force which pass through the coil of wire on the magnet 
 is increased or decreased at the same time. It is an experimentally 
 determined fact that when a change occurs in the number of lines of 
 force passing through a coil, an electric pressure is set up in the coil. 
 This pressure is in one direction when the number of lines of force 
 passing through a coil is increased, and in the opposite direction 
 when the number is decreased. Consequently the movements of the 
 Bell telephone diaphragm set up electric pressures in the telephone 
 coil, and when this coil is connected by wire to the coil of another 
 telephone, waves of current flow through the circuit which corre- 
 spond in a general way to the waves of sound set up in front of the 
 diaphragm of the first telephone. As these current waves flow 
 through the coil of the second telephone, they increase and decrease 
 the strength of its magnet. This alters the amount of the attraction 
 which the magnet exerts on its diaphragm, and the diaphragm is 
 therefore thrown into vibrations which correspond with the current 
 waves. The result of these vibrations of the second diaphragm is 
 to send out waves of sound like those which set the diaphragm of the 
 first telephone to vibrating. 
 
 The original Bell telephone is not sufficiently powerful as a trans- 
 mitter to give satisfactory service, but it is an extremely sensitive 
 and satisfactory receiver. The transmitters which are now generally 
 used are therefore based on an entirely different principle. 
 
 When two bits of carbon are permitted to lie loosely against 
 each other, the electrical resistance of their contact is very much 
 changed when changes occur in the pressure of the contact ; and if a 
 blunt metal point lie loosely against a carbon surface, differences 
 of pressure of the contact cause variations in its resistance. 
 Advantage is taken of this principle in the common telephone 
 transmitter ordinarily known as the Blake Transmitter. Fig. 101 is 
 a diagram of such a transmitter. M is a mouthpiece, and D is the 
 diaphragm. Touching the back of the diaphragm is a piece of plat- 
 inum wire about ^ inch in diameter and ^ inch long, which is 
 soldered into a hole at the end of a very fine german silver spring. 
 The other end of this piece of platinum makes a loose contact at C 
 with the polished face of a carbon button which is suspended on a 
 piece of very flexible watch spring. The amount of pressure at the 
 contact is exceedingly -small and may be very delicately adjusted by 
 the screw which is shown near the bottom of the figure. 
 
 Platinum and carbon electrodes are used in this transmitter in 
 preterence to two carbon electrodes because there is less sparking 
 between them than there would be between two carbon surfaces, 
 and the conductivity of the surfaces is preserved for a longer time. 
 The carbon button and platinum piece are represented by the two 
 heavy black spots at C in the figure. 
 
 120 
 
PUj^UMSA 
 
 FIG. 102. 
 
 FIG. 103. 
 
 If the diaphragm of the Blake transmitter is spoken to, it 
 vibrates and causes the platinum point to press more or less lightly 
 upon the carbon button and thus varies the resistance of the contact. 
 If this transmitter be connected in a circuit including a battery and 
 telephone receiver, as shown in Fig. 101, the current flowing in the 
 circuit will vary with the resistance of the carbon contact when the 
 transmitter diaphragm vibrates. The current in the circuit is there- 
 fore thrown into waves which correspond wim the vibrations of the 
 diaphragm. As these waves of current pass through the coil of the 
 receiver they increase and decrease the strength of the magnet and 
 its diaphragm is thrown into vibration so that the original sounds are 
 reproduced, as already explained. 
 
 Such a carbon contact as is used in a transmitter is called a 
 microphone and a transmitter in which it is used is often called a 
 microphone transmitter. A very easily made microphone in which 
 both electrodes are carbon, is shown in Figs. 102 and 103. In these 
 figures, C represents a short stick of carbon with pointed ends, 
 which is held loosely between the carbon blocks B B. These blocks' 
 are a little countersunk so as to keep the carbon stick from falling 
 out and they are fastened to a thin piece of pine board, R. The 
 whole is mounted on a board, S. When this microphone is con- 
 nected in circuit with a cell of battery and a telephone receiver by 
 means of the wires, w w, which are attached to the carbon blocks, 
 it will transmit sounds to the telephone. Such a rough microphone 
 will not transmit speech so that it can be understood, but it will 
 cause the telephone to sound for the slightest whisper. 
 
 The ordinary microphone transmitter which is used in telephony 
 is a rather delicate affair, and it cannot be worked with more than 
 one or two battery cells. In order that long lines may be satisfac- 
 torily spoken over, the effect of the transmitter is intensified by means 
 
 121 
 
 
FIG. 104. 
 
 FIG. 106. 
 
 FIG. 105. 
 
 of electromagnetic induction^ which will be explained in a later les- 
 son. The induction coil (Fig. 104) which is used for this purpose also 
 has the advantage of shutting out a scratchy sound which is caused 
 by the transmitter. 
 
 The commercial telephone system consists of much more than 
 the transmitter and receiver with their accompanying battery and 
 line. When telephones are used simply to connect two points, there 
 must be located at each point a transmitter, a receiver, a battery cell, 
 a means of operating an electric call bell at the other point, and a 
 local call bell. This outfit is usually put up in a set like the familiar 
 form shown in Fig. 105. Here A represents the transmitter, B, the 
 receiver, C, a box containing the battery cell, D, the electric call bell, 
 and B, a box containing a small dynamo with permanent magnets 
 called a magneto which may be operated by a crank. The latter is 
 used for operating the call bells. When the receiver is not in use 
 it hangs on a hook (as shown) which is depressed by the weight of 
 the receiver and moves electrical contacts which connect the bells 
 and magneto into the circuit and disconnect the telephone instru- 
 ments. When the receiver is taken from the hook the latter rises so 
 
 122 
 
FIG. 107. 
 
 B, call bell. M, magneto generator. S, hook switch. N, O and P, contact springs, 
 ^receiver. T, transmitter. C, induction coil. D, battery. Land L 1 , line terminals. 
 
 that the contacts cut the bells and magneto out of circuit and the tele- 
 phone instruments into the circuit. Fig. 107 shows a diagram of the 
 circuits in an ordinary commercial telephone set. When the receiver 
 is hung on the hook, the end of the hook lever comes in contact with 
 P, thus connecting the magneto generator and bell into the line cir- 
 cuit which connects with the telephone set at L and L 1 . When the 
 receiver is removed from the hook a spring lifts the hook and brings 
 the lever into contact with N and O, thus connecting the secondary 
 of the induction coil and the receiver into the line circuit and clos- 
 ing the battery circuit through the transmitter. The method of 
 using the telephone is too well known to require description. 
 
 The use of telephones simply to connect two points is only a 
 small part of the field of usefulness of the telephone. The great ma- 
 jority of telephones are used in connection with a central exchange. 
 This is a place where many telephone lines center and are brought 
 to a switch board so that they may be readily connected with each 
 other. By this arrangement each telephone user in a great city may 
 have his telephone quickly connected with that of any other person. 
 Each telephone user or subscriber, is supplied with a set such as is 
 shown in Fig. 105, and his line is run from the telephone set to .a 
 
 123 
 
section on the switch board at the exchange which bears the sub\ 
 scriber's individual number. When a subscriber wishes to speak 
 with another, he turns the crank of his magneto, thus causing a signal 
 at the switch board. He then takes his telephone receiver from its 
 hook, and when the switch board attendant speaks, he asks her to 
 connect him with the number of the second subscriber. This being 
 done, the attendant rings the telephone bell of the second subscriber 
 by means of a magneto, which calls him to the telephone. When 
 the conversation between the two subscribers is completed, one of 
 them notifies the switch board attendant by means of his magneto. 
 
 The earth is very commonly used as one-half of telephone cir- 
 cuits, so that only one wire need be used to connect an instrument 
 with the exchange. The ground terminals of the instruments are 
 then connected by wire to gas or water pipes, or to iron bars driven 
 into the ground. The telephone receiver is such an exceedingly 
 delicate instrument, that outside currents are likely to affect its oper- 
 ation when the ground return is used, so that important lines are 
 nowadays constructed with a complete metallic circuit, that is, with 
 wires for both the outgoing and incoming part of the line. A special 
 transmitter, called the long distance transmitter, is usually used with 
 metallic circuits. This transmitter, which is shown in Fig. 106, 
 is a microphone transmitter in which the loose contact between a bit 
 of platinum and a carbon button, is replaced by a short tube faced 
 with carbon buttons and containing powdered carbon. The carbon 
 granules contained in the tube are so arranged that the vibrations of 
 the diaphragm vary the pressure with which they lie against each 
 other, and the total resistance of the tube passes through wide varia- 
 tions. This transmitter is therefore very powerful, and is especially 
 used on lines of great length. 
 
 Copyrighted, 1894, 
 
 124 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XVI. 
 
 Points for Review. 1. How does a Bell telephone work? 
 
 2. Why is the Bell telephone not commonly used as a transmitter? 
 
 3. What is a microphone? 
 
 4. How does a microphone work? 
 
 5. How does a Blake transmitter differ from an ordinary microphone? 
 
 6. What apparatus is placed in telephone sets? 
 
 7. What is the purpose of each part of the set? 
 
 8. What is a central exchange? 
 
 9. What is the purpose of a telephone switchboard? 
 
 10. What is meant by a subscriber? 
 
 11. What is meant by a ground return? What is meant by a metallic circuit? 
 
 12. How does the essential construction of a long distance transmitter differ from 
 that of an ordinary transmitter? 
 
 XVII. 
 
 THE CONSTRUCTION OF TELEGRAPH AND TELE- 
 PHONE LINES AND INSTRUMENTS. 
 
 An overwhelming proportion of the telegraph and telephone 
 lines in the United States is carried on the wooden poles which are 
 such common sights in city streets and along railroads and high- 
 ways. Usually these poles are made of white cedar or chestnut, 
 though in some parts of the country, pine, cypress . and tamarack 
 are used. The poles differ in length from twenty-five feet to as 
 much as one hundred feet, and in diameter at the upper end from 
 four or five inches to eight or ten inches. The sizes which are most 
 commonly used are from twenty-five to forty feet long and from five 
 to six inches in diameter at the top. These are set in holes dug in 
 the ground to a depth varying from four to seven feet, depending 
 upon the length of the poles and the importance of the lines which 
 they carry. The wires which are supported by the poles are usually 
 tied fast to glass insulators, which are screwed on oak or locust pins 
 (Fig. 1 08) fastened to pine cross-arms. The arms are usually 3^ x 
 4J^ inches and as long as required. Fig. 109 shows the general 
 arrangement of a line of eighteen wires. The cross-arms fit into 
 notches called gains, which are cut in the sides of the poles, and 
 they are then fastened in place by means of lag screws (Fig. no) or 
 bolts. Poles are usually erected with the cross-arms facing each 
 other on alternate pairs of poles (Fig. in). If they are set in this 
 manner, the arms are not likely to be pulled off; but when all the 
 
 125 
 
FIG. 108. 
 
 FIG. 109. 
 
 RE I55UFJVICH. I87g. 
 
 FIG. 110. 
 
 - if -X.-I2T l2 : -*-l : %- 15 - 
 
 ft 3 g g ,_! 
 
 10- 
 
 8 3 ft ft 
 
 
 8 8 8 
 
 
 -ft 9 ft 8 8 
 
 e a 
 
 9 ft ft ft ft 
 
 9 8 ft 8 ft 
 
 FIG. ill. 
 
 FIG. 113. 
 
 FIG. 112. FIG. 114. 
 
 arms face in one direction, it is possible for all of them to be pulled 
 off, one after another, on account of the breaking of a pole or of one 
 f the arms. Fig. 112 shows the top of a pole arranged to carry 
 fifty long distance telephone wires. The numbers marked on the 
 
FIG. 115. 
 
 figure show the dimensions. In this figure the cross-arms are shown 
 braced with iron braces. These are ordinarily used when the arms 
 are intended to carry heavy or particularly important wires. The 
 braces are quite commonly used in cities, but are not often used in 
 the country, except upon long distance telephone lines. 
 
 In erecting the poles, care must be taken that they be set in 
 straight lines as much as possible. If curves or corners are turned, 
 the poles at the turn must be braced or guyed to prevent their being 
 pulled over by the strain of the wires. Figs. 113 and 114 show a 
 braced pole and poles guyed in two ways. When the guy crosses 
 a street or other passageway it is not uncommon to make the post 
 to which the guy is attached eight or ten feet high. The figures 
 show the poles somewhat tipped or inclined. This is an additional 
 safeguard against the poles being pulled over by the strain of the 
 wires. In cities the poles are ordinarily shaved all over with a draw 
 knife before being set, and they are then painted, but in the country 
 this refinement is not necessary. The distance between poles varies 
 from about 120 feet to 300 feet or the number of poles, to the mile 
 varies from 45 to 18. Only in the case of important lines carrying 
 many wires are the shorter distances between poles used, and the 
 average number of poles used for lines running through the country 
 is from 20 to 30 per mile. 
 
 In stringing wire various methods are used, but the commonest 
 way is to string the wire out on the ground, after which it is carried 
 to its place on the insulators by linemen who climb the poles by 
 means of spurs, or else the wire is draw from a reel over the cross- 
 arms. Wire which is intended for use on pole lines is usually fur- 
 nished in coils which may be laid on a hand reel (Fig. 115). When 
 a considerable length of wire has been laid on the cross arms, a block 
 and tackle is attached to its end and it is stretched up tight. While 
 the wire is held tight, it is tied by linemen to the insulator upon 
 which it is placed at each cross-arm. The tie is usually made of 
 wire like that in the lines but it is often somewhat smaller in size. 
 The line wire is laid in the groove of an insulator and one end of 
 the tie wire is twisted tightly around it close to the insulator. The 
 tie wire is then carried around the groove in the insulator and its 
 
FIG. "116. 
 
 FIG, 117. 
 
 FIG. 118. 
 
 FIG. no. 
 
 other end is twisted tightly around the line wire close to the 
 insulator. The appearance of the line wire and loop of tie wire with 
 the insulator removed, is shown in Fig. 116. 
 
 Wire is furnished from the wire mills in coils which contain 
 lengths varying from a few hundred feet to a half mile or more. A 
 great many joints are, therefore, necessary in a long line. The com- 
 monest form of joint used in this country is that known as the Wes- 
 tern Union or twist joint, which is shown in Fig. 117. The figure 
 shows the way a twist joint appears when made of iron wire of 
 medium size and also when made of small copper wire. In impor- 
 tant lines, these joints are always soldered in order that their electri- 
 cal conductivity may be as great as possible, and that the wires at 
 the joint may not be corroded by the effect of gases which are in the 
 air. The soldering is done by dipping the twisted joint into a pot of 
 melted solder. Another form of joint which is often used with cop- 
 per wire is that known as the Mclntyre sleeve joint. For this pur- 
 pose a ' 4 Mclntyre sleeve n made of two short pieces of copper tube 
 brazed together side by side (Fig. 118) is furnished. The tubes are 
 of such size that the line wire slips easily into them. The ends of 
 the wire are slipped through the tubes from opposite directions, and 
 are then turned up and cut off close. The joint is then twisted by 
 special pliers as shown in Fig. 119. The twisting draws the tubes 
 so closely around the wire as to insure a good electrical contact and 
 
 128 
 
FIG. 120. 
 
 no soldering is required. The latter point is of particular advantage 
 when the line is composed of what is known as hard drawn copper 
 wire, as heat due to soldering spoils the temper of such wire and 
 reduces its strength. 
 
 The insulation of long telegraph and telephone lines is a matter 
 of much importance. The effect of poor insulation is illustrated in 
 Fig. 1 20, where A and B are two telegraph stations connected by a 
 wire strung upon poles. The electrical circuit through the stations 
 is completed by a ground return. All the battery is supposed to be 
 located at A. If the wire is not well insulated at each point where 
 it is supported at the poles, an appreciable proportion of the cur- 
 rent leaks out of the line into the earth without having passed 
 through the distant station. Each point of leakage gives a branch 
 circuit, and if the line is long, so that many such branch circuits are 
 in parallel, the total leakage may be so great that sufficient current 
 does not reach station B to work its relay when signals are made at 
 A by means of the key. The actual proportion of the current which 
 escapes by leakage depends upon the ratio of the line resistance to 
 the combined resistance of all the leakage paths. 
 
 The effect of leakage may even be so great as to permit enough 
 current to flow through the relay at A to keep its armature attracted 
 when the key is open at B. This makes it impossible to send' sig- 
 nals from B to A. To avoid the reduction of current in the relay at 
 one end of a line, which may be caused by leakage when all the battery 
 is placed at the other end, it is usual to divide the battery and 
 put one-half of it at each end of the line, care being taken that the 
 two halves are properly connected in series. In this case the full 
 amount of current will always flow through both relays when the 
 keys are closed, but the leakage may be so great that the relay 
 armature at either station is continuously attracted without regard 
 to whether the circuit is made or broken by the key at the other 
 station, in which case signals cannot be transmitted from one station 
 to the other. 
 
 To make the resistance of the leakage paths as great as possible, 
 the insulators to which the line wire is attached are commonly made 
 
 129 
 
J S 
 
 FIG. 121. 
 
 FIG. 125. FIG. 122. FIG. 124. 
 
 of glass in the form of bells (Fig. 121) which may be screwed on 
 the wooden pins in the cross arms. These insulators are made in 
 various slightly different forms and sizes, as shown in the figure. 
 Glass is an excellent insulator when it is dry (L,esson i, page 4) but 
 in damp weather its surface becomes covered with a thin layer or 
 film of water. This film of water makes a path through which, at 
 every insulator, a small portion of current may leak from the wire to 
 the wooden supporting pin, and thence over the damp wood of the 
 cross arm and pole to the ground, or to some other wire, Water is 
 a comparatively poor conductor and the quantity of current which 
 escapes at each insulator is very small, but the total loss at all the 
 insulators, on a line several hundred miles long, may be a very 
 serious matter. As the leakage at each insulator is along the 
 film of water which covers the surface of the glass, the effective insu- 
 lation is increased by increasing the length of the path over which 
 the current must pass on the insulator's surface. This is done by 
 adding a second petticoat or bell to the glass as shown in Fig. 122. 
 Rubber hook insulators (Fig. 123) which may be screwed into 
 the bottoms of cross arms or in other similar positions are sometimes 
 
 130 
 
used for special work. In Europe, porcelain bell insulators which 
 are quite similar in shape to the American glass insulators, are used. 
 The dense white porcelain of which these insulators are made is 
 better for the purpose than is glass, but it is more expensive. 
 
 Where telegraph or telephone lines run inside of buildings, it is 
 usual to use copper wire of a small size which is insulated by a 
 braided covering of cotton thread thoroughly soaked with paraffine. 
 Where single lines need support, as in passing from the poles to a 
 building, it is usual to use an oak bracket (Fig. 124} with glass insulator 
 or a porcelain knob (Fig. 125) fastened at some convenient point 
 
 It has already been said that in telegraphy and telephony the 
 earth is commonly used as one-half of the electric circuit (Lesson XIV, 
 page 104, and Lesson XVI, page 131). The connection to' the earth in 
 telegraphy is made by means of ground plates, which may be sheets of 
 copper or galvanized iron T V inch thick and three or four feet square. 
 The plates are buried in the earth in a vertical position where the 
 soil is damp. Very often a connection to gas and water pipes may 
 be substituted for the ground plate. In this case, the pipe is care- 
 fully cleaned at the point to which the conductor is to be attached 
 and the wire is soldered fast to it. For telephony a sufficiently sat- 
 isfactory ground may usually be made for individual lines by driving 
 an iron rod several feet into the ground, though a connection to 
 water or gas pipes is better. At central exchanges a ground plate is 
 almost always used. 
 
 In large cities telephone and telegraph conductors are often 
 placed underground, and in some places they are run over house tops. 
 When electric wires are placed underground they must be continu- 
 ously insulated by some material which is sufficiently flexible to per- 
 mit of the wires being easily handled. For telegraph wires this 
 insulation commonly consists of a thickness of a vulcanized rubber 
 compound which has been placed on the wire under hydraulic pres- 
 sure. In the case of telephone wires.it is particularly important that 
 their capacity be the smallest that is possible, on account of the 
 delicacy of the telephone current, and crinkled paper is often used 
 for their insulation (refer to Lesson XII, page 88). When a fibrous 
 insulation such as paper is used, it is necessary to protect it from 
 absorbing moisture, and a lead covering over the insulation is there- 
 fore used. In fact, the lead covering is generally used with rubber 
 covered wires also, in order that the rubber may be properly pro- 
 tected from mechanical injury and from the injurious action on its 
 insulating qualities of gases or liquids which may come in contact 
 with it when under ground. 
 
 Before the lead covering is put on the insulated wires, a number 
 of them are usually ' 'laid up' ' or bunched into a cable, and the lead 
 is put around the whole. This may be done by pulling the cable 
 into a lead pipe, or by making a pipe around the cable by means of 
 
 131 
 

 FIG. 126. 
 
 FIG. 127. 
 
 FIG. 128. 
 a lead press. The end of a telephone cable is shown in Fig. 126. 
 
 Underground cables are usually not buried directly in the 
 ground, but are placed in what are known as electric conduits. 
 These consist of pipes or ducts made of iron, terra cotta, cement, 
 wood and sometimes other materials. The ducts are sometimes laid 
 singly but. they are usually laid in sets surrounded by concrete, as 
 shown in Fig. 127, which is an end view of a conduit containing 
 twelve ducts. The ducts are commonly circular in cross section and 
 from two to three inches in diameter, though ducts of rectangular 
 cross sections are often used. 
 
 In order that cables may be placed in the conduits, arrange- 
 ments for getting at the ducts must be made. This is done by build- 
 ing cable manholes at intervals along the conduit. These are usually 
 brick vaults, six or seven feet deep and several feet in diameter, 
 which are covered at the street surface by cast iron covers. Sections 
 of the conduit terminate on opposite sides of the manholes as shown 
 in Fig. 128. The manholes are placed at intervals of about 300 feet 
 in straight parts of a conduit and also at the turns which may occur 
 in the conduit. 
 
 When a conduit with its manholes is all built, the cables are 
 drawn into the ducts, one section at a time. The sections of each 
 cable must be jointed together in the manholes. To do this the 
 conductors are first jointed in the usual manner and their joints are 
 insulated. Finally a short piece of lead pipe is placed over the joints 
 and is soldered at both ends by a wiped joint to the lead coverings. 
 This makes the joint moisture-proof if it has been properly made. 
 Making cable joints requires the greatest care to avoid the entrance of 
 moisture into the cable, and it is always necessary to handle open 
 cable ends with the greatest care. The ends should always remain 
 sealed except when work is to be done on them. 
 
 132 
 
FIG 131. 
 
 FIG. 129. 
 
 FIG. 130. 
 
 In cities where the telephone wires are underground it is usual 
 to distribute the lines to the subscribers from the manholes. The 
 wires for all the subscribers in a block may be taken in a cable from 
 a manhole to the basement of a building, and they may then be dis- 
 tributed to the subscribers by passing through or over the buildings. 
 Where cables terminate in buildings, or on poles at points where the 
 underground wires connect to overhead lines, it is usual to carry each 
 cable into a small water-tight box, called a cable head or terminal. 
 Connections pass through the sides of the box so that the overhead 
 wires may be connected to those in the cable. Fig. 129 shows a tel- 
 ephone pole with a large box arranged to contain a cable head. Fig. 
 130 shows a terminal head designed to terminate a cable containing 
 one hundred wires. When overhead and underground wires are 
 joined they are always connected to a lightning arrester, which 
 usually consists of two brass plates with saw tooth edges like Fig. 
 131. One of the plates is connected to the wire and the other plate 
 to the ground. If the wire becomes charged by lightning a dis- 
 charge jumps from the wire to the ground across the space between 
 the teeth, and thus relieves the wire. 
 
 Where cables terminate at city telegraph offices or telephone 
 exchanges the wires are carried from the terminal heads to distribu- 
 tion boards and from there to the switchboard. When overhead 
 wires enter large offices or exchanges they are also carried through 
 a distribution board to the switchboard. 
 
 The object of the distribution board is to enable any connection 
 between the line wires and the switchboard to be made without 
 changing the permanent connections either at the switchboard or 
 the cable terminals. The wires are connected to lightning arresters 
 at the distribution board and also to very delicate fuses. The latter 
 are made of bits of gold leaf or very fine wire, and are used to pro- 
 
 133 
 
FIG. 132. 
 
 FIG. 133. 
 
 FIG. 134. 
 
 FIG. 135 
 
 tect the switchboard apparatus and office instruments from being 
 burned out by too great a current. A burn out would be the certain 
 result of a telephone or telegraph wire making contact (crossing it is 
 called) with a live electric light wire, were it not for the fuse, which 
 melts and breaks the circuit when too large a current flows, and thus 
 saves the intruments. 
 
 Telegraph switchboards are quite simple in construction except in 
 the largest terminal stations. For small way stations they consist 
 simply of a spring jack (Fig. 132) and a divided brass plug attached 
 to a double conducting cord (Fig. 133). The line enters and leaves 
 the switch through the binding posts, one of which is connected to 
 the spring seen in the figure and the other of which is connected 
 to a contact against which the spring presses. When the divided 
 plug is inserted between the spring and the fixed contact, the cur- 
 rent is forced to flow through the conductors attached to the plug and 
 through the station instruments. For stations which are entered by a 
 number of lines, a spring jack switchboard is arranged as shown in 
 Fig- 134. A plain plug board is more common for way stations at 
 which a number of lines enter. The appearance of this is shown in 
 Fig. 135. The board shown in the figure is provided with four 
 vertical brass bars to which the line wires are connected. Between 
 the bars are rows of brass discs. The discs and the binding post 
 which are on a horizontal line are connected together by copper 
 wires at the back of the board, and to these binding posts the station 
 instruments are connected. By properly connecting the discs and 
 
 134 
 
J[ J^ 
 
 ?*w T*v 
 
 o 
 
 G O 
 
 
 
 
 
 3! 
 
 
 
 
 
 
 E W E \V 
 
 o o o o 
 
 
 
 
 
 
 
 
 
 n 
 
 
 
 
 (%-j 
 
 
 
 
 0- 
 
 
 
 
 . . . 
 
 FIG. 136. FIG. 137. 
 
 bars together by inserting plugs in the holes shown in the figure, 
 the station instruments may be connected into the circuits or the 
 station may be entirely cut out of service, at the will of the operator. 
 Fig. 136 shows more plainly the effect of plugging the circuits. In 
 this the vertical lines represent the bars to which the line wires 
 are connected and the horizontal lines represent the rows of discs to 
 which the instruments are connected. In the left hand cut of the 
 figure, instruments A are connected to line number one, and instru- 
 ments B to line number two; in the right hand cut the A instruments 
 are connected to line number two, and the B instruments are cut out 
 without interrupting line number one, since the current can flow in 
 upon one bar to a plug, then across the disc to another plug and out 
 again on the other bar. The upper horizontal line marked G in the 
 figure, is used in grounding the circuits when desired. In large ter- 
 minal stations a combination of the spring jack and plug switch- 
 boards are made so that the lines, batteries, and instruments may be 
 connected together as desired. 
 
 Telephone switchboards are, as a rule, more complicated than 
 telegraph boards, since an exchange is always connected to a 
 larger number of wires than is a telegraph office of equal importance, 
 and since the connections of the telephone wires must be arranged 
 so that the operator and subscribers are able to signal and talk to 
 each other, as well as so that the subscribers' lines may be quickly 
 connected together. 
 
 In the earlier and simpler forms of telephone switchboards the sub- 
 scribers' wires on entering the exchange are each connected to a switch- 
 board circuit which contains a spring jack and an electromagnet, which 
 controls a drop shutter, and which terminates at the ground plate. 
 One form of this electromagnet is seen in Fig. 137. The armature, 
 A, of the electromagnet, M, has a hook, D, which ordinarily sup- 
 ports the shutter or drop, P, which is hinged at the bottom, but 
 when the subscriber sends current over the line from his magneto the 
 armature is attracted and the drop is released. When released, the 
 drop falls on its hinge and shows the subscriber's number, which is 
 painted at its back. When a subscriber calls, the exchange 
 operator inserts a plug in the subscriber's spring jack, which connects 
 her telephone instruments to his line. She then learns what connec- 
 tion is desired, and makes it by inserting the plugs at the ends of ,a 
 
 135 
 
 ! 
 
 XTNIVER3I 
 jRNiA; 
 
SUBSCRIBERS 
 
 
 
 SECTION N 
 
 SECTION N?3- 
 
 SECf ION N? 2. 
 EXCHANGE 
 
 FIG. 139. 
 
 cord into the spring jacks belonging 
 to the lines of the two subscribers that 
 are to be connected together. When 
 the subscribers have finished talking, 
 one of them turns his magneto crank, 
 which causes a drop to fall in the ex- 
 change and calls the operator's atten- 
 tion to the fact that the lines may be 
 disconnected. Boards of this general 
 type are used in nearly all small ex- 
 changes in the country. One is shown 
 in Fig. 1 38. It will be noticed that 
 the cords are held down under the 
 table by weights running on pulleys. 
 This keeps the cords from getting tan- 
 gled, and the plugs are held in a con- 
 venient position for the operator to pick up. 
 
 As one operator can take care of the calls from only a limited num- 
 ber of subscribers (50 to 100 subscribers is the usual number per 
 operator), in the larger exchanges a great many boards of the kind 
 described would be required, and much difficulty and waste of time 
 would be experienced in making connections between the line of 
 a subscriber connected to one board and the line of a subscriber con- 
 nected to another board in another part of the room. Hence, what 
 are known as multiple switchboards are used in exchanges having 
 
 FIG. 138. 
 
 136 
 
many subscribers. The multiple board with its numerous details can 
 be explained here only in the briefest outline. The principle upon 
 which it is based is to divide the total number of subscribers' lines 
 into sets, each of which is brought to a different sectipn of the switch- 
 board where the lines belonging to the set may be looked after by 
 an operator. The lines are connected to a drop and a spring jack in 
 their proper sections, so that the operator may communicate with 
 the subscribers by means of her telephone set. In addition to enter- 
 ing its own section through a drop and spring jack, every subscriber's 
 line is also connected to a spring jack in every other section. Con- 
 sequently each operator attends to the calls of a limited number of 
 subscribers whose lines are connected to drops in her section, and 
 since all other lines have spring jacks in her section she can connect 
 any of her subscribers' lines to the line of any other subscriber 
 which enters the exchange. Fig. 139 shows the principle of the 
 multiple board. The dots marked "local jacks" in each section 
 represent the spring jacks belonging to the lines which are looked 
 after by the operator at the section^ For simplicity the drops, keys 
 for ringing up subscribers, operator's telephone set, etc., are omitted 
 from the figure. The dots marked "ordinary jacks' ' represent the mul- 
 tiple spring jacks, by means of which the operator may connect her 
 subscribers with any others. It will be seen, for instance, that sub- 
 scribers' lines, numbers i, 2, and 3, enter the local jacks of section 
 number one, but they also enter the ordinary jacks of the other sec- 
 tions. If an operator in the second section wishes to connect one of 
 her wires, say number 6, with one of those belonging to the first sec- 
 tion, say number 3, she is able to do so at once on her own part of 
 the board, as shown in the figure. An ingenious arrangement by 
 which the operator can tell when a line is in use, prevents switching 
 three subscribers together. Exchanges with multiple switchboards 
 have been planned to give telephone service to the enormous number 
 of 10,000 subscribers from one board, but the telephone service has 
 never yet risen in any place to such magnitude that any exchange 
 has reached the number of 10,000 subscribers. In the large cities it 
 is usual for the purpose of economizing in the construction of lines to 
 locate several sub-exchanges to serve districts at a considerable dis- 
 tance from the main exchange. This practice causes the number of 
 subscribers' lines attached to any one exchange to be smaller than 
 might be otherwise expected. , 
 
 Copyrighted, 1894, 
 
 137 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XVII. 
 
 Points for review. 1. What wood is usually used for telegraph and telephone 
 poles? For cross arms? For pins? 
 
 2. Why are corner poles braced or guyed? 
 
 3. How are joints in line wire made? 
 
 4. Why is good insulation so important for telegraph lines? 
 
 5. Of what material and in what form are the insulators which are used on pole 
 lines usually made? 
 
 6. Why is the insulation of telegraph and telephone lines likely to be poor in damp 
 or wet weather? 
 
 7. Why is insulated wire used for telegraph and telephone lines where they arc 
 inside of buildings? 
 
 8. What are ground plates? 
 
 9. What materials are used for insulating underground telegraph and telephone 
 wires? 
 
 10. Why are lead coverings put on underground cables? 
 
 11. What are electric conduits? What are manholes? 
 
 12. How are cables terminated in buildings? 
 
 13. What is the purpose of fuses in telegraph and telephone circuits? 
 
 14. How are the common forms of telegraph switchboards arranged? 
 
 15. How are the simpler forms of telephone switchboards arranged? 
 
 16. What is the principle of multiple telephone switchboards? 
 
 17. Why are multiple switchboards necessary in large exchanges? 
 
 WESSON XVIII. 
 
 TESTING LINES FOR INSULATION AND CONDUCTIVITY 
 AND THE LOCATION OF LEAKS AND BREAKS. 
 
 On account of their exposed positions, overhead telegraph and 
 telephone wires are particularly liable to injury. It is therefore 
 necessary to make careful, systematic and continued tests of import- 
 tant lines in order that they may be kept in satisfactory condition. 
 The troubles to which lines are heir may be divided into four classes: 
 
 1. Grounds; 
 
 2. Crosses; 
 
 3. Poor connections; 
 
 4. Breaks. 
 
 A line is said to be grounded when so much current leaks from 
 it to the ground as to interfere with its proper use. Grounding may 
 be caused by a general leakage all along the line, or a large leak 
 may exist at one point, where the line comes in contact with trees, 
 etc. When the insulation of the line becomes so low that practically 
 all the current leaks off, it is said to be dead grounded. 
 
 13& 
 
Lines are said to be crossed when they make contact with each 
 other, so that messages sent over one line may be received on the 
 other. Only one of the crossed lines can then be used to send inde- 
 pendent messages. The most fruitful cause of crosses is the swinging 
 of loose wires in the wind, by which means they become tangled up. 
 Sometimes crosses or grounds will appear and disappear at intervals, 
 when they are often called swinging crosses or grounds. These may 
 be caused by a swinging wire which touches another wire or a 
 ground contact at intervals, but does not remain continuously in 
 contact. 
 
 Poor connections result from various causes, such as corroded 
 joints in the wire, a corroded connection to a ground plate or water- 
 pipe, a poor contact between the ground plate and the earth, loose 
 connections at binding posts of instruments, at switchboards or at 
 batteries. Poor connections may very seriously reduce the con- 
 ductivity of the line, and thus reduce the distinctness of messages 
 sent over it unless extra battery power is used. 
 
 A break may be caused by a binding post connection working 
 entirely loose, by a wire breaking at an instrument, or by the line wire 
 breaking. It may also be caused by defective contacts in the work- 
 ing parts of an instrument, or in the case of a telegraph line by a 
 careless operator leaving his key open. When a line wire breaks, 
 the circuit may be entirely opened, or if one or both of the ends get 
 on the ground it may be possible to get current through one or both 
 portions. 
 
 The simplest way of determining the condition of a line is by 
 comparing the distinctness of the signals which are transmitted over 
 it from day to day. Thus, in the case of a telegraph line, if the sig- 
 nals which are sent out from a terminal station where half the bat- 
 tery is located, are found to be strong and good on a certain day, 
 while signals which are received at the station over the same wire 
 are weak and indistinct, it is evident that the insulation of the line 
 is poor (compare Lesson XVII, page 129). If the signals which are sent 
 and received are equally indistinct, while the battery is in good con- 
 dition, the conductivity of the line is probably less than usual. If 
 signals sent over one wire can be received on another, the lines are 
 either crossed or sufficient current leaks from one wire to the other to 
 give the effect of a cross. In the case of a break which opens the cir- 
 cuit, the armatures of the relays in the line fall back from their mag- 
 nets; but if the ends of the line at the break become grounded, it may be 
 possible to send signals between stations upon the same side of the 
 break. 
 
 The section between two stations upon which trouble exists, may 
 usually be located in the case of a local telegraph line passing through 
 stations which are close together. To do this, the station nearest one 
 end is called up from the end station, either by means of the faulty wire 
 or by means of another wire, and is told to ground the faulty wire. 
 
 139 
 

 ) 
 
 FIG. 140. 
 
 This being done, signals are transmitted between the two stations 
 over the faulty wire. If the wire works all right, the next station is 
 called up and the test of the working condition of the wire is again 
 made. This is continued from station to station until the signals 
 fail in transmission. The trouble is* then on the last section tested, 
 and a line-man may be sent out to exactly locate and correct it. 
 
 Trouble on telephone lines is usually shown to the exchange 
 operator through difficulty or impossibility in communicating with a 
 subscriber. Crosses make themselves evident through the fact that 
 when a subscriber on one of the crossed wires calls the exchange by 
 working his magneto, not only does the drop fall which is attached to 
 his wire, but the drops attached to the wires which are crossed with 
 his also fall. What is known as cross talk between telephone wires 
 is not a certain sign of a cross, as it may be caused by either electro- 
 magnetic or electrostatic induction (Lesson I). The latter is a very 
 common cause of cross talk. To avoid cross talk in telephone cables, 
 the two wires of each metallic circuit are twisted together, and such a 
 cable is therefore said to be made up of twisted pairs. 
 
 In the case of long trunk telegraph or telephone lines con- 
 necting cities at considerable distance apart, methods are 
 required for the location of faults by direct electrical measurements. 
 It is usual to make careful daily or weekly measurements of the 
 insulation and conductivity and sometimes of the capacity of such 
 lines. The results of these measurements are carefully recorded in a 
 book; and the records are a material aid in the location of faults by 
 electrical measurement. The commonest instruments to be used in 
 testing lines are a sensitive galvanometer (Lesson IX, page 58) and a 
 Wheatstone bridge (Lesson X, page 68) . 
 
 To measure the conductivity of a metallic circuit is very sim- 
 ple. At one end of the line the two wires are connected together, 
 and at the other end of the line the two wires are connected to the 
 bridge (Fig. 140). Half the resistance measured by the bridge is 
 the resistance of one of the wires composing the circuit, if the wires 
 are of equal length and size. 
 
 When only one wire is available, as may be the case with tele- 
 graph circuits, its far end is connected to ground, the near end is 
 connected to one of the binding posts of the bridge, and the other 
 
 14,0 
 
EARTH 
 
 FIG. 141. 
 
 binding post to which the unknown arm should be connected, is 
 connected to ground as shown at B 1 , in Fig. 141, which shows the 
 arrangements of the connections for a postoffice pattern bridge (lyes- 
 son X, page 70). With this arrangement the arm of the bridge 
 marked A in the diagrams, Fig. 140, and Lesson X, Fig. 47, is 
 between A and B. The arm B is between A and E, in the zigzag 
 part of the bridge rheostat. The arm C is between B and C ; and the 
 unknown resistance D is connected to the bridge at the points C 
 and E. The resistance measured by the bridge may be taken as 
 equal to the resistance of the line, provided the ground connections 
 are good. 
 
 KVo JL 
 
 FIG. 142. 
 
 When the individual conductivity of three wires running 
 between the same points is desired, the measurement is very simple. 
 The resistance of the wires taken in pairs (Fig. 142), is measured 
 exactly as in the case of a metallic circuit. From these measure- 
 ments the resistance of each wire may be calculated. For instance, 
 if wires i and 2 taken together, measure 4, 500 ohms, i and 3 taken 
 together measure 3,750 ohms, and 2 and 3 taken together 
 measure 4,700 ohms, then the resistance of all three wires is 
 1 2 ~ 6>475- Wire number i then measures the difference 
 between the resistance of all, and that of numbers 2 and 3 together, 
 
 141 
 
FIG. 143. 
 
 or the differences between 6,475 an ^ 4>7 oa Number i therefore 
 measures 1,775 ohms. In the same way wires numbers 2 and 3 are 
 each found to measure 2,725 and 1,975 ohms. 
 
 When resistance measurements are made with the earth as part 
 of the circuit, currents flowing in the earth may interfere with the 
 results by entering the wire and flowing along it. Such currents are 
 called earth currents. At exceptional times, as, for instance, during 
 the continuance of the so-called magnetic storms, earth currents flow- 
 ing on the wires may be so strong that telegraphing may be carried 
 on without any battery attached to the wires. When earth currents 
 interfere with the measurements made on a grounded circuit, the 
 tests must be postponed until a more favorable opportunity, if ad- 
 ditional wires cannot be used in making the measurements by the 
 last method given above. 
 
 Insulation measurements are made with the line disconnected 
 from its ground plates (the line open, Fig. 143). As a general rule, 
 the insulation resistance is higher than an ordinary Wheatstone 
 bridge will measure, and the method explained in Lesson X, page 
 72 is used. The condition of the insulation of a line from day to 
 day may also be roughly determined by means of a milliamperemeter 
 (Lesson XI, page 75), which is placed in the circuit at one end of 
 the line, and then a battery of a fixed number of cells is connected 
 in the circuit at the other end of the line. If the resistance of the 
 circuit and the pressure of the battery be known, a certain standard cur- 
 rent which may be calculated according to Ohm's Law (Lesson VIII, 
 page 5 2), should flow through the line when the insulation is perfect. 
 The difference between the standard current and that indicated by 
 the amperemeter is a measure of the leakage from the line. 
 
 A comparison of the periodical measurements of conductivity 
 and insulation shows whether or not the line is in good order, or 
 whether or not any poor connections are developing or its insulation 
 is deteriorating. 
 
 The location of the position of a ground or a cross on a line may 
 be determined in various ways. If the fault is a dead ground, a 
 measurement of the resistance of th? line is made by bridge from one 
 end of the line, the other end of the line being open, (Fig. 144), and 
 the distance to the ground is calculated at once from the resistance 
 of the line per mile. Thus, suppose a line 500 miles long ordinarily 
 measures 4,500 ohms or 9 ohms per mile, and the resistance meas- 
 ured through a dead ground is 1,800 ohms, then the ground is 200 
 miles from the station where the measurement is made, since 9 times 
 
 142 
 
FIG. 144. 
 
 200 is equal to 1,800. When the ground is only partial its location 
 is not so simple, since the resistance of the leakage path comes into 
 the measurements. Several methods may be used in making the 
 measurements, but the two following are the simplest. The resis- 
 tance of the line through the fault may be measured from each end, 
 the other end being open at the time. To find the resistance of the 
 line between one end, A, and the fault, the resistance of the line in 
 good order is added to that measured through the fault from A. 
 From this is subtracted the resistance measured through the fault 
 from B and the result is divided by two. For instance, suppose the 
 resistance measured through the fault from A, as shown in Fig. 144, 
 is 3,500 ohms, and a similar measurement made from B shows 5,000 
 ohms, the line itself from A to B measuring 4,500 ohms, then, the 
 resistance of the line from A to the fault is 3>50Q+4 'f ) ~ 5 ' 000 ^ 1,500. 
 If the line measures 9 ohms to the mile, the distance from A to 
 the fault is 167 miles. The reason for this is readily seen, since 
 the total resistance of the line is equal to that from A to the fault, 
 added to that from B to the fault, F. The measurement from A 
 through the fault gives the resistance from A to F added to the 
 resistance of the leak. The measurement from B through the leak 
 gives the resistance from B to F added to the resistance of the leak. 
 Adding together the resistance of the line in good order and the 
 resistance from A through the fault, gives a sum which is equal to 
 the resistance of the leak plus the resistance of the line from F to B, 
 plus twice the resistance of the line from A to F. Subtracting the 
 resistance from B through the leak leaves a remainder equal to twice 
 the resistance of the line from A to F. 
 
 The second method of locating a fault is by what is called the 
 loop method. This can be used only when the leaky wire can be 
 looped with a good wire so that both ends may be connected to a 
 "bridge for testing. In this case the connection is made up as shown 
 in diagram in Fig. 145, where EP is the leaky wire and CP is the good 
 one. Af makes one bridge arm and Cf another, while A B and B C 
 are the other two arms. When AE or A B and A C are adjusted until 
 the bridge is balanced, the resistance from C to f and from A to f 
 are to each other as B C is to A B, while the total resistance of Cf 
 
 143 
 
FIG. 145. 
 
 FIG. 146. 
 
 "YVo* 
 
 FIG. 147- 
 
 plus Ef plus A E are known from the records of the wire conductiv- 
 ities and the reading of the rheostat, A E. The way in which the 
 connections are made to a postoffice pattern bridge is shown in 
 Fig. 146. 
 
 When two wires are crossed, the location of the point where 
 they make contact with each other is carried out in very much the 
 same manner as the location of grounds, except that the measure- 
 ments are made over a circuit made up of the two crossed wires 
 (Fig. 147) instead of over a circuit made up of the ground and the 
 grounded wire. The distance from the measuring station to the 
 cross is calculated from the measured resistance and the resistance 
 per mile of the two wires together. Thus, suppose the resistance 
 measured through the cross at X, as shown in Fig. 147, is 4,400 
 ohms, and the resistances of the two wires are 9 and 13 ohms per 
 mile. Then the cross is 200 miles from the measuring station, since 
 the resistance per mile of the two wires together is 9 plus 13, or 22 
 ohms. In this measurement it is assumed that the resistance at the 
 cross itself is too small to be taken into account. When this is not 
 the case, special measurements have to be made as in the case of a 
 partial ground. 
 
 In making test measurements it is usual to disconnect all telegraph 
 or telephone instruments from the circuit, though they may be per- 
 mitted to remain in circuit and a correction made on account of theil 
 resistance or insulation. 
 
FIG. 148. 
 
 In testing underground wires and submarine cables, practically 
 the same methods are used as in the testing of overhead wires. 
 Systematic, periodical tests are quite essential for the preservation of 
 the life of cables, since their usefulness may be quickly destroyed after 
 a leak starts. Fig. 148 shows the permanent testing arrangements as 
 they are set up in the testing-room at the end of an ocean cable. 
 
 Copyrighted, 1894, 
 
 145 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XVIII. 
 
 Points for Review. 1. What are the classes of trouble which occur on telegraph 
 and telephone lines? 
 
 2. What is a ground? What is a cross? 
 
 3. How can the position of trouble be located on a local telegraph line? 
 
 4. What are earth currents? 
 
 5. How is the conductivity of a line measured? 
 
 6. How is the insulation of a line measured? 
 
 7. How can the position of a ground be located by electrical measurements? 
 
 8. How can the position of a cross be located by electrical measurements? 
 
 I^KSSON XIX. 
 
 PRINCIPLES OF CONTINUOUS CURRENT DYNAMOS 
 
 AND MOTORS. 
 
 The experiments of Oersted (Lesson VI, page 35), Sturgeon 
 (Lesson VI, page 40), and others, showed the intimate relation 
 existing between electricity and magnetism, and also showed 
 that the flow of an electric current always produces magnetism 
 (Lessons VI. and VII). It remained for the brilliant experimental 
 studies of Prof. Joseph Henry, of Princeton College, New Jersey, and 
 Michael Faraday, of the Royal Institution, London, to make the 
 most important additions to our knowledge of the mutual action 
 between electric currents and magnetism. Within two years after 
 the publication by Oersted that a magnetic needle may be deflected 
 by bringing near it a wire carrying an electric current, Faraday had 
 succeeded in producing a continuous motion by means of the effect 
 of an electric current upon a permanent magnet, and it was soon 
 after learned that a wire hung over the pole of a magnet and 
 with its ends in mercury troughs, as shown in Fig. 149, would 
 continuously revolve around the pole on account of the mutual 
 attraction of the lines of force belonging to the magnet and to a 
 current in the wire (Lesson VI, page 38).^ In the motion thus pro- 
 duced, by means similar to those utilized in many of the electrical 
 instruments which have already been described, lies the principle of 
 the operation of the electric motors which prove so useful at the 
 present day. At the time of Faraday, the best method of generating 
 n\an electric current was by means of an electric battery, but the use- 
 res'fllness of the electric motor could be but small as long as it depended 
 its power upon the consumption of zinc in a battery (Lesson IV, 
 
 24). 
 
 146 
 
FIG. 149. 
 
 FIG. 150. 
 
 To the vigorous minds of Faraday and Henry, the production of 
 motion when an electric current was brought into the influence of a 
 magnet, seemed to suggest a reverse action through which an electric 
 current might be produced by the motion of a wire in a magnetic 
 field. This thought led, shortly after 1830, to the magnificent dis- 
 covery by Faraday that a tendency for electric currents to flow is pro- 
 duced in a conductor which is moved in a magnetic field so as to cut 
 through the lines of force of the field. That is, an electric pressure is 
 set up in the conductor when it cuts the lines of force. The two 
 great experimenters also independently discovered the fact that any 
 change in the magnetic field around a wire tends to set up an electric 
 current in the wire, exactly as any change in an electric current 
 which flows in a wire causes a corresponding change in the magnetic 
 field about it. In this great discovery lies the principle of the oper- 
 ation of dynamo electric generators or dynamos, as they are usually 
 called. Faraday himself made in 1831 what may be called the first 
 model of a dynamo. This consisted of a disc of copper rotated between 
 the poles of a strong magnet (Fig. 150). From this disc a current 
 was collected by copper brushes which rubbed on the edge of the 
 disc and on its shaft. 
 
 Faraday's discovery was quickly turned into commercial service 
 and many small machines were made for generating electric currents 
 by rotating coils of wire between the poles of permanent magnets. 
 These machines with permanent magnets are ordinarily spoken of as 
 magneto electric generators or magnetos, to distinguish them from the 
 ordinary dynamo electric generators or dynamos which have electro- 
 magnets. The magnetos which are used for ringing telephone call bells 
 (Lesson XVI, page 122), belong to the same class as the early machines. 
 
 When a wire is moved in a magnetic field so that it cuts 
 lines of force, the action which occurs causes a difference of electric 
 pressure between the two ends of the wires. The magnitude and 
 direction of the pressure which is thus induced depends upon certain 
 fixed relations. 
 
 The magnitude of the pressure depends upon the rate at which the 
 
 147 
 
FIG. 151. FIG. 152. 
 
 wire cuts lines of force, that is, upon the total number of lines of force 
 cut by the wire in a second of time. When the wire cuts one hundred 
 million (100,000,000) lines of force in every second during its motion, 
 an electric pressure of one volt is set up, and if the wire (like C in Fig. 
 151) be laid across conducting rails which are electrically connected 
 through a galvanometer (shown at G in the figure) the galvanometer 
 will indicate, while the wire moves, the flow of a current, having a 
 strength which is equal to the induced pressure divided by the resist- 
 ance of the electric circuit made up of the galvanometer, rails, and 
 moving wire. If the wire cuts through the lines of force at the rate 
 of two hundred millions (200,000,000) to the second, the induced 
 pressure is equal to two volts, and if the wire cuts only 75 million 
 lines each second, a pressure of only ^ volts is set up, which is ac- 
 cording to the rule given above. 
 
 The number of lines of force which are cut in a second by a 
 wire moving in a magnetic field depends upon four items: i, upon 
 the strength of the field, or the number of lines of force which it con- 
 tains in each square centimeter; 2, upon the length of the wire which 
 is in the field; 3, upon the speed with which the wire moves; 4, upon 
 the angle with which the wire moves across the lines of force. If 
 the wire moves diagonally across the lines of force it does not cut 
 through as many lines in a given time as when it moves equally 
 fast at right angles to the lines. 
 
 The direction of the induced electric pressure depends upon the 
 direction of the lines of force in the magnetic field and the direction in 
 which the wire cuts through them. In Fig. 152, if the vertical 
 arrows show the direction of the lines of force and the horizontal 
 arrow between the rails shows the direction in which the wire A B 
 moves, then the end B of the moving wire is positive and the 
 other end negative in pressure. That is, a current will flow around 
 the circuit, composed of the wire and the rails, from B through C 
 and D to A and from A through the wire to B. The current flows 
 in the external circuit, B C D A, from the positive or high pressure 
 end to the negative or low pressure end of the wire, and within the 
 moving wire the current flows from the low pressure end to the high 
 pressure end. The motion of the wire across the lines of force 
 causes it to act like a pump, which lifts the electric current from its 
 low pressure or suction end to its high pressure or discharge end. In 
 this respect the moving wire acts exactly like a friction machine or a 
 
 148 
 
FIG. 154. 
 
 FIG. 153. 
 
 primary battery (Lesson II, page 10, and Lesson III, page 16). 
 
 If the direction of the wire's motion be reversed, the direction 
 of the current will also be reversed. Reversing the direction of the 
 lines of force also reverses the current. 
 
 There are various ways of remembering the relation between 
 the direction of the electric current, the direction of the wire's 
 motion, and the direction of the lines of force. One of them is to 
 hold up the right hand, with the thumb sticking straight up, the 
 first finger sticking straight out, and the middle finger turned off to 
 the left (Fig. 153). Now, if the hand be turned in such a direction 
 that the thumb points in the direction of motion of the wire and the 
 first finger points in the direction of the lines of force, then the mid- 
 dle or central finger will point in the direction of the current which 
 is set up in the wire by the induced pressure. 
 
 Another way of remembering this relation is by a modification 
 of Ampere's rule (Lesson VI, page 37). If a man lies in the mov- 
 ing conductor so that he looks down along the lines of force (his face 
 is towards the south pole), and the motion is towards his right hand, 
 he will be floating head first down the current which is set up in 
 the wire. 
 
 It has already been explained (Lesson V, page 31) that the earth 
 is a great magnet, and that its lines of force, therefore, reach out 
 through all the space within which we live. The induction of elec- 
 tric pressure by a wire cutting lines of force may, therefore, be illus- 
 trated by swinging a long wire in the earth's magnetic field. If a 
 wire be suspended across a room and its ends be attached to a sensi* 
 
 149 
 
tive galvanometer, the needle 01 the galvanometer will be deflected 
 from side to side when the wire is set to swinging. When the^wire 
 moves in one direction, the needle will move to one side of its zero 
 point; and when the wire moves in the other direction, the needle 
 will move to the other side of the zero. This shows that the direc- 
 tion of the pressure induced by the cutting of the earth's lines offeree 
 depends upon the direction in which the wire moves across the lines. 
 
 If the wire be caused by some means to swing more slowly, the 
 deflections of the galvanometer needle will be smaller, showing that 
 the magnitude of the induced pressure depends upon the velocity of 
 motion of the wire. 
 
 If half the wire be now replaced by a piece of string, and the 
 ends of the remaining half be connected to the galvanometer with- 
 out practically altering the resistance of the circuit, and the wire be 
 set swinging at about the same speed as before, the galvanometer 
 deflections are reduced to about one half their former value, showing 
 that the induced pressure depends upon the length of the wire. 
 
 These experiments can only be successfully carried out in some 
 such favorably equipped place as a college laboratory, but their 
 description serves to illustrate the effect of moving a conductor 
 across magnetic lines of force. An experiment illustrating the same 
 thing may be made by a permanent magnet, a coil of wire, and any 
 galvanometer with a light needle which is obtainable. If the coil 
 made up of a few turns of wire be slipped along one end of the magnet 
 at a fixed speed, the galvanometer needle will show a certain deflec- 
 tion. Now if more turns be added to the coil, which is then moved 
 exactly as before, the galvanometer deflection will be proportionally 
 greater, showing that a greater electric pressure has been induced. 
 In the case of the coil we have the following condition; each turn 
 cuts the lines of force at a certain rate as the coil is slipped along 
 the magnet, and a corresponding electric pressure is set up in it. 
 Since the turns of the coil are all connected in series and the elec- 
 tric pressures set up in them are all in the same direction, the elec- 
 tric pressure induced in the whole coil is equal to the sum of the 
 pressures developed in all of its turns. This is exactly similar to the 
 case of an electric battery with its cells connected in series, where 
 the battery pressure is equal to the sum of the pressures of all the 
 cells. Adding additional cells to the battery increases the battery 
 pressure, and adding additional turns to the moving coil increases the 
 total pressure induced in it. 
 
 If the connections of some of the cells in the battery are reversed, 
 the pressure at the battery terminals is reduced and becomes equal 
 to the difference of the pressures which are developed by the cells 
 connected in one way and those which are connected in the reverse 
 way. In the same way,* if part of the turns of the moving coil be 
 wound in one direction and part in the other direction, the pressures 
 
 150 
 
FIG. 155. 
 
 FIG. 156. 
 
 developed in the two parts are opposite, and the effective pressure 
 developed by the coil is equal to the difference of the pressures which 
 are developed in the parts. If half the turns are right handed and 
 half left handed, no current will flow in the coil when it moves in 
 the magnetic field, because the pressure developed in one half of the 
 turns tends to cause the current to flow one way, and the equal pres- 
 sure developed in the other half of the turns tends to cause the cur- 
 rent to flow in the opposite direction. These two tendencies neu- 
 tralize each other, and no current flows. 
 
 For the same reason, if a coil of wire be moved straight across the 
 lines of force of a uniform field (Fig. 154) no current will flow in the 
 coil, since the pressures developed in the two halves of each turn are 
 in opposition, as shown by the arrows, and are of equal value. The 
 truth of this may be easily proved by applying one of the rules given 
 earlier (page 150). If the coil be mounted on an axis or shaft, so that 
 it may be revolved in the field (Fig. 155), a different condition exists. 
 Now, the two halves of the coil cut the lines of force in such a way 
 that the pressures are in the same direction as shown by the arrows, 
 and a current therefore flows in the coil. Fig. 156 shows the coil after 
 it has turned through a half revolution from its first position. From 
 this figure it is seen that the two sides of the coil are now both cutting 
 the lines of foioe in a direction which is opposite to that in which 
 they cut the lines before. The direction of the current in the coil is 
 therefore reversed. As the coil continues revolving the current in 
 it is reversed in every half revolution. Such a current, which flows 
 first in one direction and then in another, is called an alternating 
 current. 
 
 151 
 
FIG. 157. 
 
 FIG. 158. 
 
 If, instead of being short circuited on itself, the coil be connected 
 to an external circuit by means of such sliding contacts as are shown 
 in Fig. 157, the alternating current may be led off to be used for any 
 desired purpose. The rings A A, to which the ends of the coil are 
 attached, in this case are called collecting rings or collectors, and the 
 parts B B, which bear on the collectors, are cabled brushes. In an 
 actual machine made up for the purpose of generating electricity by 
 a coil revolving in a magnetic field, the revolving part is called an 
 armature. Telephone magnetos, which have already been referred to, 
 consist of a coil of wire wound on an iron core, which is revolved in 
 the magnetic field between the poles of a horse shoe magnet (Fig. 
 158). Such machines produce an alternating current. 
 
 FIG. 159. FIG. 160. 
 
 It is possible to arrange the collector which is attached to a coil 
 that is revolved in a magnetic field in the manner shown in Fig. 159. 
 With this arrangement, the collector segments connect each brush 
 first with one end of the coil and then with the other end as the coil 
 revolves. If the brushes are properly set, that is, if they bear on the 
 collector at proper points, this arrangement causes the current to 
 flow continuously in one direction in the external circuit, though 
 in the coil itself, its direction of flow reverses with each half revo- 
 lution as before. Such an arrangement of the collector is called a 
 commutator, and the current in the outside circuit is said to be 
 commutated or rectified. Fig. 160 shows one of the early dynamos 
 
 152 
 
FIG. 161. 
 
 FIG. 162. 
 
 with a single coil armature and commutatpr of two segments. This 
 machine looks quite like the magneto shown in Fig. 158, but the 
 collector is different and the magnetic field is set up by an electro- 
 magnet instead of a permanent magnet. 
 
 An armature with one coil furnishes a current consisting of a 
 series of waves or pulsations, which may be represented by Fig. 161. 
 This is easily understood after a little consideration. When the coil 
 stands up and down between the pole pieces like the full lines in 
 Figs. 155 and 156, it is in such a position that when it is revolved a 
 small amount, the conductors move practically parallel to the lines 
 of force and no lines are cut. When the coil is in continuous 
 revolution, no pressure is induced at the instant that it is in the 
 positions shown by the full lines in Figs. 155 and 156 (A., C. and E., 
 Fig. 161). 
 
 When the coil stands as shown by the dotted lines in Figs. 155 
 and 156, it is in such a position, that when it is moved a little, the 
 conductors cut squarely across the lines of force and the largest pos- 
 sible number of lines of force are cut for a given amount of motion. 
 The dotted positions of the coil correspond with the points B and D 
 in fig. 161. 
 
 Direct current dynamos having armatures with one coil are not 
 satisfactory for general use for two reasons: ist, the wavy character 
 of the current is a disadvantage for some purposes; ad, the commuta- 
 tion of large currents at the full pressure which is required for most 
 commercial uses is not practical. To overcome these difficulties the 
 armature coils must be uniformly distributed over the surface of the 
 armature, and the windings must be connected at equal intervals to 
 commutator segments. The first armature of this kind that was put 
 into commercial service was invented by a Frenchman named 
 Gramme. The core of Gramme's armature consisted of a ring made 
 of iron wire. This ring had a winding of insulated copper wire 
 wound uniformly over its surface and at equal intervals the windings 
 were electrically connected to commutator segments. The arrange- 
 ment is shown in Fig. 162. When this armature is placed in a mag- 
 
 153 
 
FIG. 163. 
 
 N 
 
 FIG. 164. 
 
 netic field the lines of force pass through the iron core from one pole 
 to another (Fig. 163) so that the revolution of the ring causes the 
 outer conductors to cut lines of force but the inner conductors are 
 entirely shielded. When the armature is revolved the wires of the 
 armature winding which are under one pole piece cut lines of force 
 in one direction, and those under the other pole piece cut lines in 
 the opposite direction. The effect of the opposing electric pressures 
 which are thus set up in the windings of the armatures, is to cause a 
 point at one side of the armature to come to a high electrical pres- 
 sure and a point on the opposite side to come to a low electrical 
 pressure. If brushes bear on the commutator at these points (A and 
 B in Fig. 164) a current will flow in the external circuit from the 
 high to the low pressure side of the armature, that is, from A to B. 
 The path of the current through the armature itself is from B to A, 
 through the two halves of the armature in parallel. This is plainly 
 shown by the figure. Since the number of conductors under the pole 
 pieces is practically the same for every position of the armature 
 during the revolution, the armature produces a practically continuous 
 current when it is continuously revolved at a uniform rate, as when 
 it is driven by a steam engine. 
 
 Copyrighted, 1894, 
 
 154 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XIX. 
 
 Points for review. 1. Who was Joseph Henry? Who was Michael Faraday? 
 
 2. What is the result of moving an electric conductor in a magnetic field? 
 
 3. What is the effect of changing the strength of the magnetic field which is around 
 a. conductor? 
 
 4. Upon what depends the magnitude of the electric pressure which is induced 
 when a conductor is moved in a magnetic field? 
 
 5. Upon what does the direction of the induced pressure depend? 
 
 6. How may the direction of the current set up by the induced pressure be 
 remembered? 
 
 7. How may the effect of moving a conductor in a magnetie field be illustrated? 
 
 8. What is the result of moving a coil of wire straight across a uniform magnetic 
 field? 
 
 9. What is the result of revolving the coil in the field? 
 
 10. What is an alternating current? 
 
 11. How may the current induced in a revolving coil be taken off for use in an 
 external circuit? < 
 
 12. What is the difference between dynamos and magnetos? 
 
 13. What is the purpose of a commutator? 
 
 LESSON xx. 
 
 PRINCIPLES OF CONTINUOUS CURRENT DYNAMOS 
 
 AND MOTORS: THEIR CONSTRUCTION, CARE 
 
 AND ATTENDANCE. 
 
 As a rule, commercial Gramme or ring armatures are not wound 
 with a continuous wire but the divisions of the armature windings, 
 the ends of which are connected to adjacent commutator segments or 
 bars, are wound as separate coils. This makes it possible to insu- 
 late the different parts of the winding more effectively from each 
 other, and thus prevent the current from jumping by a short path, or 
 short circuiting, directly from one coil to another instead of follow- 
 ing all the way around the coils. The separate coils are connected 
 to the commutator segments, and to each other, so that the wind- 
 ing is in effect the same as though made with a continuous wire 
 connected at intervals to the commutator segments. 
 
 The armature core may be an iron cylinder or drum, made out 
 of discs of sheet iron laid together (Fig. 165), instead of an iron ring. 
 In this case the winding seems more complicated, but its general 
 plan is similar to that of the ring armature. The winding consists 
 of a number of coils wound uniformly over the surface of the drum, 
 which are connected together in such a way that the zvinding is elec- 
 trically the same as though it had been made zvith a single long wire. 
 
 155 
 
FIG. 165 
 
 FIG. 166. 
 
 The coils are connected to the commutator bars exactly as in the 
 ring armature, and their effect in producing electrical pressure when 
 the armature is revolved is just the same as has already been explained 
 in the case of the ring armature. Armatures with drum shaped cores 
 are called Siemens or drum armatures. A Siemens armature with 
 four coils is shown in Fig. 166, from which may be seen the way in 
 which the wires are wound on the core and connected to the commu- 
 tator. The same figure shows one coil wound upon an armature core 
 which is intended for sixteen coils. Commercial armatures usually 
 have from thirty to one hundred coils. 
 
 It has already been said that the early Gramme armature cores 
 were made out of iron wire coiled up to form a ring. In modern 
 machines the cores for both Gramme and Siemens armatures are 
 built up of discs, which are punched out of sheet iron (Fig. 165). 
 These discs are usually insulated from each other by thin tissue 
 paper. The object of dividing the cores into discs or laminating 
 them, and of insulating the discs from each other, is to prevent currents 
 from being set up in the core itself when it is revolved in the . mag- 
 netic field. The rule that electric pressures are set up when a con- 
 ductor cuts lines of force, applies equally as much to the core of the 
 armature as to the windings. Currents tend to flow in armature cores 
 from one end to the other at the surface, and back again near the. 
 center of the core. By properly laminating the cores these currents 
 are nearly all prevented, while the passage of lines of force all the way 
 through iron from one side of the core to the other, is not interfered 
 with. The great objection to permitting currents to circulate in 
 armature cores is the fact that it takes power to keep them circulat- 
 ing, and all this power is converted into heat in the armature core, 
 and is wasted. Compare Lesson VIII, The heating of the core has 
 
 156 
 

 FIG. 167. 
 
 another disadvantage since a high temperature is likely to injure the 
 cotton and shellac insulation which is used between the coils them- 
 selves, and between the coils and core. Even with the best of lamina- 
 tion a certain amount of power is lost, and heating is caused, by cur- 
 rents circulating in the core discs. These currents are ordinarily called 
 eddy currents because they eddy uselessly through the core, mfoucault 
 currents after the name of a scientist who made some investigations 
 many years ago relating to the generation of currents in masses of 
 metal. 
 
 There is an additional cause -of lost power and heating in the 
 cores of armatures which cannot be reduced by lamination. This 
 seems to be due to a sort of friction between the molecules as they 
 are caused to turn over by the attraction of the magnetic field while 
 the armature revolves. Every time the molecules are caused to 
 turn around under the influence of a magnetic field, a certain 
 amount of power is used, which is converted into heat; conse- 
 quently, for every revolution of the armature a certain amount of 
 power is used and converted into heat. This effect is called hys- 
 teresis. The amount of power wasted and heat produced in a core 
 on account of hysteresis depends upon the amount of iron in tne 
 core, the number of revolutions made by it in a minute, the density 
 of magnetism in the iron, and the quality of the iron. It may be 
 said that, in general, the softer the iron the less is the loss due to 
 hysteresis; consequently, the iron used in armature cores is very soft 
 wrought-iron or steel which has been carefully annealed. 
 
 The magnetic field in which the armature revolves is ordinarily 
 produced by a great electromagnet, as has been said in the preceding 
 lesson. The frame of the electromagnet is so arranged that it can 
 hold the windings required to set up the lines of force, and in order 
 that the lines may be caused to pass through the armature the poles 
 are arranged to embrace the armature. These expanded poles are 
 called polepieces (P P in Fig. 167), and the whole of the magnet 
 frame is called the field of the machine. The parts of the field upon 
 which the windings are placed are often called the field-cores (m. m. 
 in Fig. 167.) 
 
 It is always necessary to allow a certain amount of space 
 
 157 
 
between the pole-pieces and the surface of the armature, and in 
 addition a certain amount of space is occupied by the armature wind- 
 ings, so that a considerable depth of non-magnetic material exists 
 between the iron of the pole-pieces and the iron of" the armature core. 
 This space is usually called the air space or gap (G, Fig. 167). 
 
 The number of ampere turns (L,esson VI, page 39) which are 
 required to give the magneto-motive force which is needed to set up 
 the lines of force necessary to induce a given electrical pressure in the 
 armature windings depends upon the reluctance of the armature core, 
 of the air gap and of the magnet frame. Since there is no insulator ot 
 magnetism, some of the lines of force which are set up in the field will 
 leak around the armature instead of passing through it, and the 
 cross-section of iron in the path of the lines of force through the 
 field must be sufficiently large to hold these leakage lines as well as 
 the useful ones which pass through the armature. It is the leakage 
 or stray lines of force which magnetize watches when they are 
 carried near a dynamo. In order that the proportion of the total 
 number of lines of force that leak around the armature shall be 
 as small as possible, the reluctance of the air gap, which is always a 
 considerable part of the total reluctance in the magnetic circuit, 
 must be made as small as possible. 
 
 It is also of advantage to make the air space reluctance, and 
 therefore the total reluctance of the magnetic circuit as small as pos- 
 sible because the number of ampere turns which are required to set 
 up the field magnetism, are thereby reduced, and the expense of 
 building the machine is consequently decreased. For this purpose, 
 the armature core is often made toothed and the windings are placed 
 in the slots or grooves between the teeth. It is sometimes thought 
 that placing the armature conductors in grooves between teeth in the 
 core permits some of the lines of force to pass through the core In 
 such a way that they are are not cut by the conductors as the arma- 
 ture revolves. This is a mistake, however, and armatures with the 
 conductors wound in slots give exactly the same electrical pressure 
 when revolved in a magnetic field as is given by an armature with 
 the same number of conductors wound on the surface of its core when 
 it is revolved at the same speed in a field of the same strength^ 
 
 We have seen that the operation of dynamos is a direct applica- 
 tion of Faraday's discovery that an electrical pressure is generated in 
 a conductor when it is moved in a magnetic field. Electric motors 
 work on the principle that a conductor carrying a current tends to 
 move when placed in a magnetic field, on account of the mutual 
 action of the lines of force of the field and of the ciirrent. The reasons 
 for these actions we do not know, but we know their existence as the 
 result of experiment and are able to apply their results to practical 
 use. These two principles are practically the reverse of each other, 
 and the action of dynamos and motors is therefore a reversible one. 
 
 158 
 
That is, a machine which is designed to be used as a dynamo to gen- 
 erate electric currents when driven by mechanical power, may usually 
 be used equally well to generate mechanical power when driven as a 
 motor by an electrical current. It is a fact that the best dynamos 
 usually make the best motors, and manufacturers sell their standard 
 machines to be used either as generators or motors. We shall there- 
 fore treat them as entirely similar in construction. It is only when 
 the machines are built to be used for some special purpose that they 
 cannot be conveniently interchanged in their action. 
 
 The points required in a good dynamo or motor for general use 
 are a powerful magnetic field, which requires a small magnetic reluc- 
 tance in the magnetic circuit; as little waste as possible of power by 
 heating, which requires that the windings shall be well designed and 
 that a good quality of iron which is well laminated shall be used in 
 the armature core; and good insulation of the windings from elec- 
 trical contact with the iron cores and of the various turns of the 
 windings from each other. L" a machine is striped with gold paint, 
 it does not necessarily follow that it is a well built machine. A good, 
 plain finish is of advantage in an electrical machine, because it gen- 
 erally shows the good quality of the' workmanship which is always nec- 
 essary in a satisfactory machine. A good finish is also desirable because 
 it quickly shows dirt and bad treatment and thus makes evident any 
 neglect on the part of the dynamo attendant. Dirt and dampness are 
 two great enemies of the insulation of dynamos and motors, and the 
 machines must therefore be kept perfectly clean and dry in order that 
 they may operate well and last indefinitely without unnecessary 
 repairs. 
 
 When a dynamo armature is revolved in a magnetic field so as to 
 produce a current, the lines of force belonging to the current are attract- 
 ed by the lines of force of the field. This attraction tends to stop the 
 motion so that power has to be exerted to keep the armature moving, 
 and the total electrical power produced is eaus 1 to the power exerted 
 on the armature less that which is lost by mechanical and magnetic 
 friction (Lesson VIII, page 52). The useful electrical power which is 
 delivered by the dynamo to its external circuit is less than the total 
 electrical power generated, by the amount which is lost in heating 
 the armature core by eddy currents and in heating the armature and 
 magnet windings by the useful currents. 
 
 When a machine is operated as a motor by furnishing current 
 to it from an external source, the same losses exist, so that the 
 amount of electrical power which must be furnished to it is greater 
 than the mechanical power which is taken from its pulley. When 
 the motor armature is caused to revolve by the magnetic attractions, 
 its conductors cut the lines of force of the field, and an electric pressure 
 is therefore set up in them. The direction of this is opposite to that 
 of the external source which sends the current through the armature. 
 
 159 
 
FIG. 168. FIG. 169. FIG. 170. 
 
 The electric pressure which is thus set up in the armature conductors 
 of the motor is called a counter electric pressure or counter electro- 
 motive force. The wor^k which is done by the motor is dependent 
 upon it, and a useful electric motor which does not produce a counter 
 electric pressure is as impossible of existence as is a perpetual motion 
 machine. Seekers after either are looking for the impossible. 
 
 Dynamos may be divided into three classes depending upon the 
 way in which their field magnets are wound. These are: i. Series 
 wound (Fig. 168), in which the field winding is connected in series 
 with the external circuit, and all the current generated by the 
 dynamo passes through a thick wire which is wound a comparatively 
 few times around the field cores; 2. Shunt wound (Fig. 169), in 
 which a field winding of high resistance is connected in parallel, or 
 as a shunt, to the external circuit, and only a portion of the current 
 generated by the dynamo passes around the field cores through a 
 great many turns of fine wire; 3. Compound wound (Fig. 170), 
 which is a combination of the first two, so that the fields are magne- 
 tized in the same direction by both a shunt and a series winding. 
 If three dynamos of the same size and shape have fields wound in 
 the three different ways, the number of ampere turns in the magnet- 
 izing coils must be the same in each. Since the series winding 
 carries a large current, the number of times the current must pass 
 around the magnet core to make a given number of ampere turns is 
 comparatively small, and the winding has comparatively few turns. 
 The shunt winding carries a comparatively small current and this 
 current must therefore pass many times around the core in order that 
 it may have the same magnetizing effect as the large current passing 
 a few times around the core. In the compound winding, the number 
 of series turns and of shunt turns must be so proportioned that the 
 number of ampere turns made up by both together shall be the same 
 as in the other cases. 
 
 The purpose for which a dynamo is to be used, almost always 
 fixes the style of its field windings. Series wound dynamos are 
 ordinarily used for furnishing a current of constant strength to arc 
 lamps which are connected in series (Fig. 179). Series windings are 
 
FIG. 180. & FIG. 179. 
 
 also used on the fields of street railway motors. Shunt or compound 
 wound dynamos are used for furnishing the current to incandescent 
 lamps or electric motors which are all connected in parallel (Fig. 
 1 80) between wires which are kept at a constant difference of press- 
 ure, and shunt wound motors are commonly used to furnish power 
 for stationary purposes. Compound dynamos have quite an advan- 
 tage for furnishing current to be used by electric motors, that is, for 
 power distribution, because they automatically keep the pressure con- 
 stant through the combined action of the shunt and series field wind- 
 ings. The pressure supplied by shunt dynamos decreases to a cer- 
 tain degree, as the current furnished by the armature increases, on 
 -<. ccount of the resistance of the armature, and because the magnetism 
 stt up by the current in the armature coils interferes with the field 
 magnetism. The magnetizing power of a series winding, of course 
 increases with the current which is furnished by the machine, and 
 the natural fall of pressure in a shunt dynamo may be entirely over- 
 come, or even reversed, by the addition of series turns. When shunt 
 dynamos are used, it is necessary to regulate the strength of the field 
 magnetism by means of a variable resistance which is connected into 
 the field circuit (Fig. 180). This resistance is often called afield 
 rheostat or hand regulator. 
 
 In order that the number of ampere turns required to set up the 
 magnetism in a dynamo shall not be excessive, it is important to 
 make the magnetic reluctance (Lesson VI, page 41) in the path of the 
 lines of force as small as possible. On account of this, the magnet 
 frame composing the magnetic circuit of the field is substantially 
 made of iron. In many machines good wrought iron is used because 
 its permeability is greater than that of cast iron, but cast iron costs 
 less per pound than wrought iron, so that some manufacturers use 
 cast iron in the fields of their machines. In this case a greater 
 
 161 
 
 MCTNIVF 
 
 ^^C 
 
162 
 
FIG. 176. 
 
 163 
 
FIG. 177. 
 
 FIG. 178. 
 
 13-J.. 
 
weight of cast iron is used to make up for its lower permeability, 
 but on account of the smaller cost of cast iron the heavier machine 
 may not be any more expensive than the lighter one in which 
 wrought iron is used. Fig. 171 shows a very common form of 
 machine in which the fields are made of wrought iron, except the 
 pole pieces which are of cast iron. In Fig. 172 is shown a machine 
 in which the fields are made wholly of wrought iron. The form of 
 these machines makes it necessary to support the magnet frame and 
 armature bearings by a cast iron bed plate. The one horse-power 
 " Letter Type n generator of the Westinghouse Company is a machine 
 in which the fields are wholly of cast iron. 
 
 In some machines the magnet frames are made of very soft steel 
 castings. This metal has fine magnetic qualities and therefore is 
 specially excellent for use where light weight is important. The 
 field of the great 2,000 horse-power dynamo which was used to fur- 
 nish current to the electric motors of the Intramural Railway at the 
 World's Fair and which is now furnishing current to electric street 
 car motors, is made of steel. Fig. 173 shows a street railway motor 
 with a steel magnet frame. 
 
 Not only does the material of which the frame of a dynamo is 
 made depend to some extent upon the use for which the machine is 
 intended, but the form of the machine is also a matter of choice 
 which depends to a considerable extent upon the object of the ma- 
 chine. For instance, the motor shown in Fig. 173 is iron clad, that 
 is, the steel frame surrounds the field windings and armature. This 
 arrangement protects the windings from danger of mechanical injury 
 and from the danger of being splashed by water thrown by the car 
 wheels from puddles in the street. Water will quickly ruin the 
 insulating qualities of the cotton thread and canvas which are largely 
 used to insulate the wires on dynamos and motors. 
 
 The commonest forms of dynamos and motors which are in 
 general use are shown in Figs. 171, 172, 173, 174, 175, 176, 177 and 
 178. In Figs. 177 and 178, the fields have four and eight poles. 
 These are called multipolar to distinguish them from the more com- 
 mon two pole or bipolar machines. Multipolar machines may have 
 any number of pairs of poles which their dimensions will admit. 
 The armatures which are used in multipolar machines are wound 
 upon the same principle as those used in bipolar machines, which 
 have been explained. The number of sets of brushes required to 
 take the current from the commutator of a multipolar machine is 
 commonly equal to the number of poles, but sometimes certain 
 special connections are made in the armature, which make it possible 
 to use only two sets of brushes as shown in Fig. 178. Machines 
 having the form shown in Figs. 174 and 175, are often spoken of as 
 consequent pole machines because the lines of force appear to enter 
 the armature from the center of the frame. Nearly all dynamos and 
 
FIG. 181. 
 
 FIG. 182. 
 
 motors have forms which are simply variations of those shown here. 
 
 When a dynamo is started for the first time, it is necessary to 
 magnetize its fields from some other machine. The iron usually holds 
 sufficient residual magnetism to afterwards start the machine into 
 operation, Jand whenever started it will quickly build up its magnet- 
 ism to full strength. In order that a dynamo may properly magnet- 
 ize itself, it is necessary that the field windings be connected to the 
 brushes so that the current generated by the residual magnetism will 
 pass around the fields in the proper direction. If the connections be 
 made properly, but the direction of rotation of the armature be then 
 reversed, the connections must also be reversed. This is illustrated 
 in Fig. 181, which shows the difference in the connections of a 
 shunt dynamo when the direction of the armature rotation is re- 
 versed. 
 
 The most important detail to look after when a dynamo is in 
 operation is the condition and position of the brushes. Dynamo and 
 motor brushes are sometimes made of copper, in which case a bunch 
 of copper wires carefully laid up together and soldered at one end, or 
 a number of thin copper sheets laid together and soldered at one end, 
 is commonly used. Copper brushes usually touch the commutator 
 on a bevel (Fig. 182). Sometimes carbon brushes are used. These 
 are usually blocks of copper-plated carbon which touch the com- 
 mutator either on a bevel or radially. The brushes are held against 
 the commutator by means of spring brush holders. When in proper 
 position they are exactly opposite each other on a certain diameter of 
 the commutator of a two pole machine. With the brushes in the 
 proper position, a good machine will usually deliver its current with 
 little or no sparking, while the machine may spark badly if the brushes 
 are in any other position. The position of no sparking may change 
 with the load on the machine, in which case the brushes on a dynamo 
 must be moved forward as the load increases, and the brushes on a 
 motor must be moved backward under the same conditions. 
 
 Further questions relating to the handling of dynamos and 
 motors naturally enter into the following lessons. 
 
 Copyrighted, 1894, 
 
 166 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XX . 
 
 Points for Review. 1. How is a Gramme armature wound? 
 
 2. How is a Siemens armature wound? 
 
 3. In what way does the current which is produced by the operation of a Gramme 
 or Siemens armature flow through the armature itself? 
 
 4. What is the object of laminating armature cores? 
 
 5. What are foucault or eddy currents? 
 
 6. What is hysteresis? 
 
 7. Why is a watch likely to become magnetized when brought near a dynamo? 
 
 8. Why are some dynamos more likely than others to magnetize watches which are 
 brought near them? 
 
 9. What is the fundamental principle of the operation of dynamos? 
 
 10. What is the fundamental principle of the operation of electric motors' 
 
 11. What is the relation between generators and motors? 
 
 12. What points are necessary in a good dynamo? 
 
 13. Why is it necessary to keep dynamos clean and dry? *" 
 
 14. What is the relation between the mechanical work done by an electric motor and 
 its counter electric pressure? 
 
 15. Why would an electric motor which did not develop a counter electric pressure 
 be useless? 
 
 16. What are the three types of windings which are placed on dynamo field magnets? 
 
 17. For what purposes are series wound machines ordinarily used? 
 
 18. For what purposes are shunt wound machines ordinarily used? 
 
 19. For what purposes are compound wound machines ordinarily used? 
 
 20. What are multipolar machines? 
 
 21. How are compound and shunt machines regulated? 
 
 22. What must be done to the field connections of a dynamo when the direction of 
 rotation of its armature is reversed? 
 
 23. Why must the brushes of a dynamo which is in operation be carefully looked 
 after? 
 
 WESSON XXI. 
 
 ARC LIGHTING AND ARC LIGHT MACHINERY. 
 
 The arc lights which are so much a necessity today for illum- 
 inating the streets of cities and all large spaces which require a high 
 degree of illumination, whether in-doors or out, are the direct com- 
 mercial outgrowth of another magnificent discovery which was 
 announced shortly after 1800. This discovery was, indeed, nothing 
 less than the possibility of producing the common electric arc. The 
 discoverer of the electric arc, Sir Humphrey Davy, an English scien- 
 tist, exhibited it on a grand scale in 1808 in a lecture before the 
 Royal Institution in London, when he connected the electric circuit 
 of two thousand or more cells through two pieces of charcoal and 
 then gradually separated them. The result was an arch or " arc" of 
 
 167 
 
dazzling light between the charcoal tips such as had never before 
 been artificially produced. Sir Humphrey Davy's experiments cre- 
 ated a great deal of interest but the real usefulness of the electric arc 
 was not seen until Faraday's later discoveries had laid the founda- 
 tion for the development of the dynamo and the economical produc- 
 tion of electricity. 
 
 The means for producing this arc of light are comparatively sim- 
 ple. When two pointed pieces of carbon (made from charcoal, coke, 
 etc.) are joined to opposite poles of the circuit from a powerful gen- 
 erator of electricity and are touched together, a current flows between 
 triem. Where their points are in contact a considerable resistance 
 exists, and the points are heated by the current unless they are 
 pressed very tightly together! If the contact is quite loose the points 
 become so hot as to cause the carbon to pass off as vapor. Now if 
 the carbon points be separated the current will continue to flow 
 across the space between the points which is filled with carbon vapor, 
 forming the electric arc. Carbon vapor is a much better conductor 
 of electricity than air, and the current can therefore be caused to 
 flow across a space filled with it, though it could not readily be 
 caused to flow continuously through the same space filled with 
 air. 
 
 It seems strange to speak of the vapor of carbon, but the 
 temperature of the electric arc is so great that it boils and vaporizes 
 the most refractory materials. The vaporizing of any material is 
 merely a question of temperature and the vaporization of carbon, 
 platinum, gold, iron, copper, etc., in the electric arc is just as simple 
 as the conversion of water into steam (the vaporization of water) 
 over a common coal fire. The vaporization of "refractory" 
 materials like carbon, platinum, etc., simply requires a much higher 
 temperature than that which is reached by the coal fire which is 
 amply sufficient to boil water. 
 
 After the electric arc has existed for a little time between the 
 carbon points, the points look very much as shown in Fig. 183. 
 Both points become quite hot and give off light, but the positive 
 point becomes much hotter than the negative, and from it comes 
 the greater part of the light of the arc. In an arc which is set up 
 with a continuous current, carbon is carried off by the current from 
 the positive point and deposited on the negative point. The posi- 
 tive point therefore becomes a little hollowed out on the end as 
 shown in the figure. This hollow is called the crater of the arc. 
 As the greater part of the light of the arc comes from this positive 
 end or crater, the positive carbon in arc lamps is almost always put 
 at the top, in order that the light may be thrown downward. When 
 an arc is set up with an alternating current, both points become some- 
 what crater-like and light is given off about equally from the two 
 points. 
 
 168 
 
FIG. 185. 
 
 FIG 186. 
 
 FIG. 194. 
 
 1G9 
 
FIG. 188. 
 
 FIG. 189. 
 
 FIG. 191. 
 
 170 
 
Since the arc is surrounded by air the carbon of which the points 
 are composed is gradually burned up, and it the carbons are fixed 
 in position the arc gra^s longer and longer until its resistance 
 becomes so great that the Current can not pass through it; the current 
 then stops and the arc goes out. Since carbon is carried away from 
 the positive point and deposited on the negative point, the former 
 wastes away at a rate which is just about double that of the latter. 
 
 In order that the electric arc may be used for commercial light- 
 ing an automatic device must be used to keep the carbons fed 
 towards each other as they waste away, so that the arc shall always 
 have the proper length. This is included in the mechanism of what 
 are known as arc lamps. These consist of a case which contains 
 the feeding mechanism, below which is a frame to support the lower 
 or negative carbon and a glass shade. The feeding mechanism has 
 two duties to perform: i. To separate the carbons, or strike the arc, 
 when the lamp is thrown into circuit; 2. To regulate the movement 
 of the upper or positive carbon downwards towards the negative one 
 as the carbons wear away. The lower carbon is usually clamped 
 solidly at the bottom of the lamp frame, and the upper one is 
 clamped at the end of a polished brass carbon rod the motion of 
 which is controlled by the mechanism. 
 
 Fig. 184 shows the familiar form of an arc lamp. Figs. 185 to 
 1 88 show the mechanism of different lamps. 
 
 The mechanism of the lamp is. usually caused to operate by the 
 opposing action of two electromagnets. The windings of one of 
 these are composed of few turns of comparatively coarse wire which 
 are connected directly into the circuit in series with the arc. The 
 windings of the other magnet are made of many turns of compara- 
 tively fine wire which are connected as a shunt to the arc. The two 
 electromagnets may be plainly seen in Figs. 185 and 186. 
 
 In some lamps, both windings are put on the same magnet as is 
 shown in Fig. 187. The purpose of the windings is the same in the 
 two arrangements and may be explained by reference to Figs. 185 
 and 1 86. A brass lever which runs across the lamp, carries an iron 
 armature or plunger at each end. The armatures are in such posi- 
 tions that they are attracted by the two electro-magnets, and the 
 lever is attached to the mechanism which controls the carbon rod. 
 The lamp is trimmed or carboned with the tips of the two carbons 
 resting against each other. When the lamp is thrown into circuit 
 the full current of the circuit flows through the series winding and 
 the lever is lifted by the attraction of the series magnet. This causes 
 the mechanism to grip and raise the carbon rod sufficiently to strike 
 the arc. As the carbons burn away the electrical pressure between 
 their points becomes greater so that the current in the shunt coil 
 increases. The armature of the shunt coil is attracted more and 
 more strongly and the lever is slowly tipped until the clutch releases 
 
 171 
 
the carbon rod sufficiently for it to slide slowly downward and thus 
 feed the positive carbon towards the negative. In order that the 
 lamp may burn smoothly and quietly v ?it is necessary for the 
 feeding mechanism to keep the carbons at a uniform distance 
 apart while the lamp is burning. This can only be accomplished 
 when the magnetizing coils are properly balanced against each 
 other and the strength of the spring, which acts on the lever, 
 is properly adjusted. Even when all the adjustments are exactly 
 right arc lamps will not burn well unless the carbons are of uniform 
 quality. In some arc lamps, the carbon fod is controlled by a clock 
 work, which in turn is controlled by the differential magnets (Fig. 185 
 and 1 86,) while in others, a simple clutch is caused to act on the carbon 
 rod by the magnets (Fig. 187). 
 
 In another style of lamp the differential action of the magnets is not 
 utilized, but the pull of the shunt magnet is arranged to act against the 
 force of a spring. The winding of the magnets is quite similar to 
 those already referred to, as shown in Figs. 188 and 189, in which 
 both the lamp as a whole and a diagram of the windings are 
 exhibited. This style of lamp is trimmed so that a little space 
 remains between the carbon points. Its action is as follows: 
 
 When the lamp is thrown into circuit the current flows through 
 the series winding P and thence through the contact at N to the 
 other side of the lamp. This causes the magnet to attract the arma- 
 ture A and the hold of the clutch on the carbon rod is released. The 
 upper carbon at once drops against the lower one, thus throwing the 
 starting coil J into circuit in shunt with the coil P. The starting 
 coil attracts the armature just above it and opens the contact at N. 
 This throws the shunt coil K and the series coil P into series with 
 each other, and they form a shunt across the poles of the lamp. The 
 large magnet is thus sufficiently weakened to allow the spring S to 
 raise the armature A which actuates the clutch mechanism and 
 strikes the arc. Then as the arc* increases in length the electrical 
 pressure between the carbons increases, the current flowing through 
 the combined coils K and P increases and the armature A is suffi- 
 ciently attracted to slightly release the carbon rod and thus cause the 
 lamp to feed. 
 
 As a general rule, arc lamps are connected in series (Lesson 
 XX, Fig. 179), so that the same current passes through all. This 
 current is usually furnished by a series dynamo which automatically 
 keeps the magnitude of the current constant. The constancy of the 
 current is a very important element in the proper regulation of the 
 lamps. Nearly all arc lamps are now adjusted so that the pressure 
 required to pass the current through the arc is from 45 to 50 volts. 
 If the pressure is made smaller the arc becomes shorter and gives less 
 light, and it produces a continuous hissing or frying sound. If the 
 pressure is greater, the arcjflames and flickers, which makes it unsat- 
 
FIG. 195. 
 
 FIG. 196. 
 
 FIG, 190. 
 
 FIG. 197. 
 
 UNIVERSITY 
 
 -173 
 
FIG. 199. 
 
 FIG. 184. FIG. 193. 
 
 174 
 
isfactory. The current used usually approximates 9.6, 6.5, or 4 
 amperes. Arc lamps which are intended to be used with 9.6 amperes 
 are usually spoken of as 2,000 candle-power or 450 watt lamps, while 
 those intended to be used with 6. 5 and 4 amperes are usually called 
 1, 200 candle-power and 600 candle-power lamps. A candle power is 
 equal to the light given off by a sperm candle of fixed size and form. 
 The actual candle power given off by the lamps is much less than 
 these figures, and in fact the light given off in different directions 
 varies from a hundred candle-power or thereabouts to nearly the rated 
 value of the lamp. Fig. 190 shows by the curve the amount of light 
 given off by arc lights in different directions when using various 
 currents. The greatest amount of light is given off at an angle of 
 about 45 from the direction of the carbons. For this reason the 
 best effect may be gained from arc lights used in illuminating streets by 
 hanging them from 25 to 35 feet from the ground over the center of 
 the streets, or mounting them at street corners on tall poles such as 
 that shown in Fig. 191. Inside of buildings they are usually hung 
 from small boards fastened to the ceiling-. A switch similar to that 
 shown in Fig. 192 is usually placed in arc lighting wires where they 
 enter a building. 
 
 As the carbons which are ordinarily used in arc lamps are of 
 such a length that they will only burn for seven or eight hours, 
 double lamps (Fig. 193) are used for all night lighting. These con- 
 sist of a modified mechanism which controls two carbon 
 rods, one of which does not come into service until the car- 
 bon held in the first has burned out. The carbons that are 
 ordinarily used vary from ^ to ^ inches in diameter and are 
 usually coated with copper to reduce their resistance. The positive 
 carbon is about twelve inches long and the negative is about six 
 inches long. The carbons are made from finely ground coke or 
 lampblack which is mixed with syrupy compounds and then baked 
 in moulds. The copper coat is put on by electroplating. Some- 
 times oval carbons about one inch broad and a half inch thick are 
 used in single lamps for all night burning. 
 
 The number of successful manufacturers of arc light machinery 
 is comparatively small. The earliest to enter the business in this 
 country with commercial success was the Brush Electric Co. To 
 this company is probably due the introduction of lamps with differ- 
 ential magnets, which are still so much used. The Brush arc 
 dynamo is shown in Fig. 194. 'Figs. 195, 196, 197 and 198 show- 
 respectively the Thomson-Houston, Wood, Standard, and Western 
 Electric arc dynamos. The regulation of each of these is performed 
 by moving the brushes around the commutator, so that as lamps are 
 cut into and out of circuit the pressure is varied so that the current 
 is always kept of constant value. 
 
 In order that the dynamos in an arc light generating station 
 
 175 
 
may be properly managed, it is necessary to have some arrangement 
 by which any dynamo in the station may be connected to any one of 
 the circuits which run out to the lamps. The number of dynamos 
 and circuits may be quite large in a plant which is located in a large 
 city. The arrangement that is usually used for the purpose is a 
 switch-board (Fig. 199) fitted with a heavy spring jack for each wire 
 leading to the dynamos and another similar spring jack for each wire 
 leading to the lamp circuits. The spring jacks may be connected as 
 desired by plugs and cords, very much as a telephone operator con- 
 nects two subscribers (Lesson XVII, page 135). The figure shows a 
 switchboard arranged for three lamp circuits marked i, 2, 3, and for 
 three dynamo circuits marked A, B, C. Each dynamo is shown to 
 be connected to a lamp circuit by means of plugs and cords. The 
 amperemeters at the top of the switchboard are connected in the 
 dynamo circuits and serve to show the dynamo attendant whether 
 or not the machines regulate properly. 
 
 Copyrighted, 1894, 
 
 176 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXI. 
 
 Points for Review: 1. What is the electric arc? 
 
 2. Why was the arc not put into service for commercial illumination immediately 
 after its first production? 
 
 3. What are arc lamps? 
 
 4. What is the principle of operation of arc lamps? 
 
 5. How are arc lamps usually connected in circuit and what kind of a dynamo is 
 used to produce the current supplied to them? 
 
 6. How much current is used in the ordinary arc lamps, and what pressure as 
 required for each arc? 
 
 7. Why are arc lamps which are used for street illumination placed at a consider- 
 able height from the ground? 
 
 8. How are arc light carbons made? 
 
 9. What is the object of the regulator on arc light dynamos? 
 10. How are arc light switchboards generally arranged? 
 
 XXII. 
 
 INCANDESCENT LIGHTING AND POWER TRANSMIS- 
 SION: TWO, THREE AND FIVE WIRE SYSTEMS 
 OF DISTRIBUTION FOR ELECTRIC 
 LIGHTS AND MOTORS. 
 
 Illumination by arc lights is very satisfactory in streets or open 
 spaces out of doors or in large rooms such as shops or halls, but its 
 intense brilliancy causes it to cast dense shadows which totally unfit 
 it for satisfactory use in general indoor lighting. Its unavoidable 
 flickering and occasional hissing also make it unsatisfactory for 
 general use in small rooms. If the faults of the arc when used for 
 general indoor lighting were not so evident, the use of small arcs in 
 office and house lighting might have been attempted as early as 1880, 
 by which time the arc lamp had begun to prove its value for outdoor 
 lighting. 
 
 By 1880 the disadvantages of the arc for general illumination 
 had become known and inventors were using every effort to find 
 some substitute. Many years earlier, inventors had made electric 
 lamps which consisted of a loop of wire made of platinum or iridium, 
 two metals which melt only at exceedingly high temperatures, and 
 in which the light was produced by heating the wire white hot, or 
 to incandescence, by means of a current. The light was therefore 
 
 177 
 
produced by means of the great heat caused in the wire when a cur- 
 rent flowed through the high resistance of the wire (L,esson VIII, 
 page 53). This is a case where the C 2 R loss was turned to a useful 
 account but the lamps were not successful, though the same principle 
 is used in the incandescent lamps of today. Just previous to 1880 
 many prominent inventors, including Edison, Maxim, Farmer, and 
 Sawyer and Man in this country, and Swan in England, were mak- 
 ing every effort to construct a satisfactory lamp to operate by the 
 incandescence of some material. It was found that loops of platinum 
 and iridium were unsatisfactory because they soon melted or gave 
 out when continuously subjected to the high temperature which is 
 necessary to produce a satisfactory light. The only conducting 
 material which would stand the high temperature of incandescence 
 was found to be carbon. Unfortunately carbon burns away when 
 heated to a high temperature in the air, and therefore could not be 
 used in a lamp in the same way that the metallic wires had been. 
 
 As early as 1845 a lamp had been made in which a thin stick of 
 carbon was enclosed in a glass globe from which the air had been 
 exhausted. This lamp produced an excellent light, as the carbon 
 could not burn away in a vacuum, however hot it became, but no 
 satisfactory arrangements then existed for making proper carbon 
 sticks or for exhausting the air from the glass globes. Shortly 
 before 1880 the inventors turned from their efforts to [make a satis- 
 factory loop from a metal wire, to make another attempt to use carbon. 
 By 1880 Edison, Sawyer and Man and Swan had made lamps which 
 produced light through the incandescence of a thin strip or filament 
 of carbon. The lamp made by Edison looked very much like the 
 incandescent electric lamps of the present day, and it is no doubt to 
 his industry and ingenuity that we owe the cheap and economical 
 form of incandescent lamp which we now use. One of Edison's 
 early lamps is shown in Fig. 200. The globe or bulb of the lamp 
 contained a filament of carbonized paper in an arched or horse-shoe 
 form. The ends of the carbon horse-shoe were connected to short 
 pieces of platinum wire which passed through the glass of the bulb. 
 By means of these wires current could be led to the filament. The 
 bulb was exhausted (that is, the air was removed) by means of a 
 form of mercury air-pump, which is used in a modified form for the 
 same purpose at the present day, and which is capable of producing 
 a very perfect vacuum. 
 
 Figs. 201 and 202 show the two forms of air-pumps which have 
 been commonly used in exhausting lamps. These are often called 
 vacuum pumps because they are used to produce a vacuum. The 
 first is called the Geissler pump after its inventor, who was also the 
 maker of the vacuum tubes known as Geissler tubes, which display 
 such pretty color effects when an electric spark is passed through 
 them. The pump shown in the second figure is called a Sptengel 
 
 178 
 
FIG. 200. 
 
 FIG. 201. 
 
 pump, also after the name of its inventor. The Geissler pump may 
 be briefly described as an air pump made of glass in which mercury is 
 used as a plunger in order that leakage of air may be entirely avoided. 
 In Fig. 201, B 1 and B 2 are two glass bulbs which are connected by a 
 long U of glass. From the tube just below B 2 , a tap leads off to the 
 lamps which are to be exhausted. In this tap is a valve, C, which 
 closes by being pushed upwards, and a bulb, D, containing some 
 material which absorbs all the moisture from the air which passes 
 through it. This is used because it is necessary to keep the pump 
 perfectly dry and the air in the lamps before they are exhausted 
 always contains some moisture. Now, if by means of suction at P, 
 the mercury is caused to rise up in B 2 , it pushes all the air out of 
 B 2 through the valve V. The mercury is prevented from reaching 
 the lamps by the valve C. When the mercury is caused by suction 
 at I to drop down again to its old level, the valve V, as the mercury 
 leaves it falls back into its seat, so that no air can get in and the 
 bulb B 2 is left entirely free from air. It is therefore ready to 
 receive a new supply of air from the lamps when the mercury level 
 falls below the level of the lamp tap. The operation of alternately 
 exhausting the bulb B 2 and putting it into connection with the 
 lamps from which it receives a new portion of air, is continued until 
 the lamps are properly exhausted. The glass tube to which the 
 lamps are connected is then melted or sealed off at T, and the lamps 
 are finished. It is this sealing off from the pump that causes the 
 sharp tip at the top of commercial incandescent lamps. 
 
 179 
 
FIG. 206. 
 
 FIG. 202. 
 
 The operation of the Sprengel pump (Fig. 202), is quite similar 
 in principle to the operation of some injectors. The mercury is 
 allowed to flow in a jet through the nozzle J, and air is drawn from 
 the lamps by the suction of the drops of mercury rushing past the 
 end of the lamp tap. 
 
 The carbon filaments of incandescent lamps are now usually 
 made from bamboo strips or from silk or cotton threads. These are con- 
 verted into carbon by baking, in very much the same way that wood 
 is converted into charcoal in a kiln. The material is first made into 
 exactly the proper size to produce a filament. After proper treatment 
 it is then bent around blocks of carbon and is packed in a crucible 
 filled with powdered carbon. After baking for many hours the 
 material is converted into black carbon hairpins, the hairpin form 
 coming from the form of the blocks around which the material was 
 wrapped. To bring the filaments to the proper resistance and at the 
 sanie time put them into condition to stand the strain of the high 
 temperature of "burning," they are commonly "treated" by a process 
 which deposits very hard grey carbon upon their surfaces. The 
 filaments are then each mounted upon two short pieces of platinum 
 wire which are sealed into a bit of glass. The connection between 
 the carbon and the platinum is usually made satisfactory from an 
 electrical point of view by means of a cement. The filament thus 
 mounted is sealed into the bulb by a glass blower. The bulbs are 
 usually purchased ready-made from a glass factory. One of these 
 
 180 
 
FIG. 208. 
 
 FIG. 209. 
 
 bulbs is selected and a piece of glass tube is connected to the top of 
 the bulb. This serves as a handle for the workmen and also for con- 
 necting the lamp to the pump. The carbon is then inserted into the 
 neck of the bulb and the glass at the base of the carbon is so care- 
 fully welded into the glass base of the bulb that the union becomes 
 absolutely perfect. After exhausting, as already explained, the lamp 
 is complete. 
 
 For convenience in use, incandescent lamps are mounted on 
 bases to which they are fastened with plaster. These bases contain 
 two contacts which correspond to two contacts in a socket which may 
 be connected to an electric circuit. In Fig. 203, b is the lamp bulb, 
 c is the carbon filament, w w are the platimum leading in wires, j is 
 the cement connecting the carbon and platinum, f is the brass base 
 which is attached to the lamp by the plaster p, d and r are the two 
 contacts by which the carbon is brought into connection with the 
 electric circuit when the lamp is inserted in a socket, and t is the 
 point where the lamp was * 'sealed oft' ' the pump. 
 
 The bases used on lamps have various external forms depending 
 upon the manufacturer. Certain forms, known as the Edison, 
 Thomson-Houston, and Sawyer-Man, which have come into very 
 general use, are shown in Figs. 204, 205 and 206. The Westing- 
 house lamp, which is shown in Fig. 207, has a glass stopper in which 
 the leading-in wires are fixed, which is not welded or sealed fast to 
 the bulb, but the long joint between the stopper and the neck of the 
 bulb is ground until it fits so closely that it is air tight. As an addi- 
 tional precaution the outside of the joint is covered with cement. 
 
 In order that incandescent lamps may be as conveniently turned 
 on and off as gas lights, the sockets often contain switches as shown 
 in Fig. 208, which represents a socket with its brass shell removed. 
 
 181 
 
FIG. 207. 
 
 Where lamps are arranged to be controlled by wall switches, plain or 
 keyless sockets are generally used (Fig. 209). 
 
 Incandescent electric lamps and electric motors are sometimes 
 operated upon series circuits, but they are much more satisfactory when 
 connected in parallel (L,esson XX, Fig. 180) as is usually done. 
 The difference between the connection of lamps in parallel and lamps 
 in series may be illustrated by comparing the methods of utilizing 
 water power. Suppose a series of dams is placed in a stream and a 
 mill is placed at each dam. The water which passes through the 
 waterwheels of the first mill flows down to the second mill and passes 
 through its wheels and thus continues to flow through the wheels of 
 one mill after another. The wheels of each mill are therefore turned 
 by the same water that turns the wheels of every other mill. In 
 order that this may be the condition, each mill must be located on a 
 lower level than the one up stream from it. Then the total fall of the 
 stream is so divided that each mill gets advantage of a proper portion. 
 In series arc lighting the same current flows through all the lamps 
 one after the other, and the total pressure at the dynamo is divided 
 amongst the lamps. If an arc dynamo is capable of producing 1,000 
 volts it will operate twenty lamps in series since it takes about 50 
 volts to send the current through each arc. If a portion of the lamps 
 are cut out of circuit, the pressure at the dynamo must be reduced or 
 the current will increase above its proper value. 
 
 If a large dam is built on the stream, and the mills are located 
 so that they all take water from the same canal and discharge water 
 
 182 
 
FIG. 210. 
 
 into the same tailrace, the water of the stream is divided between 
 the mills in proportion to their needs, and their wheels are in par- 
 allel. The amount of water flowing through the wheels of each mill 
 in this case is directly proportional to the work being done in the 
 mill. If one mill is shut down the gate through which water Is 
 admitted to the wheel is closed, and no water flows through. The 
 water used by each mill is entirely independent of the amount used 
 by the others. In the same way, when electric lamps are connected 
 in parallel the current flowing through each lamp is entirely inde- 
 pendent of that flowing through the others, and simply depends upon 
 the resistance of the lamp and the pressure at its terminals. When 
 it is desired to cut out of circuit a lamp which is {connected in par- 
 allel with others, its connection with the circuit is Broken by a 
 switch (Fig. 208) so that no current can flow through it. This is 
 equivalent to closing the gate through which water enters a mill, as 
 already explained. 
 
 When it is desired to shut down one of a number of mills in 
 series, it evidently will not do to simply close the gates which admit 
 water to the wheels, as that would prevent the water from flowing to 
 the other mills, but it is necessary to arrange a short path for the 
 water to flow around the mill which is shut down. In the same 
 way when it is desired to turn off an electric lamp which' is operated 
 in series circuit, the lamp must be short circuited (Fig. 210). Some 
 special switches used on arc lighting circuits (Lesson XXI, Fig. 
 192) short circuit the lamp which is to be turned off, so that the main 
 line is properly completed, and then disconnect the lamp terminals 
 from the line. 
 
 Since the current which flows through incandescent lamps con- 
 nected in parallel depends upon the pressure at the lamp terminals, 
 and the light given by each filament depends upon the current flow- 
 ing through it, the pressure at the terminals of the lamps must be 
 kept perfectly constant or they will not give a steady light. If the 
 electrical pressure at the terminals of an incandescent lamp is changed, 
 
 183 
 
the light given off by the filament changes at a much faster rate. If 
 a lamp, for instance, which is intended for a pressure of no volts and 
 to give 1 6 candle-power, be connected to a 105 volt circuit, the light 
 which it gives is no more than about 12 candle-power and is of a 
 poor red color. If the same lamp be connected to a 115 volt circuit, 
 the light which it gives becomes about 20 candle-power and is of a 
 brilliant whitish color. The great candle-power and whiteness of 
 the light in the latter case shows that the filament is so excessively 
 hot that even refractory carbon cannot last long under the strain, and 
 the filament will soon give out. The length of time during which 
 the filament of an incandescent lamp will last that is, the life of 
 the lamp decreases very rapidly as the temperature at which the 
 filament burns is increased above its proper value. On the other 
 hand, the power required to produce light increases as the working 
 temperature of the filament decreases. It should, therefore, always 
 be the aim- to work incandescent lamps at the exact pressure for which 
 they were designed. 
 
 We have already seen that there is always a loss of pressure when 
 an electric current flows through a wire, this loss being equal to the 
 product of the amperes of current and the resistance of the wire 
 (L<esson VII, page 42). Consequently, when a number of incandescent 
 lamps in parallel are connected to a circuit at some distance from the 
 dynamo which supplies the current, the wires of the circuit must be 
 quite heavy in order that the loss in pressure shall not be too great. 
 
 When incandescent lamps and motors are connected to wires which 
 lead from a central generating station, it is common to allow a loss 
 of pressure or "drop" amounting to as much as ten percent to twenty 
 per cent of the dynamo pressure when all the lamps are turned on. 
 The circuits must be s v o arranged that all the lamps shall be fed with 
 current at as nearly the same pressure as possible, and if the drop 
 is allowed to be greater than twenty per cent, this becomes a difficult 
 matter. When the drop in the wires is too great, it is also difficult 
 to regulate the dynamo so that the pressure at the lamps shall not 
 vary when lamps are turned on or off. It is easy to see that every 
 lamp which is turned on or off changes the current flowing through 
 the wires of the circuit, and therefore changes the pressure lost in the 
 wires between the dynamos and lamps. In plants which are confined 
 to a single building the drop, when all the load (lamps and motors) 
 is turned on, is usually made to be from five to ten per cent of the 
 pressure at the dynamo. 
 
 On page 43 of Lesson VII, it is stated that the resistance of a wire 
 is directly proportional to its length and inversely proportional to the 
 square of its diameter. The diameters of wires are usually measured 
 in thousandths of an inch or mils, and the square of the diameter of a 
 wire when it is given in mils is called the cross section of the wire 
 in circular mils. The resistance of a commercial wire which is one 
 
 184 
 
foot long and has a cross section of one circular mil is about 10.5 
 ohms at ordinary temperatures. The resistance of any copper wire is 
 therefore equal to 10.5 times its length in feet divided by its cross 
 section in circular mils. As a formula which is easy to remember 
 this may be written ^R=^~^- (Tables giving the sizes of wires for 
 electric lighting nearly always have a column which shows their 
 cross-sections in circular mils.) In electric lighting and power dis- 
 tribution, the current goes out from the dynamo to the lamps and 
 motors on one wire and back on another of equal length, so that the 
 total length of the wire in the circuit is twice the distance along the 
 line from the dynamo to the lamps. We may put twice the 
 distance in place of the length, or 2D in place of L, and we have 
 ^__ axioox __2ix_D. -^ ow ^ ^e drop of pressure in a line of resistance R 
 
 when current C flows through it, is v^RxC= 81x ^ xa This finally 
 shows that the circular mils cross section required in a wire which is 
 intended to carry a current through a circuit of a given length with a 
 fixed number of volts drop is M 21x ^ xc - This may be stated in the 
 following way: The wire which conveys the current from dynamo to 
 lamps must have as many circular mils in its cross section as is equal 
 to twenty-one times the Distance from dynamo to lamps multiplied by 
 the Current in amperes and divided by the Drop in volts. 
 
 As an example, suppose that it is desired to find the size of wire 
 in the Brown & Sharpe (B. & S. ) Guage which will carry 50 
 amperes from a dynamo to lamps which are 500 feet away, the drop 
 to be about 10 volts. . Then, M^ 21 * 5 ?* 50 ^ 52500. A No. 36. & S. 
 wire has a cross section of 52634 circular mils and so should be 
 used, as it is the wire of nearest siz*e. 
 
 The volts drop in pressure in any case is fixed by the pressure at 
 the dynamos and the percentage loss of pressure which may be 
 allowed. For instance, if the pressure at the dynamos is 125 volts 
 and the loss is tec percent, then the volts drop is 12.5. Up to the 
 present time, it has not been found commercially possible to 
 produce incandescent lamps for a higher pressure than about 115 
 volts, and, consequently, nearly all incandescent lighting with contin- 
 uous currents is done at a pressure between 100 and 115 volts and 
 each 1 6 candle-power lamp takes about one-half an ampere of cur- 
 rent. If by any means the pressure at the lamps could be doubled 
 without any change in the amount of light given out for each 100 
 watts, each 16 candle-power lamp would require only about one- 
 fourth of an ampere. The pressure being doubled, the number of 
 volts in a given percentage loss would also be doubled. We see, 
 therefore, that the current divided by the drop is only one-fourth as 
 great with the double pressure, so that the wires required to carry 
 current a fixed distance for 200 volt lamps need only be one-fourth as 
 heavy as those required to carry current for the same number of 100 
 
 185 
 
FIG. 211. 
 
 volt lamps, or putting the statement in another way, the weight of 
 copper which is required to supply current at a fixed percentage loss 
 of pressure to a number of 100 volt lamps at a certain distance from 
 the dynamo will serve to supply four times as many lamps of double 
 the pressure. In the same way, it may be seen that if the pressure is 
 increased from 100 volts to 300 volts, the wires required to convey a 
 given supply of power a certain distance may be reduced to one-ninth 
 as great a cross-section, and therefore to one-ninth the weight, of 
 those required for the 100 volt distribution. 
 
 The general rule may be given as follows: When a given amount 
 of power is transmitted by electricity over a certain distance at afixea 
 percentage loss, the cross-section of the wires, and therefore their weight, 
 is in inverse proportion to the square of the pressure. This rule applies 
 equally whether the current is used for producing light, operating sta- 
 tionary motors or running street cars; the transmission of electricity 
 along a wire from a dynamo at one point to be used at another point 
 is electric transmission of power, whatever may be the purposes for 
 which the current is used. Electric lamps, arc or incandescent, may be 
 operated on the same circuits with electric motors or electric heaters, 
 or electric lamps may be taken out of a circuit and electric motors 
 put in their place, or vice versa, without altering the conditions. It 
 is well known that in many cities electric arc and incandescent 
 lamps, stationary motors and electrically heated flatirons and curling 
 irons are all furnished with the power necessary for their operation 
 from the same circuits. Electric street cars are often furnished with 
 light, heat and power, from the current conveyed to the car by the 
 trolley wire. 
 
 It is very easy to make the pressure quite high in circuits which 
 are arranged to transmit power from a central station to electric 
 motors alone, and thus keep the weight of the wires required within a 
 reasonable limit, since the electric motors may have their windings 
 designed for any reasonable pressure. Five hundred volts is quite 
 commonly used for circuits which are specially intended to supply 
 current to stationary motors and street car motors. Incandescent 
 lamps may be used on the circuits, but it is necessary to use them in 
 sets of five 100 volt lamps connected in series (Fig. 211.) This is 
 the arrangement which is used for lighting electric cars. For gen- 
 
 186 
 
0- 
 
 
 o- 
 
 KH 
 
 o 
 
 
 
 o 
 
 FIG. 212. 
 
 eral purposes, such an arrangement is not at all satisfactory, because 
 all the lights of each set must either burn or be extinguished. It is 
 not possible to have only one or two lamps of a set burn at once. 
 
 Putting only two 100 volt lamps in series on a 200 volt circuit is 
 more satisfactory than having five lamps in a set, but even this is 
 not satisfactory for general use. By means of the three wire system, 
 nearly as much saving in copper may be effected as by doubling the 
 pressure, and yet the individual lamps are entirely independent. A 
 diagram of the arrangement of the three wire system is shown in Fig. 
 212. A and B are two dynamos: the positive terminal of the first is 
 connected to the positive line wire, its negative terminal is connected 
 to the positive terminal of the second, and the negative terminal of 
 the second is connected to the negative line wire. A third wire 
 called the neutral wire is connected at a point between the two 
 dynamos and runs out on the line with the positive and negative 
 wires. The electric lamps are connected between the positive and 
 neutral wires and the negative and neutral wires, the lamps being 
 arranged so that the number on each side of the system is as nearly 
 equal as possible and so that the number of lamps likely to burn at one 
 time is equal on the two sides. When this condition is fulfilled as shown 
 in the figure, the system is said to be balanced, and the current flows 
 from the positive pole of the first dynamo through the positive wire to 
 the lamps on the positive side, through these lamps to the neutral wire 
 and thence directly through lamps on the negative side to the nega- 
 tive wire, and it returns to the negative terminal -of the second 
 dynamo. If the system is balanced, no current returns to the dynamos 
 through the neutral wire, and the dynamos operate exactly as 
 though they were simply connected in series. The function of the 
 neutral wire is to distribute the current from the lamps on the posi- 
 itive side of the system to those on the neutral side, and if more lamps 
 are in use on one side of the system than on the other, the extra cur- 
 rent is deli vered or returned to the dynamos through the neutral wire. 
 Fig. 213 shows by the arrows the way in which the current is distributed 
 to the lamps on a balanced system in which the lamps are not exactly 
 opposite to each other. Fig. 214 is a diagram of a three wire sys- 
 
 287 
 
 OF THE 
 TJNIVERSI 
 
 OF 
 
hOH 
 
 -o 
 
 
 
 -o- 
 
 
 
 k>J 
 
 0- 
 -0- 
 
 o 
 
 -0- 
 -0 
 
 o- 
 
 -0 
 
 FIG. 218. 
 
 FIG. 214. 
 
 FIG. 215. 
 
 tern with more lights connected to the positive than to the negative 
 side of the system. The arrows show the direction in which the 
 current flows in the wires. The positive wire carries enough current 
 to supply the lamps on the positive side of the system, and the dif- 
 ference between the current required to supply the two sides returns 
 through the neutral wire. The positive dynamo therefore carries 
 more load than the negative dynamo. In Fig. 215 the dynamos of 
 Fig. 214 are replaced by pumps and the lamps by water motors. 
 Again the arrows show the direction of the streams in the system of 
 piping. 
 
 18S 
 
FIG. 216. 
 
 The plan for increasing the dynamo pressure used to supply in- 
 candescent lamps, by connecting the lamps practically in series and 
 yet making them really independent of each other by means of a 
 neutral wire, may be extended. Fig. ' 2 16 is a diagram of the arrange- 
 ment with four lamps in series and five wires. This is known as the 
 five wire system. 
 
 The weight of wire required in a three wire system amounts to 
 a little more than one-fourth of the weight required for a two wire 
 system because of the introduction of the neutral wire. The actual 
 weight required is about three-eighths of that in a two wire system. 
 This saving in the weight of copper is a very important factor to 
 large electric lighting companies, as their copper feeders and mains 
 cost a great deal of money. The saving by the five wire system is 
 proportionally greater than that effected by the three wire system, 
 but it causes greater difficulty in keeping the pressure perfectly con- 
 stant at the lamps. The three wire system is used in a great many 
 plants in this and foreign countries which have been constructed by 
 the Edison Co. All the large Bdison illuminating plants in large 
 American cities use the three wire system. The five wire system is 
 constructed by the Siemens & Halske Co. It is used in a large plant 
 in Berlin and elsewhere. 
 
 Electric motors are usually operated on two wires except when 
 they are connected to electric lighting circuits, because, as already 
 explained, they may be wound for any desired pressure and it is not 
 necessary to use low pressure motors connected in series in order to 
 get an economical pressure for distribution. 
 
 Since the armatures of electric motors are usually of low resist- 
 ance, it is necessary to connect a resistance box in series with the 
 armature when starting a motor on a constant pressure circuit. This 
 resistance box contains sufficient resistance so that a little more 
 than the ordinary full load current is allowed to pass through the 
 armature when it is standing still. The machine consequently starts 
 
O.P. CUfrOUT BOX 
 
 .COMMUTATOR. 
 
 FIG. 217. 
 
 easily. As the armature speeds up, its counter electric pressure 
 grows, and the resistance in series with the armature may then be 
 slowly reduced and finally be cut out altogether. Fig. 217 shows 
 the connections to constant pressure mains of a shunt motor with its 
 starting box. 
 
 Copyrighted, 1894, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXII. 
 
 Points for Review: 1. What is the principle of the incandescent lamp? 
 
 2. Why do not the carbons of incandescent lamps quickly burn away? 
 
 3. How is the vacuum of incandescent lamps produced? 
 
 4. Of what materials are incandescent lamp filaments ordinarily made? 
 
 5. How is the original material of the filament converted into carbon? 
 
 6. Why are incandescent lamps better than arc lamps for general indoor lighting? 
 
 7. Upon what kind of circuits are incandescent lamps and motors usually operated? 
 
 8. Why is a constant pressure necessary for the operation of incandescent lamps 
 connected in parallel? 
 
 9. What is the effect on an incandescent lamp of "over-running" it that is, of 
 running it at too high a pressure? 
 
 10. Why is there always a loss of pressure in the wires which convey current from 
 dynamo to lamps? 
 
 11. What is meant by circular mils? 
 
 12. Suppose it is desired to convey 150 amperes from a dynamo to lamps which are 
 800 feet away, and the drop is not to exceed 25 volts; what must the cross-section of the 
 wires be? To what number in the B. & S. gauge does this most nearly correspond? 
 
 13. If the pressure at the dynamos is 130 volts, and the loss is 20 per cent, what is 
 the pressure at the lamps? What is the pressure at the lamps if the loss is 10 per cent? 
 
 14. If the pressure desired at the lamps is 104 volts and the drop on the line is 20 
 per cent of the dynamo pressure, what must the pressure be at the dynamos? 
 
 15. How does the weight of copper required in transmitting a given amount of power 
 over a given distance, at a fixed percentage loss, vary with the pressure at the dynamos? 
 
 16. What is meant by the 3-wire system? 
 
 17. Why is the 3-wire system used in electric light plants? 
 
 18. Why are starting resistances used with electric motors which are operated on 
 constant pressure circuits? 
 
 LBSSON XXIII. 
 
 CONSTRUCTION OF ELECTRIC LIGHT , AND 
 POWER CIRCUITS AND THEIR TESTING. 
 
 The wires of nearly all electric light and power plants which 
 are not located in large cities are carried on wooden poles. The con- 
 struction of the pole lines is quite similar to that of telegraph and 
 telephone lines explained in L,essqn XVII, but as a general rule 
 electric light and power lines carry fewer but much heavier wires. 
 The sizes of the wires depend upon the current transmitted over 
 them, their length, and the drop of pressure which is permitted to 
 occur in them, and are determined by the method explained in the 
 previous lesson. The wires are always of copper of the highest ob- 
 tainable conductivity, and they ordinarily vary in size from No. 10 
 to No. oooo B. ,& S. gauge, or from about T V of an inch in diameter 
 to nearly y 2 of an inch in diameter. The former is the smallest 
 wire of soft copper which can b^ depended upon not to break from 
 mechanical strains caused by the wire swaying in the wind, other 
 
 191 
 
wires falling upon it, etc.; while the latter is the largest solid wire 
 which can be conveniently handled. Where a number of large wires 
 are run on the same pole line, extra heavy poles and cross arms are 
 used. The glass insulators which are used for electric light and 
 power lines are rather heavier than those illustrated in Lesson XV II, 
 which are used for telegraph and telephone lines, and have a deeper 
 groove (Fig. 219). The groove in these insulators is so large com- 
 pared with that in other insulators that they are commonly called 
 deep groove insulators. 
 
 The wires used upon overhead telegraph and telephone lines are 
 not covered with insulation and the same is true of low pressure 
 electric light lines. For instance, the overhead wires used to dis- 
 tribute current for incandescent lighting by the ordinary 3-wire 
 system are almost always bare, and the glass bells at the point of 
 support are depended on to give a satisfactory insulation. This is 
 perfectly safe when the pressure is as low as in the ordinary 3-wire 
 system, where the pressure between the positive and negative wires 
 is seldom higher than 260 volts. When the pressure used on overhead 
 lines is higher than 300 volts, it is usual to use insulated wires. The 
 insulation consists of a continuous braided cotton covering of two or 
 three thicknesses, which is thoroughly soaked in some insulating com- 
 pound. As the insulation is supposed to be partially waterproof, such 
 wire is often called weather-proof 'wire. The insulating compound 
 which is used is almost always black. Black weather-proof wire is used 
 for the overhead lines of power plants which distribute current to 
 moto;s at a pressure of 500 volts, for the overhead lines of alternating 
 current electric light plants which use a pressure of 1,000 volts, for the 
 feeders of electric railway plants, arc light wires, etc. It has become 
 an almost universal custom in this country to use No. 6 B. & S. 
 gauge weather-proof wires for arc light lines. As the arc current 
 seldom exceeds 10 amperes, the loss of pressure in a No. 6 wire 
 several miles in length is not very great,- and it is a convenient and 
 economical size to use. 
 
 The circuits for electric lighting and power are always complete 
 wire circuits, as the use of the ground for returning large currents is 
 likely to cause difficulties from the uncertain resistance of ground 
 plates, and &g>ounded electric light circuit always introduces a risk 
 of fire in each house that it enters. For the latter reason fire insur- 
 ance men or Underwriters refuse to approve the use of a ground 
 return for the distribution of electric light and power where the wires 
 enter buildings insured by them. 
 
 In the large cities, electric light wires are put underground. In 
 this case, two entirely different systems may be used. The first is 
 like that described in Lesson XVII, and is often called the drawing in 
 system, because the lead covered cables are pulled or drawn into con- 
 duits from manhole to manhole. Electric light cables differ very much 
 
 192 
 
from telephone and telegraph cables, as they usually contain only 
 one wire, and seldom contain more than two wires. Fig. 220 shows a 
 single conductor electric light cable and a two conductor or duplex 
 cable. The electrostatic capacity of electric light cables is not a 
 matter of great importance, and the choice of insulating material for 
 such cables may therefore depend almost wholly on mechanical and 
 insulating qualities. In some cables, rubber compounds are used foi 
 the insulating material. In this case, the copper conductor is 
 covered with a layer of rubber compound, and over this is pressed the 
 lead sheathing. The thickness of the rubber insulation depends, to 
 some extent, upon the electrical pressure at which current is trans- 
 mitted through the conductors, but it is usually between y& and J^ of 
 an inch, while the thickness of the lead covering is sufficient to give 
 a satisfactory protection to the insulation against mechanical injury 
 and to protect the insulation from contact with moisture or harmful 
 gases. 
 
 The insulation of some cables is made by closely wrapping the 
 conductor with strips of paper which have been soaked in an insulat- 
 ing compound so as to make it quite soft and flexible. The thick- 
 ness of this paper wrapping is made about the same as that of rubber, 
 and a lead sheathing is put on in exactly the same manner as on the 
 rubber insulated cables. A third style of insulation consists of a 
 thick braiding or wrapping made up of several layers of cotton or jute 
 which is soaked in an insulating compound quite similar to that used 
 for weather-proof wires. This is also covered with a lead sheathing. 
 The latter cables are often said to have fibrous insulations on 
 account of the character of the materials used. As fibrous material 
 will rapidly absorb moisture and its insulating qualities thus become 
 ruined, it is absolutely necessary that the lead sheathing shall con- 
 tain no holes, however small, and the ends of the cables must be 
 protected from moisture with extreme care. The protection of rubber 
 insulation from moisture is not so important, but moisture may even 
 here have a serious effect, so that the most careful inspection and 
 handling of the cables is advisable. 
 
 The second method of laying underground conductors for the 
 distribution of electric current is often called the solid or built in sys- 
 tem, because the insulated conductors with their protecting conduit 
 are laid in the ground together. In this case, if any harm comes to 
 either the conductor or its insulation, the street must be dug up at 
 the place of "trouble" before repairs can be made. With the "draw- 
 ing in" system, repairs maybe made by simply pulling out that sec- 
 tion of cable between two manholes which contains the injury, and 
 replacing it with a piece of good cable. 
 
 The "built in" system is commonly used for low pressure dis- 
 tribution of electric current, and for this purpose gives excellent sat- 
 isfaction. Nearly all the great electric illuminating companies in 
 
 193 
 
our large cities which use the 3-wire system have their conductors 
 laid in this manner. For high pressure distribution, the "built in" 
 system of underground conductors is not as satisfactory as the "draw- 
 ing in" system. 
 
 The most commonly used arrangement of the "built in" system 
 is that known as "Edison tubing." This was introduced about a 
 dozen years ago, and was used in its original form in the laying of 
 the conductors connected with the old Pearl street central station in 
 New York city, the first great central station for the general distri- 
 bution of the electric current. Edison tubing was the earliest, and for 
 many years, the only scheme, in which the details of a general 
 underground system for distributing electric current were satisfac- 
 torily worked out. 
 
 On account of the experience gained in laying the conductors 
 for the various large Edison electric illuminating companies, the 
 system of tubing has been considerably changed since its first intro- 
 duction. As the tubes are now made, they usually contain three 
 copper rods the positive, negative and neutral conductors of the 
 3-wire system. These rods, which are somewhat over 20 feet long, are 
 each wound with a spiral of manilla rope, and are then laid side by 
 side but separated from each other by the ropes. Another spiral of 
 rope is wound around the bunch to hold the conductors firmly 
 together. The bunch of three conductors is placed in an iron pipe 
 twenty feet long, in such a way that the copper rods stick out a few 
 inches at each end. One end of the pipe or tube is then connected to a 
 pump by means of which a vacuum is created in the tube, and, finally, 
 hot black insulating compound is pumped into the tube until all the 
 open space inside of it is filled. The insulating compound is of a 
 bituminous nature and hardens when it is permitted to cool. Fig. 
 221 shows a cross-section of a "tube" in which AAA are the copper 
 conductors, C is the iron pipe, and B is the insulating compound. 
 Fig. 222 is a complete length of the completed tubing, showing the 
 form in which it is delivered from the factory to be laid in the 
 ground. For the purpose of laying the tubes a trench is dug, and 
 the 20 feet lengths are laid down end to end. The conductors 
 in successive tubes are joined by means of flexible copper connectors 
 (Fig. 223) having solid copper heads with holes which slip over the 
 ends of the rods where they are soldered fast. Ball-like caps are 
 bolted fast to the tube ends and over these is bolted a split coupling 
 box which covers the joint (Fig. 224). Only one-half of the coup- 
 ling box is shown in the figure. In the top of this coupling box is 
 a hole through which hot insulating compound may be poured when 
 the joint is completed, and the hole is then covered with an iron cap. 
 
 The arrangement here described is very satisfactory since it 
 offers an electric company the same ease as a gas company or a water 
 company in making connections to houses. A branch to a house, or 
 
U ) 
 
 FIG. 219. 
 
 FIG. 224. 
 
 195 
 
service connection, as it is called, may be connected to the main con- 
 ductors at any coupling box by simply changing the plain box to a 
 T box (Fig. 225). 
 
 Several different arrangements of "built in" conductors have 
 been used in England, France and Germany. One of these consists 
 of a simple brick, concrete or cast-iron trench, or culvert, in which the 
 copper rods or bars used for conductors are placed on porcelain insula- 
 tors. Fig. 226 shows an end view and a side view of such a culvert at a 
 point where a set of insulators is located. One of the most remarkable 
 arrangements of the u built in" system is that used in London to 
 distribute electric current by the two wire system from the noted 
 Deptford central station. The conductors in this case are enclosed 
 in an iron pipe, as are the conductors in the Edison system, but the 
 conductors themselves are copper tubes placed one inside of the 
 other instead of being rods placed side by side. The space between 
 the conductors is filled with insulation which consists of brown 
 paper soaked in an insulating compound (Fig. 227). The same 
 kind of insulation is also placed between the outer conductor and 
 the iron protecting pipe. This conducting system was designed and 
 laid down to transmit current at the enormous and unusual pressure 
 of 10,000 volts, and it has served its purpose very well. As the 
 tubes could not be made in lengths much greater than 20 feet, joint- 
 ing the lengths together was a matter of much difficulty on account 
 of the concentric arrangement of the conductors. 
 
 In order that the electrical pressure may be kept the same 
 at all points on a system of conductors which cover a large district, 
 the conductors must be divided into feeders and mains. The mains 
 consist of the conductors to which lamps or motors are directly 
 connected. These are carried all through the streets of the district to 
 which current is to be supplied and are often joined into a network by 
 means of fuses located in manholes or junction boxes at street corners. 
 The current is supplied to the mains at certain central points called 
 feeding points by means of feeders which run directly to the feeding 
 points from the central station where the current is generated. Fig. 228 
 is a diagram representing the arrangement of feeders and mains. The 
 points marked i, 2, 3, 4, are the feeding points, and A, B, C, D, are 
 houses to which current is supplied through service connections. 
 The figure shows three wires in each main and feeder, as is required 
 in a 3-wire system. By carefully calculating the resistance of each 
 feeder and main before the system is constructed it is possible to get 
 a very uniform pressure over the whole distributing system. In 
 order that the dynamo-men may regulate the pressure of the dyna- 
 mos in the central station so as to keep the pressure uniform at the 
 feeding points, it is necessary to have voltmeters, or pressure indi- 
 cators, in the dynamo room, which show the pressure at the feeding 
 points. For this purpose wires called pressure wires are run from 
 
 190 
 
FIG. 225. 
 
 $&?&tt&X&ttSm 
 
 FIG. 226. 
 
 FIG. 228. 
 
 197 
 
 S^Z* OF THE 
 
 (-0NIVERS: 
 
 \^c> 
 
 
the feeding points to the voltmeters in the dynamo room. A some- 
 what similar network of pipes is sometimes used in the distribu- 
 tion of gas and water in large cities. 
 
 The importance of that part of electric lighting circuits which 
 is inside of buildings cannot be overestimated. A central station may 
 be built upon the best plan to supply current through a perfect dis- 
 tributing system, but a safe and satisfactory light will not be given 
 if the inside wiring is poorly planned and put in place. Fires which 
 occur on account of the electric light wires in houses are always caused 
 either by the use of poor material, careless planning, or bad work- 
 manship when the inside wiring was put in, and if the wiring is done 
 properly it is almost impossible for Jires to be caused by an electric 
 lighting system. On the other hand, poorly constructed wiring is a 
 constant danger aud should not be permitted anywhere. On account 
 of the danger which may be caused by unscrupulous .or untrustworthy 
 wiremen, it is usual in large cities to have official inspectors to exam- 
 ine and test all electric light work placed within buildings. It is the 
 duty of these inspectors to see| that the work is safely and properly 
 done in accordance with rules fixed by the city authorities and 
 approved by the fire underwriters. Even with such inspection the 
 work is not always done in the best manner, yet comparatively few 
 important fires have been caused by electric wires and a great 
 majority of the accidents laid to the door of electricity are due to some 
 other cause. 
 
 For ordinary wiring inside of a building only the very best rubber 
 covered wire should be used. A great many factories produce rubber 
 covered wire for use in inside wiring and much of it is very poor, 
 so that great care is necessary in selecting material. 
 
 The wires may be run in buildings according to three entirely 
 different methods. In the first, the wires are run upon the surface of 
 ceilings or walls in plain sight and are held in place by means of 
 cleats made of wood or porcelain (Fig. 229). This is the commonest 
 arrangement of wiring in stores and other buildings where the 
 position of the wires in plain sight is not objectionable. As the wires 
 are in plain sight and therefore can be easily inspected at all times, open 
 work or cleat work, as this arrangement is called, is a safe and satis- 
 factory arrangement of the wiring, provided the wires and appliances 
 are all out of reach so that they cannot be tampered with. In damp 
 places or in places where .there are fumes which attack the insulation, 
 the wires are often supported on porcelain knobs (Fig. 125, Lesson 
 XVII), instead of being held against the walls by cleats, as an addi- 
 tional safeguard. 
 
 There are many places where the appearance of open work is 
 objected to, but where the wires may be placed in wooden casings or 
 mouldings which are fastened to ceilings or walls in plain sight. This 
 is an exceedingly safe and satisfactory arrangement, since the wires 
 
 198 
 
r 7 nJ! 
 
 
 
 FIG. 229. 
 
 FIG. 231. 
 
 FIG. 238. 
 
 o 
 
 O 
 
 O 
 
 FIG. 232. 
 
 FIG. 230. 
 
 FIG. 233. 
 
 199 
 
are well protected from mechanical injury or from being tampered 
 with, and yet the condition of the wiring may be easily seen at any 
 time by a simple inspection. Common forms of mouldings are shown 
 in Fig. 230. These may be made of any desired wood, though pine 
 is most commonly used. 
 
 In the third method of running wires, they are placed entirely 
 out of sight, or concealed. This may be done in various ways; the 
 commonest and at the same time the least safe and satisfactory way 
 is to fasten the wires to the ceilings and walls of the building before 
 the plastering is put on. The wires are then entirely covered by the 
 plaster, so that it is impossible to examine or repair them without 
 injury to. the walls, and, indeed, the position of the wires in the walls 
 is often forgotten in a few months after the building is finished, so 
 that repairs are doubly difficult to make. This arrangement of the 
 wires is made more unsafe because the plaster upon the walls often 
 spoils the insulating qualities of the rubber coverings and the wires 
 become grounded as a consequence. 
 
 In buildings with wooden floors and partitions the wires are 
 often fastened to the floor joists or partition studding by means of cleats 
 or porcelain knobs. When this is properly done, the wires are 
 not likely to be injured by plaster or dampness, but the disadvantage 
 that they cannot be examined is still present. They are also liable 
 to injury by plumbers, carpenters or other workmen who are engaged 
 in making repairs or alterations to the building. 
 
 When it is necessary to conceal electric light wires it is much 
 better to arrange a hidden moulding or conduit to contain them. 
 This may be hidden behind decorations or other objects on the walls 
 or may be laid neatly under the floors or the plaster (Fig. 231.) 
 Special tubes are made to be used as conduits for inside wiring. 
 These are called "interior conduit," "vulca duct," etc. (Fig. 232), 
 and are used to a considerable extent. When properly used they are 
 excellent, but are no better than any strong, watertight insulating tube, 
 such as an iron or brass pipe with an insulating lining. Interior tubing 
 made of insulating material was originally intended to take the place 
 of the rubber insulation on the wires so that they could be used with 
 a cheap cotton covering, but it has been found to be necessary to use 
 the best rubber insulation on wires in the tubes in order that the 
 wiring may give satisfaction. The advantages of tubes are that the 
 wires are protected from mechanical injury and from contact with 
 plaster, moisture, etc. 
 
 The plan of the wiring in a building depends a great deal on the 
 size and construction of the building, but in its details it should 
 always fulfill, not only in the letter but in the spirit, the require- 
 ments of the Underwriters which are laid down in special printed 
 rules. In small buildings supplied with current from a central sta- 
 tion, the simplest plan for concealed wiring is what may be called 
 
 300 
 
FIG. 234. 
 
 the "distributing system." Heavy service wires are led from the 
 street mains of the electric light company through a fuse block or 
 cut-out (Fig. 233) to a convenient central point in the building. At this 
 point the service wires terminate in a number of fuse blocks from 
 each of which a circuit of smaller wire runs out to supply a 
 limited number of lamps, usually between 5 and 15. Fig. 234 shows 
 such a plan of wiring so plainly that no additional description is 
 necessary. In the figure, s, s, s are switches for turning the lights on 
 and off, and c is a fuse block used to protect a small branch circuit 
 which for convenience is connected to one of the taps instead of 
 being run back to the distributing center. By this arrangement of 
 the distribution any serious trouble which occurs on one branch or 
 tap causes the fuses at the distributing center which belong to 
 the branch, to melt. This disconnects the defective branch from the 
 service wires without interfering with the other branches. The 
 location of all the fuse blocks at a central point makes it convenient 
 to replace fuses, and the fuse blocks can be so protected that a fire 
 cannot possibly be caused by the arc which sometimes occurs when 
 a fuse melts or blows. 
 
 201 
 
FIG. 235. 
 
 Another plan for wiring a building is shown in Fig. 235. In 
 this figure, a heavy trunk circuit runs from the main cut-out in the 
 cellar to the top of the house, and the lamp taps branch off from 
 the trunk at each floor; s, s, s are switches for controlling the lights 
 and c, c, c are fuse blocks. This plan makes it necessary to scatter 
 the fuse blocks through different parts of the house, which is a dis- 
 advantage. 
 
 In large buildings, a combination of the two plans just ex- 
 plained is used, and feeding trunks, or feeders, are run from the main 
 fuse block to several distributing centers at convenient points in the 
 building. One feeder with its mains is shown in Fig. 236, where 
 XA is the feeder running from the main cut-out, or from the dynamo 
 room if a special lighting plant is located in the building, and B, B, 
 D, D is a main which runs down and up so as to supply current to 
 the different floors of the building. Fuse blocks are placed at 
 each rectangle to protect the parts of the circuit beyond it. The 
 horizontal lines are mains which run along each floor to carry current 
 to the distributing centers which are shown by the rectangles at Y, 
 Y 1 , Y 2 , Y 3 . The lamp taps which are run from the centers to the 
 lamps are represented by the short spiral lines. Only one line is 
 used in this figure to represent the circuit, which may be either 2- 
 wire or vwire. 
 
KG" ^ Qy7 
 
 FIG. 236. 
 
 For very large buildings, the plan shown in Fig. 236 may be 
 extended by running feeders to various points in the building, from 
 which points mains run to the distributing centers. The feeding points 
 are then usually joined together by a heavy connecting circuit often 
 called a crib. This is shown in Fig. 237 where E 1 , E 2 , E 3 , E 4 are 
 feeders running to four feeding points in a building which are 
 marked K, K, K, K. These points are joined together by the 
 crib from which the mains run off to the various centers of distri- 
 bution. 
 
 The wiring plan in a large building is seen to be quite similar 
 to the plan of the feeders and mains used in distributing electric 
 current from a central station. The object to be aimed at in arrang- 
 ing the wires in either case is to keep all the lamps which are burn- 
 ing at one time as nearly as possible at the same pressure, and also 
 to make it possible to keep the pressure constant regardless of the 
 number of lamps burning. The size of wires used at any place 
 must be calculated from the amount of current which the wires 
 carry and the volts drop in pressure which is allowed. The calcu- 
 lation sometimes indicates a wire which is too small for safety, and 
 a wire smaller than No. 16 B. & S. gauge should never be used in 
 inside electric light wiring ; neither should the current passed 
 through a wire exceed the "safe carrying capacity'' given in 
 Lesson XXXII. "Wiring tables," which give the sizes of inside 
 
 203 
 
Jp* 
 
 ' E* WE* 
 
 FIG. 237. 
 
 wires required to supply current to lamps at various distances from 
 the main cut-outs, are to be found in many trade catalogues. 
 
 A great many details relating to inside wiring can only be 
 learned by observing wiring which has been completed in a proper 
 manner, but a great deal of useful information relating to the inci- 
 dental material can be obtained from the catalogues of the com- 
 panies who supply electrical material. The most important in- 
 cidental material is fuse blocks, fuses, switches and sockets. 
 Fuse blocks now invariably consist of porcelain bases of various 
 forms upon which are carried terminal screws for the con- 
 nection of the fuses and the circuit wires. Electric light fuses 
 are strips or wires of a metal alloy which melts at a temperature that 
 is so low that the melted metal cannot possibly cause harm. The alloy 
 is usually made largely of lead and tin, but varies a great deal. The 
 object of the fuse is to protect the wires beyond it from becoming 
 overheated through some accident. The size of the fuse at any point 
 is such that if anything occurs to cause an unsafe current to flow 
 through the wires protected by it, the fuse will melt and cut the wires 
 out of the circuit. Fuses of large carrying capacity usually have ter- 
 minals made of copper (Fig. 238) so that a more substantial contact 
 may be made with the fuse block terminals. Switches and sockets 
 have already been illustrated. A most important factor in locating the 
 position of centers of distribution in a building is the location of the 
 lights. The next lesson will enter into this. 
 
 Copyrighted, 1894, 
 
 2O4, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXIII. 
 
 Points for review. 1. Why are overhead electric light wires seldom smaller than. 
 No. 10 or larger than No. 0000 B. & S. guage? 
 
 2. What is "weather proof " wire? 
 
 3. Why is No. 6 B. & S. guage wire ordinarily used for overhead arc light circuits? 
 
 4. Why is the ground never used as a part of electric lighting circuits? 
 
 5. What is the difference between a "drawing in" and a "built in" system of under- 
 ground conductors? 
 
 6. What materials are used for insulating electric light cables which are intended 
 for use in underground conduits? 
 
 7. What is "Edison tubing"? 
 
 8. What are "feeders"? What are "mains"? 
 
 9. Why are the conductors of a constant pressure electric light system divided into 
 feeders and mains? 
 
 10. What is meant by inside wiring? 
 
 11. What is meant by cleat work? by moulding work? by concealed work? 
 
 12. Does properly arranged electric light wiring introduce danger from fire into a 
 building? 
 
 13. Does improperly arranged electric light wiring introduce danger from fire into a 
 building? 
 
 14. What class of insulation should always be used on wires for inside wiring? 
 
 15. Why does moulding work, when properly put in, make the best kind of wiring? 
 
 16. Why does concealed work, where the wires are laid directly under plaster, make 
 the poorest kind of wiring? 
 
 17. Why is it advantageous to wire buildings on the distribution plan? 
 
 18. Why do the Underwriters' rules usually prohibit the use of wires smaller in size 
 than No. 16 B. & S. gauge? 
 
 XXIV. 
 
 TESTING ELECTRIC LIGHT CIRCUITS, AND THE DIS- 
 TRIBUTION AND MEASUREMENT OF LIGHT. 
 
 The faults which occur on electric light lines are of the same 
 kind as those which occur on telegraph and telephone lines (Lesson 
 XVIII). The methods of testing for and locating the faults are very 
 different, however. The general condition of an electric light line 
 may be determined from the manner in which the lights burn. 
 Breaks in the line are made evident by the fact that lamps on the 
 circuit beyond the break will not burn; while crosses and short cir- 
 cuits soon make themselves evident by causing the fuses which pro- 
 tect the defective part of the circuit to melt or blow. Poor connec- 
 tions may be shown by dimness of the lamps, when the connections 
 have a sufficiently high resistance to cause a great drop in pressure. 
 It is needless to say that connections or joints of such poor conduct- 
 
 205 
 
ivity are very dangerous and should not be permitted to exist in a 
 circuit for an hour. All joints in electric light wires are soldered in 
 order that there may be no "bad joints" which may cause poor con- 
 nections. Poor connection at such points as sockets or fuse blocks 
 belonging to incandescent circuits may cause considerable heating. 
 If such heating is noticed it should be at once corrected or it may 
 cause damage. Sometimes poor connections at fuse blocks may 
 cause the fuses to blow when there is really no trouble elsewhere on 
 the circuit. This may occur when the fuse blocks have too little 
 contact surface at the connection points to properly carry the current. 
 Such fuse blocks should always be replaced by larger and better ones, 
 as they are not only an annoyance but they are dangerous. No one 
 would think for a moment of allowing poorly jointed and leaky gas 
 pipes and fixtures in a house, and defective electric wires should be 
 treated in exactly the same manner as defective gas fittings. 
 
 Series circuits, like arc light circuits, which are not in use all 
 through the twenty-four hours, are often tested for breaks, grounds, 
 and crosses by means of a magneto bell which is very much like a 
 telephone call bell (Fig. 239). The little magneto machine and call 
 bell are put in a box together and connected in series with two ter- 
 minals on the outside of the box, which are shown at the top of the 
 figure. If it is desired to test the continuity of a line that is, the 
 absence of breaks the two ends of the line are connected to the test 
 bell terminals. If the bell rings when the crank is turned the cir- 
 cuit is shown to be all right, while if the bell does not ring the cir- 
 cuit is shown to be broken, provided the test bell itself is in good 
 condition. It is easy to test the latter by short circuiting the ter- 
 minals, when the bell will ring upon turning the crank if the mag- 
 neto is all right. 
 
 If it is desired to test for grounds by means of a magneto bell, 
 one terminal of the bell is connected to earth by connecting it to a 
 gas or water pipe, jand the other wire is connected to the line to be 
 tested. If the bell rings when the crank is turned it ordinarily 
 means that the line is grounded, and if the bell does not ring, the 
 line is shown to be clear of grounds. Sometimes the bell will ring 
 a little when the line has a very high insulation, because the electro- 
 static capacity of the line is high and the current which flows into 
 and out of the line, as it is charged and discharged by the alternating 
 pressure set up by the magneto, is sufficient to ring the bell. 
 
 Most arc light lines are out of use during daylight, only those 
 which convey current to arc lamps in the buildings of large cities are 
 used during the day, and many lines are not used after midnight. It 
 is quite convenient, therefore, to use the magneto bell for testing such 
 lines. The tests can be made each day an hour or two before the 
 lines conie into service, and if anything is found to be wrong, a line- 
 man can go along the line to find the trouble and fix it. 
 
 206 
 
FiG. 239. FIG. 240. 
 
 A voltmeter is sometimes used for testing and locating grounds on 
 arc light lines while they are in use. Suppose figure 240 to represent 
 an arc light line which supplies current to five lamps and is grounded 
 at F. If the lamps are so adjusted that each requires 45 volts press- 
 ure, the difference of pressure between the fault and one terminal of 
 the dynamo is 135 volts, and between the fault and the other 
 terminal of the dynamo the difference of pressure is 90 volts. A volt- 
 meter connected to ground, as shown, indicates the difference 
 in pressure between the fault and one dynamo terminal, and so shows 
 between which lamps the fault is located. Instead of using a volt- 
 meter, 45 volt incandescent lamps may be used for testing by this 
 method. As many 45 volt incandescent lamps are connected in 
 series as there are arc lamps on the circuit to be tested. One end of 
 the series is connected to ground and the other to one dynamo 
 terminal. Then one incandescent lamp after another is short cir- 
 cuited until the lamps which remain in the circuit burn to their full 
 candle power. The number of incandescent lamps then in circuit is 
 equal to the number of arc lamps between the dynamo terminal and 
 the fault. The reason for this is evident upon examining the 
 figure. Since there are three arc lamps between the A terminal of 
 the dynamo and the fault, there is a difference of pressure of 135 volts 
 between the two points, as shown by the voltmeter, V. 135 volts is 
 the pressure required to bring a series of three 45 volt incandescent 
 lamps to full candle power, so that the number of arc lamps between 
 the fault and the A dynamo terminal is equal to the number of 45 
 volt incandescent lamps which will burn with full candle power 
 when connected in series between the dynamo terminal and the 
 ground. This test is made upon the supposition that the fault has 
 little resistance of itself. 
 
 In testing incandescent circuits for grounds, incandescent lamps 
 or voltmeters are almost always used. If one wire of a two-wire cir- 
 cuit is grounded the presence of the ground may be shown by con- 
 
 207 
 
FIG. 241. 
 
 FIG. 242. 
 
 FIG. 243. f 
 
 necting an incandescent lamp between the other wire and the earth 
 (water pipes, etc. , Fig. 241), when the lamp will burn on account of the 
 current which flows from one wire to the other through the lamp and 
 the fault. If the lamp be intended for the same pressure as that of the 
 circuit, it will burn at full candle-power if the circuit is "dead 
 grounded" and will be dimmer in proportion to the resistance of the 
 fault. Figure 242 shows a permanent arrangement of the ground 
 detector which is fixed so that the detector lamp may be connected 
 at pleasure with either of the wires. Another arrangement of lamps 
 for a ground detector is shown in figure 243. A and B are two 
 lamps connected in series between the two wires of the electric light- 
 ing system. A wire goes to ground through a fuse block and a 
 switch from a point between the two lamps. When the switch is 
 open the lamps A and B will burn very dimly but of equal bright- 
 ness, and no change will occur when the switch is closed if no 
 grounds are present on the circuit. If the wire to which the A lamp 
 is connected be grounded, current will flow from the grounded wire 
 
 208 
 
FIG. 244. 
 
 FIG. 245. 
 
 through the B lamp to the other wire when the switch is closed, and 
 the B lamp will become brighter than the A lamp. In the same way 
 the A l^mp will brighten when the switch is closed if the B wire is 
 grounded. Sometimes both wires are grounded and the faults have 
 about equal resistance. In this case the lamps will not show the 
 grounds in the ordinary way, but the test can be made by turning off 
 one lamp when the switch is closed and the other lamp will go out 
 if the circuit is not grounded. 
 
 For three- wire circuits a pair of lamps may be used as a ground 
 detector for each side of the system. 
 
 When a voltmeter is used to test for grounds on an incandescent 
 circuit it is handled in very much the same way as the incandescent 
 lamD which is used for the same purpose. The voltmeter is con- 
 nected between one of the circuit wires and the earth. If the other 
 wire of the circuit is grounded, current will flow from it through the 
 voltmeter to the wire to which the instrument is connected. If the 
 grounded wire is "dead to ground," the voltmeter will give the same 
 reading as when it is connected directly between the wires. The 
 reading of the voltmeter is less as the resistance of the ground con- 
 tact is greater, and it is zero when the insulation is good. 
 
 The methods which are used for testing for grounds on incan- 
 descent circuits show when a ground is present and upon which wire 
 it exists, but they do not give any clue to the particular portion of 
 the circuit upon which the ground is located. The ordinary method 
 of * locating" a ground which cannot be found by inspection, is to 
 
 209 
 
cut one branch after another off from the system until the ground 
 disappears. The ground is then on the last branch cut off and may 
 be found by careful inspection. 
 
 The testing of, and locating faults in, lead covered cables which 
 aie sometimes used in underground systems is done in the same way 
 as the insulation testing of telegraph and telephone cables, which has 
 already been explained (Lesson X, page 72; Lesson XVIII, page 145). 
 
 The candle-power and the best arrangement of the lamps 
 which are required to give a*satisfactory illumination in any particu- 
 lar space can be determined only by experience. The candle-power of 
 lamps is measured by an instrument called a photometer, in which 
 the illuminating power of the lamp to be measured is directly com- 
 pared with the power of standard candles (Lesson XXI, page 175), 
 or with a gas jet or lamp of known candle-power. The commonest 
 form of a photometer is that called Bunsen's photometer, which is 
 shown diagrammatically in Fig. 244. A is the standard candle, B is 
 the lamp whose illumination is to be measured, and D is a movable 
 disc of thin paper with a grease spot at its center. The photometer, 
 for practical use, is all enclosed in a perfectly dark closet, and the 
 light from A and B is carefully screened on every side except directly 
 in line with the disc. An observer measures the unknown 
 candle-power of the lamp B by moving the disc D until it 
 shows an equal illumination on both sides. The disc is generally 
 looked at by means of mirrors, so that both sides may be seen at 
 once. When the illumination of the two sides of the disc is equal, 
 the candle-powers of the lights are proportional to each other in the 
 ratio of the squares of the distances measured from the respective lights 
 to the disc. , 
 
 The reason that the squares of the distances come into the com- 
 parisons of candle-powers is illustrated in Fig. 245. If we suppose a 
 screen, A B, to be placed at a distance of one foot from the lamp, L, 
 we may consider that the screen is illuminated by a certain number 
 of rays of light falling upon it. Now, if the screen be moved to a 
 distance of two feet from the lamp the same rays of light will illumin- 
 ate an area, C D, which is four times as large as A B, and, conse- 
 quently, the intensity of the illumination on the screen is only 
 one-fourth as great as when the screen was at a distance of one foot 
 from the lamp. If the screen be moved to a point three feet from 
 the lamp, the same rays will cover the area E F, which is nine times 
 as large as A B, and the intensity of the illumination is only one- 
 ninth as great as when the screen was within a foot of the lamp. 
 Since 4 and 9 are respectively equal to the squares of 2 and 3, we see 
 that the intensity of the illumination given to a surface by a fixed 
 light is inversely proportional to the square of the distance from the light 
 to the surface. In the Bunsen photometer, the screen is placed at such a 
 
point directly between two lights that they illuminate it equally. In 
 this case, the lights must have candle-powers which are proportional 
 to the squares of their distances from the screen, as already said. 
 
 The actual illuminating effect of a given number of lamps in 
 any space depends upon a great many things. For instance, a room 
 with dark walls, which absorb a great deal of light, requires much 
 more light to give a satisfactory illumination than does a room with 
 light-colored or white walls. In a comparatively small space a num- 
 ber of lamps of small candle-power, properly distributed about the 
 space, give a much more satisfactory light than do a few large lamps 
 giving the same total candle-power. This is because the illumina- 
 tion near the large lamps is very great and at other points in the 
 space the illumination is small, while it is much more uniformly 
 distributed by the small lamps. 
 
 In ordinary rooms and stores, it is common to put from one to 
 three 16 candle-power lights for each 100 square feet of floor, while 
 in larger rooms 450 watt arc lamps may be used so that each arc 
 illuminates from 500 to 1,000 square feet of floor. Where arcs 
 are placed indoors, it is usual to surround the arc with an opal glass 
 globe which distributes the light more satisfactorily than would 
 otherwise be the case. Such globes have the disadvantage of ab- 
 sorbing nearly one-half of the light of the arc, but their effect in dis- 
 tributing the light is sufficiently important in indoor lighting to 
 counterbalance the loss of light. For outdoor lighting, arc lights 
 with clear glass globes are used almost altogether. The lamps are 
 then placed from 50 to 600 feet apart, depending upon the amount of 
 illumination desired. It is an important fact which is not very well 
 known by electric light companies, that dirty globes of clear glass 
 may absorb even more light than do opal globes, so that it is impor- 
 tant that arc light globes be kept clean. 
 
 The true measure of illumination, it may be seen from what 
 precedes, is not the candle-power but the amount of light or illumin- 
 ation obtained on a surface, and the unit for measuring illumination 
 is the amount of illumination on a perpendicular screen at the dis- 
 tance of one foot from a lamp giving one candle-power. Four candle- 
 power at a distance of two feet from the screen and 9 candle-power 
 at a distance of three feet from the screen give the same illumination 
 as i candle-power at a distance of one foot from the scren, which is 
 called a candle-foot. The illumination given by any lamp upon 
 a perpendicular surface is equal to the candle-power of the lamp 
 divided by the square of the distance between the lamp and the sur- 
 face. For instance, if we have a 32 candle-power lamp at a distance 
 of 6 feet from a wall, the illumination on the wall is 32 divided by 6 
 squared or 36, which is equal to about .9 of a candle-foot. An illum- 
 ination of i candle-foot is quite satisfactory for ordinary reading. 
 Ordinary bright moonlight gives an illumination on the ground 
 
which is equal to about r f & t of a candle-foot. The illumination upon 
 theatre stages is ordinarily from 3 to 4 candle-feet, and the illumin- 
 ation given by diffused daylight is equal to from 10 to 40 candle- 
 feet. On account of the expense of producing artificial light by the 
 common methods of the present day, it is commercially impracticable 
 to artificially produce as great an illumination as is given by day- 
 light. 
 
 Coprighted, 1895, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXIV. 
 
 Points for Review. 1. What faults occur on electric light circuits? 
 
 2. Why is it necessary to solder the joints in electric light wires? 
 
 3. How is a magneto bell used in testing electric circuits? 
 
 4. How may a voltmeter be used to " locate" grounds on arc light circuits? 
 
 5. How may incandescent lamps be used for the same purpose? 
 
 6. What is a " ground detector?" 
 
 V. How are incandescent lamps used for ground detectors on incandescent circuits? 
 
 8. How may a voltmeter be used to test for grounds on incandescent circuits? 
 
 9. How are grounds ordinarily " located" on incandescent circuits? 
 
 10. How are faults located on underground cables? 
 
 11. What is a photometer? What are standard candles? 
 
 12. Upon what does the illuminating effect of the lamps in a room depend? 
 
 13. W'hat effect does an opal globe have on the light given from a lamp? What effect 
 does a dirty globe of clear glass have? 
 
 XXV. 
 
 ELECTROMAGNETIC INDUCTION. 
 
 It has been experimentally proved that any change in the mag- 
 nectic field around an electric conductor which causes the lines of 
 force to cut the conductor tends to cause an electric current to flow 
 in the conductor (Lesson XIX, page 147). We are now sufficiently 
 acquainted with the mutual effects of electric currents and magnetism 
 (Lessons V, VI, XIX and XX) so that it is not surprising that 
 there are various conditions under which the effects of magnetism may 
 result in an electric current. One of these conditions is seen in 
 dynamos where the motion of the armature cbnductors across mag- 
 netic lines of force causes a current to flow in the electric circuit 
 of which the armature is a part (Lessons XIX and XX). It is not 
 necessary that the conductors move, but the magnetic field may move 
 so that its lines offeree cut across stationary conductors. In fact, an 
 electric pressure is set up in a conductor when it cuts lines of force, 
 whether the cutting be caused by the motion of the conductor or by the 
 motion of the lines of force. 
 
 213 
 
The magnetic lines of force which are cut by a conductor and 
 so cause an electric pressure in the conductor may not come from a 
 magnet, but may belong to an electric current in a neighboring wire. 
 When a conductor is moved towards or away from a wire carrying a 
 current, the lines of force belonging to the current are cut by the 
 moving conductor and an electric pressure is induced in it. If the 
 wire carrying the current be moved towards or away from the other 
 conductor, the lines of force belonging to the current are cut by the 
 conductor which is now stationary, and an electric pressure is set up 
 as before. The wire carrying the current may be in the form of a coil, 
 like P in Fig. 246. An electric pressure may be set up in the conduc- 
 tors of another coil, S, by simply thrusting the first coil which carries 
 a current into the second. ' After the primary coil, P, is pushed 
 into the secondary coil, S, and its movement is stopped, the electric 
 current in the secondary also stops because the conductors no longer 
 cut lines of force and the electrical pressure is no longer produced. 
 Now if the primary coil be drawn out from the secondary coil, an 
 electrical pressiire is again set up in the latter. This pressure and 
 its resulting current is opposite to that set up when the primary coil was 
 pushed into the secondary, because the lines of force are cut in the oppo- 
 site direction by the secondary coil. The same effects may be pro- 
 duced by moving a secondary coil in and out of a larger primary coil 
 or by moving one coil around near the other. The battery, C, shown 
 in Fig. 246, furnishes current to the primary coil. 
 
 The same effects may be produced by permanently fixing the 
 coil P inside of the coil S, and then varying the current which flows 
 through the coil P. When the current increases in the primary^coil, 
 the lines of force belonging to the magnetic field of the current cut 
 the conductors of the secondary coil as they are produced, and thus 
 set up an electric pressure in the secondary coil during the time 
 the magnetic field is increasing. If the primay current be reduced or 
 shut off entirely, an electric pressure is set up in the secondary coils 
 in the opposite direction during the time that the magnetic field is 
 decreasing. 
 
 Electric currents which are set up in circuits by means of cutting 
 lines of force are said to be caused by electromagnetic induction, and 
 the currents are sometimes spoken of as induction currents or induced 
 currents. The currents produced by dynamos are examples of cur- 
 rents induced by electromagnetic action. 
 
 An appliance consisting of a primary coil and a secondary 
 coil, which is used for the purpose of inducing currents in 
 the circuit of the secondary coil by varying the current in the 
 primary coil, is called an induction coil (see Lesson XVI, page 
 123). The two windings of an induction coil are usually placed on an 
 iron core which greatly increases their effectiveness. The core 
 
 214 
 
J 
 
 FIG. 246. 
 
 FIG. 247. 
 
 FIG. 248. 
 
 215 
 
must be made of iron wires, or eddy currents will be induced 
 in the core and thus heating and loss of power will result, since 
 currents are induced in all closed circuits or masses of metal 
 which are in a changing magnetic field. ' The division of the iron 
 core of an induction coil is thus seen to be necessary for the same 
 reason that it is necessary to laminate the iron cores of dynamo 
 armatures (L,esson XX, page 156). 
 
 Each turn of the secondary windings of a well built induction 
 coil cuts practically all of the lines of force which are set up by the 
 current in the primary coil, so that the total electrical pressure 
 induced in the secondary windings may be controlled by winding the 
 secondary coil with a greater or less number of turns of wire. The 
 induction coil used with a telephone transmitter is arranged to give 
 a fairly high pressure in the secondary coils and thus intensify the 
 effect of a single cell of battery. In the induction coils made for 
 scientific experiments, which are often called RuhmkorfF coils (Fig. 
 247), the secondary has so very many turns of extremely fine wire 
 that the pressure produced in the secondary, when the current from 
 a few battery cells is made and broken in the primary coil, may be 
 so great as to cause an electric spark to jump a number of inches 
 through air. In the induction coils commonly called transformers 
 or converters (Fig. 248), which are common objects on the poles of 
 electric light companies which use alternating currents, the secondary 
 coils usually have fewer turns than have the primary coils, and the 
 electrical pressure induced in the secondary coils is therefore less than 
 the pressure applied to the primary. Transformers are used to reduce 
 a high pressure which is used on the distributing circuits to a lower 
 pressure which may be safely and conveniently used in buildings to 
 operate electric lights. Transformers, as applied to electric lighting, 
 will receive attention in later lessons. By means of them we are able 
 to perform the remarkable feat of commercially transferring electrical 
 power from one circuit to another, although the circuits have abso- 
 lutely no electrical connection with each other. 
 
 If we remember the direction of the lines of force around a wire 
 which carries a current (Lesson VI, page 36), and the rule for de- 
 termining the direction of an induced current (Lesson XIX, page 151), 
 it is easy to determine the direction of the current induced in any 
 secondary circuit. By applying the rules referred to, the following 
 rules relating to induced currents may be arrived at: 
 
 1 . When a primary coil is PUSHED INTO a secondary coil, the 
 secondary induced current is OPPOSITE IN DIRECTION to the 
 primary current. 
 
 2. When a primary coil is DRAWN OUT of a secondary coil, 
 the induced secondary current is in the SAME DIRECTION as the 
 primary current. 
 
When the primary and secondary coil are fastened together and 
 current is induced in the secondary by making and breaking the 
 primary current we have the following rules: 
 
 3. When the current is MADE (started} in the primary coil, 
 a momentary OPPOSITE or INVERSE current is induced in the 
 secondary coil. 
 
 4. When a current is BROKEN (stopped) in the primary coil, 
 a momentary current of the SAME DIRECTION is induced in the 
 secondary coil. 
 
 These rules relate to the flow of current when the secondary 
 circuit is closed. If the secondary circuit be open, the electrical 
 pressure which is set up, is in such a direction that the current would 
 flow in the direction indicated were the circuit closed. 
 
 A careful examination of these rules shows a very important fact 
 which may be stated in this way: The direction of an induced cur- 
 rent is always such that the magnetic field set up by it tends to oppose 
 the change in the strength of the magnetic field belonging to the pri- 
 mary current. For instance, when the primary current of an induc- 
 tion coil is "made," an inverse current is induced in the secondary 
 coil whose magnetic field opposes the growth -of the magnetic 
 field of the primary current. When the primary circuit is 
 broken, the magnetic field of the induced current opposes the decay 
 of the magnetic field belonging to the primary current. Another 
 illustration may be taken from the primary coil which is pushed into 
 a secondary coil. When the primary coil carrying a current is pushed 
 into the secondary, an inverse current is induced which sets up a mag- 
 netic field which tends to repel the primary coil and therefore opposes 
 its motion. When the primary coil is drawn out of the secondary the 
 direct induced current sets up a magnetic field which tends to attract 
 the primary coil and therefore again opposes its motion. 
 
 In the case of a dynamo the current which is induced in the 
 armature conductors has such a direction that its magnetic effect 
 tends to stop the motion of the armature (Lesson XIX, page 147); 
 and to keep it rotating, mechanical power must be applied to the 
 armature in proportion to the amount of power represented by the 
 currents taken from the armature (Lesson VIII, page 52). 
 
 The above facts may be briefly stated in one sentence. When 
 electric currents are induced by a changing magnetic field, the mag- 
 netic field belonging to the induced currents tends to stop the change in 
 ike original field. We have also the following statement which re- 
 sults directly from the former: When electric currents are induced by 
 the motion of a conductor, the induced currents have such a direction that 
 their magnetic effect tends to stop the motion. This is called Lenz's 
 law, after a German scientist who first formally stated the principle. 
 
The principles stated in the preceding paragraph are a direct re- 
 sult of the general law of the Conservation of Energy (Lesson VIII, 
 page 52). We can transform mechanical energy into electrical energy, 
 or vice versa, or, we can transform the energy of electrical currents 
 flowing under one pressure into the energy of electrical currents flow- 
 ing under another pressure, but in every case as much energy must 
 be put into the transforming apparatus whether it be dynamo, mo- 
 tor, Ruhmkorff coil or transformer as is taken out. We have already 
 seen that the useful "output" of electrical apparatus is usually smaller 
 than the ( 'input' ' by a certain percentage of the total energy which 
 has been changed into useless heat (Lesson VIII, page 52). 
 
 A varying current may have an inductive effect upon the coil in 
 which it flows itself, in addition to its inductive effect upon adjacent 
 conductors. When a current is started in a coil it sets up a magnetic 
 field which quickly grows from zero to its full value. As the field 
 grows, its lines of force cut the turns of the coil and induce in them 
 an electric pressure which opposes the growth of the current. On 
 stopping the original current its magnetic field quickly dies away and 
 the lines of force again cut the turns of the coil, but this time in 
 such a direction that the self-induced electric pressure upholds the 
 original current. If the coil has a great many turns wound on an 
 iron core, its self-induction may be of sufficient magnitude to make 
 a brilliant spark or give a severe shock when the circuit is broken. 
 The spark at breaking a circuit is often spoken of as caused by the 
 extra current of self-induction. The effect of self-induction is made 
 use of in so-called spark coils which are used with devices for light- 
 ing gas by electricity, and which consist simply of a coil containing 
 many turns of insulated wire wound on a core of iron wire. The 
 effect of self-induction makes itself evident if the circuit of a single 
 battery cell be broken between the hands when the circuit contains 
 a spark coil, telegraph instrument, or other electromagnetic coil. 
 
 The fact that a conductor carrying an electric current is always 
 surrounded by a magnetic field (Lesson VI, page 38), would lead us 
 to expect conductors carrying electric currents to attract and repel 
 each other. This is indeed the fact. We have already seen that 
 solenoids act towards each other exactly as though they were mag- 
 nets (Lesson VI, page 38). In every case, we have learned that when 
 magnets or solenoids are brought into each other's influence, they tend 
 to move so that their lines of force shall be placed parallel and in the 
 same direction (Lesson VI, page 38). Exactly the same is true of 
 straight or curved wires which are brought into each other's influence. 
 Remembering this, we can see that two wires lying side by side must at- 
 tract each other if they carry currents flowing in the same direction. 
 This is because the lines of force can only become parallel and of the 
 same direction when the two conductors are very close together. 
 
FIG. 249. 
 
 FIG. 250. 
 
 When the currents flow in opposite directions the wires repel each 
 other. In the same way, if the wires are inclined to each other they 
 tend to turn around into such a position that the wires are parallel 
 and the currents flow in the same direction (Fig. 249). It is upon 
 this principle that the electrodynamometer acts (Lesson XI, page 
 78). The operation of Kelvin balances (Fig. 250) which have been 
 explained in Lesson XI, is based upon the attraction and repulsion 
 of parallel currents. These instruments, as stated in Lesson XIV, 
 are not sufficiently portable to bring them into common use, but they 
 are excellent for use as standards by which to calibrate commercial 
 instruments. 
 
 Copyrighted, 1895, 
 
 210 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXV. 
 
 Points for Review. 1. What is the effect of cutting magnetic lines of force by an 
 electric conductor? 
 
 2. What is the difference in the result when the conductor is part of a closed elec- 
 tric circuit, and when it is a part of an open circuit? 
 
 3. Is it necessary for the conductor to move in order that it may cut lines of force? 
 
 4. If a magnet or a primary coil is pushed into a secondary coil, does the inductive 
 effect last after the motion has ceased? 
 
 5. What are "induced currents"? What are induction coils? 
 
 6. Why cannot a solid iron bar be used for the core of an induction coil? 
 
 7. How is it possible to alter the electrical pressure induced in the secondary wind- 
 ings of an induction coil? 
 
 8. What are transformers or converters? 
 
 9. What is the general rule for the direction of an induced current? 
 
 10. Is it possible to take a greater amount of power out of the secondary of an induc- 
 tion coil or transformer than is put into the primary? 
 
 11. What is self-induction? 
 
 12. Why does self-induction cause a spark upon breaking an electric circuit? 
 
 13. Why do parallel wires carrying currents which flow in the same direction attract 
 each other? 
 
 14. Why do parallel wires carrying currents which flow in opposite directions repel 
 each other? 
 
 XXVI. 
 
 ALTERNATING CURRENTS. 
 
 We have already learned (Lesson XIX, page 151) what an alter- 
 nating current is, and how it may be produced in an armature hav- 
 ing a single coil of wire which is revolved between two pole pieces. 
 The ordinary alternating current dynamo or alternator is made up on 
 this principle, but is usually constructed with a number of coils on 
 the armature and with an equal number of poles in the field magnets. 
 In general construction an alternator is similar to a continuous cur- 
 rent dynamo, but before we enter into a discussion of the detailed 
 differences it is well to consider certain facts in regard to the alternat- 
 ing current. 
 
 If a pulsating current which varies in value like that represented 
 in Fig. 161 (see Lesson XIX, page 153) be passed through a volta- 
 meter (Lesson IX, page 62), the amount of metal, copper, for instance, 
 which is carried by the current from the anode to the cathode is pro- 
 portional to the average value of the current. In other words, the 
 electrolytic effect of a pulsating current is dependent upon the average 
 
 220 
 
or mean value of the current. The electrolytic effect of the pulsating 
 current represented by Fig. 161, is the same as that of the uniform 
 current, the magnitude of which is, represented in Fig. 251 by the 
 height of the line FG above the line AB. If the pulsating current 
 of Fig. 161 had not been commutated, but had been led into the 
 external circuit by means of collecting rings (Fig. 157, L,esson XIX) 
 as is done in telephone magnetos, the second loop of the curve rep- 
 resenting the current would fall below the line AE, because the cur- 
 rent flows alternately in one direction and then in the other. This is 
 shown in Fig. 252, where the perpendicular distances from the line OX 
 to the wavy line are proportional to the strength of the current in the 
 circuit at each instant. During the times represented by the distances 
 AC etc., in which the loops are above the line OX, the current is 
 supposed to flow in one direction, and during the intervening times, 
 CE, etc., in which the loops are below the line OX, the current is 
 supposed to flow in the other direction. Such an alternating current 
 can have no electrolytic effect, except under exceptional circumstances, 
 since the electric current which flows in one direction for one instant 
 flows in the opposite direction for the next instant and consequently 
 the voltameter plates are alternately anode and cathode. 
 
 A different condition exists in regard to the heating effect of a 
 pulsating or alternating current. It is to be remembered that the 
 heating produced by a continuous current when it flows through a 
 circuit, is equal to the current squared multiplied by the resistance of 
 the circuit (Lesson VIII, page 53). The heating produced by a pulsat- 
 ing current is equal at every instant to the value of the currrent at 
 that instant squared and multiplied by the resistance of the circuit. 
 A curve may be drawn, as is shown in Fig. 253, the height of 
 which at each point is equal to the square of the corresponding 
 height of the curve representing the current. The height of this 
 curve of squares at each point is proportional to the power expended 
 in heating the circuit at the corresponding instant. The same total 
 power would be expended in the circuit by a continuous current 
 whose square is equal to the average height of the curve of squares. 
 In Fig. 254 the line AbCdE represents the curve of squares as 
 already shown in Fig. 253, and the height of the line FG above OX 
 represents the square of the continuous current which causes the 
 same heating in the circuit as the pulsating current. The height of 
 the line FG is greater than the square of the average value of the 
 pulsating current, and consequently the heating effeU of a pulsating 
 current is greater than that of a continuous current equal to its 
 average value. 
 
 The reason for this fact may be easily seen. The squares of num- 
 bers increase in magnitude much more rapidly than do the numbers 
 themselves. For instance, 6 is twice 3, but the square of 6, or 
 
./v /*\ , 
 7 V \ 
 
 C 
 
 FIG, 251. 
 
 FIG. 252. 
 
 FIG. 254. 
 
 FIG. 253. 
 
 FIG. 255. 
 
 FiG. 256. 
 
 FIG. 257. 
 
36, is four times the square of 3, or 9. On account of this, the aver- 
 age of the squares of different positive numbers is always greater 
 than the square of the average of the numbers. For instance, the 
 average of 2, 5 and 8 is 15 divided by 3, or 5, and its square is 25. 
 The squares of these numbers are respectively 4, 25 and 64, which 
 give an average of 93 divided by 3, or 31. Now if we square the 
 values of the pulsating current at each instant we have the squares of 
 a large number of values which range from zero to a maximum, and 
 the average of these squares is greater than the square of the average 
 of the original values. 
 
 Since the heating effect of a current is entirely independent of its 
 direction, an alternating current such as that represented by Fig. 251 
 expends exactly the same power in heating a circuit or given resistance 
 as it would if commutated into a pulsating ctirrent. 
 
 When there is no self-induction or outside disturbing factor in a 
 circuit, the power expended in the circtiit is always equal to CB 
 (current times electric pressure). Here, again, when the pressure 
 and resulting current are pulsating or alternating, we have a series of 
 products of values, the average of which is greater than the product 
 of the respective averages of the current and pressure. The line 
 ABCDB, in Fig. 255, represents the electric pressure applied in a 
 circuit, and AbCdE the resulting current. At each instant the 
 power expended in sending the current through the circuit is equal to 
 the product of the corresponding heights of these two curves. The 
 height of the curve APCQE at each point is equal to the product of 
 the corresponding heights of the current and pressure curves. Curve 
 APCQE may therefore be called a power curve. Both its loops 
 are placed above the line OX because they both represent power ex- 
 pended in the circuit. The average power expended in the circuit 
 is represented by the height of the line FG, which cuts off the tops 
 of the loops so that they will exactly fill up the intervening valleys. 
 The height of the line HJ represents the product of the average 
 current by the average pressure, which is seen to be less than the 
 average power represented by the height of the line FG. 
 
 When we measure the value of an alternating current we desire 
 to find the value which, when squared and multiplied into the resist- 
 ance of a circuit, will give the heating effect of the current. This is 
 called the effective value of the current or the effective current, and it 
 is greater than the average value of the current, as we have already seen. 
 In measuring an alternating electric pressure or electromotive force we 
 likewise desire to find the value which, when multiplied into the effect- 
 ive current which it causes to flow through a circuit without self- 
 induction, will give the power expended in the circuit. This is 
 called the effective pressure or effective electromotive force, and is 
 larger than the average pressure. From the explanation given above 
 
 223 
 
it is seen that the effective value of an alternating current or an alter- 
 nating electric pressure is equal to the square root of the average of 
 all the squares of the instantaneous values of the current or pressure 
 during the time represented by one loop in the figures. The effective 
 value is, therefore, often spoken of as the u square root of the mean 
 (average) square. ' ' 
 
 Since the indications of an electro-dynamometer or of a hot wire 
 electrical measuring instrument are proportional to the square of the 
 current flowing through the instrument (Lesson XI, page 78) they are 
 excellently adapted to measuring alternating currents. The number 
 of alternations of direction made in each minute by the alternating 
 currents which are ordinarily used is so great that the movable coil 
 of an electro-dynamometer acts exactly as though it were pulled 
 around by a continuous force proportional to the averageof the squares 
 of the instantaneous values of the current. The square root of the 
 indications of the instrument is therefore proportional to the effective 
 value of the alternating current flowing through its coils. One form 
 of electro-dynamometer which is commonly used for measuring alter- 
 nating currents is shown in Fig. 56 (Lesson XI). Another form is 
 shown in Fig. 256. Alternating current voltmeters made upon the 
 same principle are shown in Figs. 256 and 257. 
 
 Since hot wire instruments also average up the squares of the 
 instantaneous values of the current, they have been used to some ex- 
 tent as instruments for measuring alternating currents. The Cardew 
 voltmeter shown in Fig. 60 (Lesson XI), which is made upon this prin- 
 ciple, has been much used as an alternating current instrument. Elec- 
 trostatic voltmeters (Lesson XI, page 82) also give indications, the 
 square roots of which are proportional to effective alternating pressures 
 when the needle is electrically connected to one pair of quadrants as 
 is usually done, and the scales of such instruments may be so gradu- 
 ated as to be direct reading. 
 
 An alternating current is said to make as many alternations per 
 minute as it makes changes in direction in each minute. Instead of 
 speaking of the number of alternations per minute of an alternating 
 current it is quite common and more scientific to speak of its fre- 
 quency, that is, the number of double alternations made per seayid. 
 The alternating current dynamos which have been generally used in 
 this country furnish currents making from 15,000 to 16,500 
 alternations per minute or having frequencies of from 125 to i3-5> 
 though frequencies only half as great, and even less, have come into 
 'use during the past two or three years. The number of alternations 
 per minute is equal to 2X60 or 120 times the frequency, since 60 is 
 the number of seconds in a minute. The fraction of a second during 
 which an alternating current makes two loops is called \\. period. 
 
 224 
 
There is one very important point in which alternating currents 
 differ radically from continuous currents. The point is so important 
 that the balance of this lesson will be taken to illustrate it. 
 
 When a continuous current is passed through an incandescent 
 lamp, the amount of power expended by the passage of the electric 
 current through the lamp filament, which is converted into heat and 
 light, is equal to C X E. In the same way, when an alternating cur- 
 rent is passed through an incandescent lamp the amount of power 
 which is expended in the lamp filament and converted into light and 
 heat is also equal to C X E, where C and E are the effective values of 
 the current and pressure measured by the proper alternating current 
 instruments which were explained in the preceding lesson. We 
 therefore see that an incandescent lamp which is intended to give 
 sixteen candle-power at a pressure of, say, no volts, will be equally 
 efficient when it is connected to a constant pressure circuit which 
 furnishes it continuous current at a uniform pressure of no volts, or 
 when it is connected to a circuit which furnishes it alternating 
 current at an effective pressure of no volts. If the current flow- 
 ing through the lamp when it is connected to the continuous current 
 circuit be measured by an accurate amperemeter of any kind, and a 
 measurement also be made when the lamp is connected to the alter- 
 nating current circuit by an accurate electrodynamometer, exactly 
 the same amount of current will be found to flow through the lamp 
 in the two cases. 
 
 Now, suppose we take 200 feet of No. 7 B. & S. gauge insulated 
 copper wire. Its resistance is almost exactly one-tenth of an ohm at 
 ordinary temperatures, and it therefore requires only one-tenth of a volt 
 to send one ampere of continuous current through it. This is true 
 whether the wire be stretched out straight, wound in a simple coil or 
 wound around an iron core, since the resistance of the wire depends 
 only upon its length, cross section, and material (Lesson VII, page 
 48), and none of these are altered by coiling or winding up the wire. 
 
 To send one ampere of alternating current of, say, a frequency 
 of 125 (15,000 alternations per minute) through this wire when it is 
 stretched straight out requires a tenth of a volt effective pressure, or 
 the same as in the case of a continuous current. The straight wire 
 therefore acts in the same way towards continuous and alternating 
 currents, exactly as does the incandescent lamp filament, which indeed, 
 is nothing more than a bent wire made of carbon. 
 
 Now, if the wire be coiled tip, a greater pressure than one-tenth of 
 a volt is required to send one ampere through the wire, while if it. be 
 wound on a big laminated iron core there may be as much as 100 volts, 
 or even more, required to send an ampere through the wire. We know 
 that the resistance of the wire is not changed by coiling it up or by wind- 
 ing it around an iron core, so that the actual resistance is one-tenth of 
 p" ^hm all the time. This is proved by the fact that coiling the wire 
 
 225 
 
and winding it around an iron core does not change the amount of 
 pressure required to send one ampere of continuous current through 
 it. It also may readily be proved by measuring the resistance of the 
 wire by a Wheatstone's bridge when the wire is stretched straight 
 out and when it is wound on an iron core. 
 
 The action of the alternating current as thus seen might lead us 
 to suppose that the flow of alternating currents did not follow Ohm's 
 law (Lesson VII, page 42). The flow of alternating currents does fol- 
 low Ohm's law, however, and the peculiar action described above is 
 easily explained as follows: 
 
 In Lesson XXV, page 217, it was stated that on account of 
 self-induction, either an increase or decrease of current in a coil is 
 retarded by the magnetic effect of the different turns of the coil tend- 
 ing to stop any change in the current. This effect is magnified to a 
 large degree when the coil is wound on an iron core, since the iron 
 largely increases the magnetic effect of the turns and therefore the 
 self-induction of the coil ; while a wire stretched out straight or bent 
 in a hairpin, like an incandescent lamp filament, has very little self- 
 induction. 
 
 When a battery is connected so as to send a current through a 
 straight wire the current rises to its full value, according to Ohm's 
 law, almost instantly. When the same wire is coiled up and con- 
 nected to the battery, the current does not rise to its full value 
 instantly on account of the retarding effect of self-induction, but the 
 delay is only a very small fraction of a second. Now, when the wire 
 is wound on an iron core and then connected to the battery, the effect of 
 self-induction is so great that it takes quite an appreciable portion of a 
 second for the current to rise to its full steady value. The final steady 
 value reached by the current is not changed by the self-induction^ but is 
 iust the same in each case, IF THE PRESSURE BE UNIFORM, because the 
 self-induction can have an effect only while the current is changing in 
 value. 
 
 An alternating current changes all the time so that it never has 
 a steady value, and the effect of self-induction is therefore felt by it all 
 the time. While the current is rising, self-induction tends to hold it 
 back or keep it from rising, and when the current is falling, self-induc- 
 tion still tends to keep it from changing. The result is that in a cir- 
 cuit having self-induction an alternating cnrrent is always retarded a 
 certain amount behind the alternating pressure which sets it up. This 
 same retardation causes the maximum value of the current to be 
 smaller than it would be were there no effect of self -induction* 
 
 We therefore see that when an alternating current flows through 
 a circuit which has such a form that its self-induction is appreciable, 
 the alternations made by the current corne a small fraction of time 
 later than those made by the electric pressure, and the value of the 
 current is smaller than if no self-induction were present. The effect 
 
 226 
 
is exactly as though the current loops of Fig. 255 were not placed 
 directly under the pressure loops, but were pushed a certain small 
 amount back of the position of the pressure loops. The effect of self- 
 induction in decreasing the amount of alternating current which 
 flows in a circuit depends upon the magnetic effect which the differ- 
 ent parts of the circuit have on each other, and also upon the "fre- 
 quency " of the current. The same result is brought about as would 
 be given by increasing the resistance of the circuit a certain amount. 
 It is therefore usual to speak of the apparent resistance of a circuit 
 through which an alternating current flows. The effective current in a?i 
 alternating current circuit is then equal to the effective electrical pressure 
 applied to the circuit divided by the apparent resistance of the circuit. 
 This may be called the Ohm's law of the alternating current circuit. 
 The apparent resistance is equal to the true resistance of the wire compos- 
 ing the circuit plus the effect due to self-induction. The true resistance 
 of the wire only depends upon its length, cross section and material, 
 while the effect of self-induction depends upon the magnetic effect which 
 the different parts of the circuit exert on each other and upon the fre- 
 qttency of the current. When a continuous current flows through a 
 circuit, the true resistance of a circuit, as measured by a Wheatstone's 
 bridge, only need be considered, but when an alternating current flows 
 through the same circuit, the apparent resistance comes into the 
 account. 
 
 The remarkable results which are brought about in alternating 
 current circuits on account of the current hanging back or lagging 
 behind the electrical pressure will be taken up in the next lesson. 
 Before entering upon the next lesson, each member of the classes 
 should study this lesson until he gets a true idea of the lagging of 
 the loops of an alternating current behind the pressure which sets up 
 the current, and the cause of this lagging. 
 
 Copyrighted, 1895, 
 
 327 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXVI. 
 
 Points for Review. 1. What is an alternator? 
 
 2. Why is the electrolytic effect of a pulsating current the same as that of a contin- 
 uous current equal to its mean value? 
 
 3. Why does an alternating current produce no electrolytic effect? 
 
 4. Why is the heating effect of a pulsating, or of an alternating current, greater 
 than that of a continuous current equal to its mean value? 
 
 5. What is the effective value of an alternating current? 
 
 6. Why is it desirable to make alternating current measurements in effective values? 
 
 7. Why are instruments based on the principle of the electrodynamometer, or on 
 the hot wire principle, always used in measuring alternating currents?' 
 
 8. Why are instruments based upon the same principles, or on the electrostatic 
 principle, always used in measuring alternating voltages? 
 
 9. What is the " frequency" of an alternating current? What is its "period " ? 
 
 10. If an alternating current has a frequency of 100, how many alternations does it 
 make per minute? 
 
 11. What frequencies are commonly used in this country? 
 
 12. Why is the apparent resistance which a coil of wire offers to the passage of 
 an alternating current greater than the resistance which it offers to the passage of a 
 continuous current? 
 
 13. Why is the resistance which a straight wire offers to the passage of an alter- 
 nating current equal to that which it offers to the passage of a continuous current? 
 
 14. Why is the apparent resistance of a coil, in which an alternating current flows, 
 increased by putting an iron core in it? 
 
 15. Why does the apparent resistance of a coil change when the frequency of the 
 alternating current which flows through it changes? 
 
 LESSON XXVII. 
 
 ALTERNATING CURRENTS AND ALTERNATING CUR- 
 RENT MACHINERY. 
 
 (Concluded^ 
 
 There is an additional difference between the effects of alter- 
 nating currents and of continuous currents, when the alternating cur- 
 rent in a circuit with self-induction lags behind the alternating pres- 
 sure which causes it to flow. When an alternating current flows 
 through a circuit which does not have self-induction, the current 
 loops and pressure loops come together as shown in Fig. 255. Then 
 we can measure the power which is used in the circuit by an alter- 
 nating current voltmeter and an electrodynamometer, because these 
 instruments measure the effective pressure and the effective current, and 
 the two readings multiplied together give the power used in the circuit 
 (Lesson XXVI, page 223). We can therefore measure the power used 
 
 228 
 
in an incandescent lamp ivhich is operated on an alternating current 
 circuit by means of an alternating current amperemeter and nn alter- 
 nating current voltmeter, exactly in the same way that we would 
 measure the power used by it when operated on a continuous current 
 circuit (Lesson XI, page 83). 
 
 If a coil of wire, having an iron core, be substituted for the in- 
 candescent lamp, the current loops are caused by the effect of self- 
 induction to lag behind the pressure loops (Lesson XXVI, page 226). 
 When this is the case we are not able to measure the power used in 
 the coil by an amperemeter and a voltmeter, as we did in the case of 
 an incandescent lamp, because in this case the product of the effective 
 current and the effective pressure is not equal to the power. The 
 actual power used in the circuit is less than the value given by the 
 product of the effective current and pressure. At EACH INSTANT the 
 power consumed in the circuit is equal to the product of the current 
 and the pressure at that instant, exactly as is the case when the cur- 
 rent and pressure loops are together (Lesson XXVI, page 223), but when 
 the current lags behind the pressure, the total power consumed is less 
 than would have been used in sending the same current under the same 
 pressure through a circuit without self-induction. The moral of this 
 is not to try to measure the power used in any alternating current 
 circuit having self-induction, by an amperemeter and a voltmeter. 
 For instance, if the alternating current flowing in the primary coil 
 of a transformer be measured and its value be multiplied by the alter- 
 nating pressure which causes the current to flow", the product does 
 not represent the power used by the transformer. 
 
 The power used when an alternating current is caused to flow 
 through a circuit which has self-induction may be measured by a 
 proper wattmeter; such, for instance, as that made out of an electro- 
 dynamometer, explained on page 85 of Lesson XII. The indications 
 of such a wattmeter when connected to the circuit as directed in Les- 
 son XII are directly proportional to the power used in the circuit, 
 because they are the AVERAGE of the values of the power given to the 
 circuit at every instant. If it is desired to find out how much power 
 is wasted in the iron core of an alternating current transformer, for 
 instance, it can be quickly done by connecting up a wattmeter as 
 shown in Fig. 258, for then, if the wattmeter has been calibrated, its 
 readings will at once give the power. 
 
 Alternating currents are widely used for the distribution of 
 electric currents for the purpose of electric lighting, because it is 
 possible to use a high pressure on the distributing lines and thus 
 make a saving in the expense of wires (Lesson XX II, page 135), and 
 the high pressure may be reduced with little loss of power by means 
 of induction coils or transformers to a pressure which it is safe to use 
 in houses (Lesson XXV, page 216). These transformers consist 
 
of two coils, the primary and secondary coils, which have well 
 laminated iron cores made of strips or ' 'stampings' ' of thin wrought 
 iron laid together in such a manner that they make a core for the 
 coils and also enclose them so as to make a complete magnetic circuit 
 for the magnetism set up by a current in the coils. The primary coil 
 usually consists of many turns of small wire, while the secondary 
 coil consists of fewer turns of larger wire. The coils are wound on a 
 " former" and are carefully insulated with mica, rubber insulating tape, 
 or other insulating materials, and the core is then built up by slip- 
 ping the stampings into position. The right-hand cut of Fig. 248 
 shows the two coils of a transformer with the stampings which form 
 the core. The left-hand cut of the same figure shows the transformer 
 after it has been placed in a water tight iron case, as is usually done 
 to protect it from injury when hung against the wall of a house or 
 on the pole of an electric light company. In the particular type of 
 transformer shown in this figure, it is usual to fill up the case with a 
 heavy paraffine oil which improves the insulation of the coils from 
 each other. In Fig. 259 is shown the coils of a transformer made by 
 another maker and also the complete transformer out of its case, 
 while Fig. 260 shows the same transformer in its case. Figs. 261 
 and 262 show other transformers. In the latter cut, the transformer 
 is shown as it hangs on the side of a house in winter and the need 
 of the protecting case is made evident. Each of the transformers thus 
 shown is made by different manufacturers, but their similarity in 
 construction is evident at a glance. There are some differences in 
 the number and shape of the iron plates used in the cores, the sizes 
 of wire and number of turns composing the coils, etc., but the greatest 
 differences apparent to the sight are differences in the shapes of the iron 
 cases. In fact, the real differences between the transformers are very 
 small, but even these small differences affect their usefulness very much. 
 The iron core of a transformer is magnetized first in one direction and 
 then in the other, by the alternating currents in the coils, and as 
 the magnetic molecules are reversed, there is a loss of power caused by 
 hysteresis (Lesson XX, page 157). There is also a loss of power caused 
 by eddy or foucault currents which are set up in the iron core. These 
 losses are quite small compared with the full load of the transformer 
 (from 3 to 6 per cent), but when a great many lightly loaded trans- 
 formers are operated all day long, as is done in many electric light 
 plants, the total power lost may cause a great expense. The losses 
 in the cores of transformers should therefore always be tested by elec- 
 tric light companies before putting the transformers into service, and 
 if, the losses are larger than they ought to be, the transformers should 
 be sent back to the makers. The tests can be made by connecting 
 up a wattmeter to a transformer as shown in Fig. 258. If the second- 
 ary circuit is left open, the reading of the wattmeter shows the loss 
 
 230 
 
100 'VOLT 
 
 CURRENT SUPPI.V 
 
 T METER 
 
 PRE.S! URE COIL 
 
 CURRENT COIL- 
 
 TRANSFORMGR 
 
 - VOLTS^- 
 
 CIRCUIT OPEN 
 
 FIG. 258. 
 
 FIG. 259. 
 
 FIG, 263. 
 
 FIG. 264. 
 
 231 
 
FIG. 262. 
 
 FIG. 265. 
 
 FIG. 268. 
 
 FIG. 266. 
 
 232 
 
FIG. 267. 
 

 
 FIG. 271. 
 
 234 
 
of power caused by hysteresis and foucault currents. The following 
 table shows approximately the amount of power which is lost in 
 transformers of the best makes. 
 
 CAPACITY OF TRANSFORMERS. LOSS IN CORE. 
 
 500 watts = 10 lights. 25 watts 
 
 1000 " = 20 " 40 u 
 
 1500 " = 30 " 50 
 
 2500 4 ' = 50 " 60 u 
 
 4500 " = 90 u 80 " 
 
 In nearly all electric lighting plants where alternating currents 
 are used in this country, the pressure generated by the alternator is 
 between 1000 and 1200 volts, while the pressure desired at the lamps 
 is between 100 and no volts, or 50 and 55 volts. The transformer 
 coils must be wound so that the number of primary turns has the 
 same relation to the number of secondary turns as the primary 
 pressure has to the desired secondary pressure. If the pressure is 
 reduced from 1000 volts to 100 volts there must be one tenth as many 
 turns in the secondary as in the primary, and if the pressure is 
 reduced to 50 volts the secondary must have one twentieth as many 
 turns as the primary. Since the power given out by a transformer 
 is nearly as great as that given to it, the current in the secondary 
 coil is nearly as much greater than the primary current as the 
 secondary pressure is smaller than the primary pressure. 
 
 We have in transformers a most striking and wonderful example 
 of the transfer of power from one electrical circuit (the primary cir- 
 cuit) to another circuit (the secondary) without the circuits being in any 
 way electrically connected with each other. The inductive action goes 
 on just as well if the two coils of the transformer are separated by 
 glass or mica as if they are wound close together. It is only necessary 
 for the magnetic circuit to be properly arranged so that the magnetism 
 which is set up by the primary coil shall all pass through the secondary 
 coil. The action of transformers is really no more wonderful than 
 the action of dynamos, but it has the striking peculiarity that no 
 mechanical motion is concerned in the transformations. 
 
 As already said, alternating current dynamos, or alternators, are 
 built upon the same principles as continuous current dynamos, but 
 the armature is wound in coils which are connected in series, and the 
 two ends are brought to separate collecting rings. The field magnet 
 usually has as many poles as there are coils on the armature, and the 
 number of alternations of the current per minute is equal to the number 
 of poles in the field magnet multiplied by the number of revolutions made 
 by the armature per minute. Fig. 263 shows a diagram of the connec- 
 tions of an alternator armature. The coils marked AAA are armature 
 coils and the rings marked CC are the collecting rings on which the 
 
 235 
 
brushes BB rub. The arrows show the way the current flows 
 through the armature. Fig. 264 sL.cws the way the magnet poles 
 are arranged for an alternator having an armature of the form shown 
 in the above diagram. This arrangement of the armature and fields, 
 which is quite common in foreign alternators, is illustrated in Fig. 
 265 which shows the form of alternator built by an English 
 maker. 
 
 In this country the coils are usually laid on the surface of a 
 laminated drum core, or in grooves cut in such a core. Fig. 266 
 shows a finished alternator armature the coils of which are laid on 
 the surface of the core. A layer of insulation is put on over the 
 coils, and over this wire bands are placed to hold the coils in place. 
 This figure also shows on the same shaft, the armature of a small 
 continuous current dynamo which is used to magnetize the field 
 magnets of the alternator. Fig. 267 shows the way in which coils 
 are sometimes fixed in grooves cut in the armature core. 
 
 Since no commutator is required with an alternator, it is not 
 necessary for the armature to revolve, and the field may be revolved 
 instead. In this case, the magnetizing current is carried to the field 
 windings through collector rings, and the armature terminals are 
 connected directly to the circuit. It is also possible to build alter- 
 nators in which neither the field nor armature revolves, but in which 
 keepers of iron are moved so as to make and break the magnetic 
 circuit of the field magnets and thus cause currents to be induced in 
 the stationary armature. The field magnets of alternators must 
 always be excited by a continuous current, which is usually generated 
 by a separate small continuous current dynamo called an exciter. 
 An exciter is shown alongside of the alternators in Figs. 265, 268, 
 269 and 270. 
 
 The general forms of alternators constructed by the different 
 American companies are quite similar to each other, as shown in Figs. 
 268, 269 and 270. Fig. 271 shows one of the great 1,000 horse-power 
 alternators which were used in 1893 to light the buildings at the 
 World's Fair. These machines, which are the largest alternators ever 
 built, are now used in various electric light stations. 
 
 Alternators cannot be worked in parallel with each other with 
 the ease which is possible with continuous current dynamos. If two 
 similar continuous current dynamos are to be connected in parallel 
 they are simply brought to their usual speeds, and their field mag- 
 netization is adjusted until the two machines produce the same 
 pressure. They may then be connected in parallel and will work 
 together very well. When two alternators are to be connected in 
 parallel, it is necessary not only to make their pressures equal, but to 
 bring them to exactly equal frequencies or to synchronism, and also to 
 arrange them so that the current loops given by the two machines are 
 
 ?S6 
 
in exact ttnison or step. On account of the difficulty in the way of 
 properly synchronising and stepping alternators they are not usually 
 operated in parallel in this country, though it is quite commonly 
 done in foreign, countries. 
 
 If an ordinary alternator is brought to synchronism with another 
 machine it may be run by the latter as a motor, but it will not start 
 itself as would a continuous current motor, nor is it possible to 
 excite the field magnets of the motor from the alternating current 
 circuits. It is therefore not convenient to use such machines, called 
 synchronous motors for common purposes (though they are sometimes 
 used for special purposes), and other methods of operating alternating 
 current motors are being sought after. The most promising of these 
 methods is coming into considerable use. It consists of combining 
 the effects of two or three separate alternating currents in what are 
 known as two-phase or three-phase systems. 
 
 A second set of windings may be placed on an alternator arma- 
 ture with the centers of its coils half-way between the first set (as, 
 for instance if another winding were put on the armature shown in 
 diagram in Fig. 263, with its coils between those shown in the Fig.), 
 then the currents generated in the second set of coils will have their 
 maximum points just one quarter of a period after the current in 
 the first winding. That is, the two currents will have a difference 
 of phase equal to quarter of a period or 90. The relation of the 
 
 A 
 
 FIG. A. 
 
 two currents to each other is shown in Fig. A where the curves 
 A and B represent the two currents. These two currents may be 
 used separately or they may be used together as a two-phase system, 
 the two currents being carried in separate circuits composed of three 
 wires very much as the three wires compose the circuits of the three 
 wire system for continuous current distribution. Instead of two 
 windings three separate windings may be placed on the armature 
 in such a way that the three currents produced in them differ from 
 each other in phase by one third of a period or 1 20. The relations 
 of these currents are illustrated in Fig. B. These currents may be 
 used separately or they may be used together as a three-phase system, 
 the three currents being carried in separate circuits composed of 
 
 237 
 
three wires, the circuits being made up of the three wires taken in 
 *iairs. Thus, if the three dots in Fig. C represent the cross section 
 of the three wires, then current A is carried 
 in the circuit composed of the wires a and 
 b. Current B is carried in the circuit com- 
 posed of the wires b and c, and current C is 
 carried in the circuit composed of the wires 
 c and a. Either a two-phase or three-phase 
 alternator which is arranged to furnish cur- 
 rents to three wires requires only three collect- 
 ing rings, though if the currents were to be 
 used separately four and six rings would be 
 respectively required. Two-phase and three- 
 phase systems are frequently called polyphase or multiphase (many 
 current) systems, and the motors which are ordinarily operated on 
 polyphase systems are called induction motors. 
 
 The action of induction motors may be explained by reference 
 to Fig. D which is an illustrative diagram of a three-phase motor. 
 The field magnet of the motor is a ring which is wound with three 
 separate coils, P, Q and R, each of which is supplied with one of 
 the currents of the three-phase system through the wires a, b and c. 
 Since the maximum values oi the three currents which thus flow 
 through the coils P, Q and R follow one another with a phase 
 difference of a third of a period their maximum points appear to 
 chase each other around the ring. The magnetic effect of each 
 coil at every instant is proportional to the current flowing in it, and 
 the combined effect of the three currents sets up a magnetic field 
 which rotates around the ring with the maximum value of the cur- 
 rents. The space inside of the field ring is occupied by an armature 
 
 238 
 
consisting of a grooved drum built up out of iron discs. Insulated 
 copper rods are laid in the grooves, and the rods are either all 
 connected together by end rings as in Fig. E or they are connected 
 in sets as indicated in Fig. D. The rotating magnetic field set up by 
 the currents in the windings P, Q and R, induces currents in the 
 armature conductors and these in turn, on account of the reactions 
 between currents and a magnetic field explained in Lesson VI. , cause 
 the armature to revolve as nearly as possible in synchronism with 
 the rotating field. The armature with the bars all connected 
 together as shown on Fig. E is called a squirrel cage armature. 
 
 FIG. F. 
 
 FIG. H. 
 
 MAIN WINDING 
 
 PRIMARY 
 
 IMMJ 
 
 rr 
 
 FIG. I. 
 
 SECONDARY 
 FIG J. 
 
The coils of polyphase machinery may be connected in three dii 
 ferent combinations, two of which are called respectively the mesh 
 and star connections and are represented in diagram by the Figs. F 
 and G. The third method of connecting the coils is a combination 
 of the mesh and star methods. The coils on the fields of Fig. D are 
 connected in the mesh method and Fig. H shows in diagram a two- 
 phase motor which may be operated with its field coils mesh con- 
 nected to a three-wire two-phase system or with the coils connected 
 separately to two separate circuits which carry alternating currents 
 with 90 difference of phase. Two-phase machinery is constructed 
 in this country by the Westinghouse Co. and the Stanley Electric 
 Co., while three-phase machinery is constructed by the General 
 Electric Co. 
 
 A special type of three-phase alternator (called a monocyclic 
 alternator) is constructed by the General Electric Co. for use where 
 electric lighting is the principal object but it is desired to operate 
 some motors from the lighting circuit. The armature of a mono- 
 cyclic alternator carries a main coil, the two ends of which go to 
 collector rings and an auxiliary (or teaser) coil, one end of which 
 goes to a third collector ring and the other end of which is connected 
 to the middle of the main coil, Fig. i. Transformers to be used for 
 electric lightning are connected in the usual way between the wires 
 running from the brushes on the main collector rings, but a three- 
 phase current may be obtained by connecting two transformers 
 between the main and teaser wires as shown in Fig. J. The three 
 circuits of the three-phase currents are between the wires A B, B C 
 & A C. If the three terminals of a three-phase induction motor 
 are connected to these wires it will run exactly as though connected 
 to a regular three-phase circuit. 
 
 Polyphase alternators may be used as synchronous polyphase 
 motors under conditions similar to those already explained for single 
 pliase machine. 
 
 Copyrighted, 1895, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXVII. 
 
 Points for review. 1. How is the power which is used in any alternating circuit 
 measured? 
 
 2. Why are alternating currents used for electric lighting? 
 
 3. What are transformers and how are they made? 
 
 4. Why are transformers put in an iron case? 
 
 5. Why should transformers be tested by electric light companies before they are put 
 into use? 
 
 6. How can this testing be done? 
 
 7. How are alternators built? 
 
 8. How can the number of alternations per minute made by the current produced 
 by an alternator be calculated? 
 
 9. If an alternator has ten poles and the armature makes 1,500 revolutions per 
 minute, how many alternations per minute are made by the current which is produced? 
 
 10. What is the frequency of the current? 
 
 11. What is an exciter? 
 
 12. What must be done to make alternators run in parallel? 
 
 13. What are synchronous motors? 
 
 14. Why can they not be commonly used? 
 
 15. What is meant by the words polyphase and multiphase? 
 
 16. What are two-phase and three-phase alternating current systems? 
 
 17. How many wires are necessary in two-phase and three-phase systems? 
 
 18. What are induction motors, and how do they work? 
 
 19. What is a squirrel cage armature? 
 
 20. What is meant by mesh connection? By star connection? 
 
 LESSON XXVIII. 
 
 MISCELLANEOUS APPLICATIONS OF ELECTRIC 
 
 MOTORS. 
 
 During five or six years past electric motors have come to be al- 
 most a necessity to people living in small cities who use small 
 amounts of power. The wonderful way in which electric motors have 
 come into general use is very striking. The number of electric 
 motors used in Chicago in the year 1889 was very small, while in 
 1894 motors to more than four thousand horse power capacity were 
 supplied with current from the distribution system of the Edison 
 Illuminating Company of that city. In addition to these motors 
 many more are supplied with current from other central or isolated 
 plants. Chicago is not at all exceptional in the number of electric 
 motors which its inhabitants use, for large numbers are also used in 
 
 241 
 
New York, Boston, Philadelphia and other large cities. In fact 
 electric motors are as necessary to the small users of power who live 
 in American cities as gas engines are to the citizens of Paris, and they 
 have also become household necessities in many places. The use of 
 electric motors in small shops and for household purposes is by no 
 means limited to the large cities, but in all places where a continuous 
 current supply is at hand throughout the day, electric motors are 
 found in many kinds of service. They are also connected with many 
 isolated plants. One of their commonest uses is to drive small fans 
 for stirring up the air in a room in the hot summer days. Such /2m 
 motors are very common in offices, theaters and public places. An 
 interesting use of fan motors is made on the electrically-lighted trains 
 of the Pennsylvania Railroad and the -Chicago, Milwaukee and St. 
 Paul Railroad, the dining cars of which are made very comfortable 
 on hot summer evenings by several fan motors, which take current 
 from the electric light circuits. An example of motors used with an 
 isolated plant is to be seen in the great plant of the Auditorium Hotel 
 and Theater in Chicago, where motors having a combined capacity 
 of several hundred horse power are in daily use. These motors are 
 used to drive ventilating fans and small blowers as shown in Figs. 
 272 and 273, to run coal and ash hoisters, meat choppers and coffee 
 grinders in the kitchen, machinists' tools for the repair shop, bel- 
 lows for the great organ (Fig. 274), to drive a small dynamo which 
 furnishes current for the hotel bells and for other purposes. Some 
 of the dynamos of this plant are required to run all day and all night, 
 so that a supply of current is always on hand by means of which the 
 motors may be operated. 
 
 The uses to which electric motors may be put are almost end- 
 less, but a few of the common applications are illustrated in the 
 figures of this lesson. In Figs. 275 and 276 are shown a sewing- 
 machine and a dentist's lathe, each with a motor connected to it. 
 Figs. 277 to 281 show various purposes for which pumps driven by 
 electric motors are used. Electric motors driving con tractors' hoists, 
 which -are used in the construction of large buildings, are shown in 
 Figs. 282 and 283, and an electric mining hoist is shown in Fig. 284. 
 Fig. 285 shows a 115 horse power Edison motor driving line shafting 
 in Machinery Hall at the World's Columbian Exposition. In Fig. 
 286 is shown a flour mill, which is driven by the electric motor which 
 appears in the figure. This list of illustrations might be extended 
 to an indefinite extent without exhausting the various purposes for 
 which electric motors may be used, and for which, indeed, they are 
 used in great numbers. 
 
 A place in which electric motors are coming to be very much 
 appreciated and widely used is in great manufactories. The ordinary 
 method of carrying power through shops by means of great belts and 
 
FIG. 274. 
 
 243 
 
 FIG. 276. 
 
 
 
FIG. 275. 
 
FIG. 277. 
 
 FIG. 283. 
 
 FIG. 279. 
 
 FIG. 281. 
 
FIG. 280. 
 
 FIG. 282. 
 
 
 FIG. 284. 
 
FIG. 285. 
 
 247 
 
FIG. 286. 
 
 FLEXIBLE DRILL 
 
 LATHE 
 
 FIG. 290. 
 
 248 
 

 FIG. 287. 
 
 249 
 
FIG. 288. 
 
 250 
 
FIG 289. 
 
 251 
 
 OF THE 
 
 VNIVERSIT 
 
FIG. 291. 
 
 jr. ar "'": IK- * I 
 
 Fi ii ii m ai ill 
 
 FIG. 292. 
 
 252 
 
heavy shafts is very wasteful of power. Prof. Flather says in his book on 
 power measurements that wherever measurements have been made in 
 even the best arranged shops, the losses of power on account of shafting 
 and belting are shown to be enormous. The attached table shows 
 the amount of power lost in belting and shafting and the amount 
 actually delivered where it is required for use, for every hundred horse 
 power developed by the engine. The table shows that from one- 
 third to three-fourths of the power of the engine is actually wasted in 
 simply making shafting revolve and causing the belts and gears to run: 
 
 POWER LOST, POWER USED, 
 NAME OF WORKS. pER ^^ pER 
 
 Union Iron Works ................... 23 77 
 
 Frontier Iron & Brass Works .......... 32 68 
 
 Baldwin Locomotive Works ........... 80 20 
 
 Wm. Sellers & Co ................... 40 60 
 
 Pond Machine Tool Co ............... 41 59 
 
 Yale & Towne Co ................... 49 51 
 
 Ferracute Machine Co ................ 31 69 
 
 Bridgeport Forge Co ................. 50 50 
 
 Shafts and belts are a great nuisance in shops, and any conven- 
 ient arrangement which can take their place would be very useful, 
 even if it did not save power. A convenient arrangement which 
 takes their place and at the same time saves much power is of the 
 greatest, service. It is in this place that the electric motor shows one 
 of its finest characteristics. In Fig. 287 is shown a large machine 
 shop in which the power is distributed by shafts and belts, which 
 give the shop somewhat the appearance of a forest, while in Fig. 
 288 is shown a .similar shop after the lathes, planers and other ma- 
 chines are arranged to be driven by electric motors. The motors are 
 close to the machines and the electric wires leading to them are put 
 out of the way so that the shop presents an appearance which is much 
 improved. The improvement is as great in fact as in appearance, 
 because the removal of shafting and belts removes a great source of 
 danger and inconvenience, and electrical distribution of the power is 
 much less wasteful than its distribution by shafts and belts. With a 
 properly arranged electrical distribution, as much as one-half or three- 
 fourths of the powei developed by the engine may be delivered at the 
 point where it will be of use. Only from one-fourth to one-half of 
 the power of the engine is wasted instead of a waste of from one-third 
 to three-fourths of the engine's power as is the case when the power 
 is distributed by shafts and belts. The reduction in the proportion of 
 the power which is wasted and lost, which may be made by using 
 electricity instead of belts and shafts is worth a great many dollars to 
 the owner of the shops, and many shops have therefore been arranged 
 for electrical transmission, while many more are being so arranged. 
 
 353 
 
Ilj Fig. 289 is shown the power house where electricity is generated 
 to operate the motors of one great manufacturing establishment. 
 Fig. 290 shows the way in which motors are applied to drive lathes, 
 drills and other machines, while Fig. 291 shows a motor which 
 drives an elevator gear and drum without the intervention of belts 01 
 pumps. 
 
 The arrangement of electric motors which will give the btst 
 results in any shop depends upon a great many things, and can only 
 be arrived at by good judgment. The ideal method would be to 
 have one or more motors built as a part of every machine in the 
 establishment, but this would make the machinery cost too much 
 money and consequently cannot be carried out, though it would prob- 
 ably be the most efficient arrangement which it is possible to make. 
 The next best arrangement, and the one which is usually adopted, is 
 to have all large machines which require considerable power fur- 
 nished with separate motors. These may be built into the machines, 
 thus doing away with all unnecessary belting or gearing, or they 
 may be directly belted to the usual driving pulleys of the machines. 
 All smaller machinery may be arranged in groups of two to six ma- 
 chines with a motor to supply power to the machine of each group 
 through a light shaft. 
 
 The amount of power required to drive different classes of ma- 
 chinery is, as a general rule, quite uncertain. 
 
 The width of the belt which is commonly used on a machine is 
 some indication of the power required, as it may be assumed that a 
 single leather belt when running at the ordinary speed used in shops 
 will satisfactorily drive from one to two horse power per inch of 
 width. A double belt will generally drive about twice as much as a 
 single one. An exact estimate of the power used by any machine 
 cannot be made from the size of its belt, however, since the driving 
 power of a belt depends, amongst other conditions, directly upon its 
 speed, and even at ordinary speeds it may transmit very much more 
 power than the rule given above would indicate, though its operation 
 would be unsatisfactory. 
 
 The ordinary manufacturers of machinery seldom have accurate 
 information in regara to the power which is required to drive the 
 machines which they build, but the following rules, when taken in 
 connection with the information presented by the widths of the driv- 
 ing pulleys, are useful : 
 
 1. Fast running machinery takes more power in proportion than 
 slow running machinery. 
 
 2. Machines which are intended to perform easy operations very 
 rapidly may require much more power than machines which are 
 required to perform much more severe operations at a slower rate. 
 Thus, wood- working machinery usually requires, on account of its 
 rapid speed, considerably more power in proportion than iron-working 
 machinery, though the latter works a much tougher material. 
 
 3. In ordinary machine shops the power required at the ma- 
 
 254. 
 
chines is about one horse-power per thousand square feet of floor, 
 motors put in on that basis will generally do the work satisfactorily, 
 provided the machines are properly grouped and the motors are so 
 arranged that losses in belts and shafting are inappreciable. 
 
 4. Engine lathes and similar tools for iron work, of sizes not 
 exceeding a swing of 20 inches, require from ^ to i horse-power. 
 Larger lathes may require as much as three horse-power, but seldom 
 more. 
 
 5. Planers and similar tools for iron work require from 2 to 5 
 horse-power, depending upon their size and the work they do. 
 
 6. Shapers, milling machines, drills, and other smaller tools for 
 iron work, ordinarily require less than one horse-power. 
 
 7. Planers for wood working require from 5 to 25 horse-power, 
 depending upon their size and work. 
 
 8. Circular saws require from i to 10 horse-power, depending 
 upon their work. 
 
 9. Smaller wood-working tools seldom require as much as one 
 horse-power. 
 
 10. Large printing presses, such as are used for book printing, 
 require from 2 to 5 horse-power. 
 
 11. Small job printing presses require from V% to y 2 horse- 
 power. 
 
 12. Sewing machines requirt irom 2 V to l /% horse-power. 
 
 13. Passenger elevators require from 10 to 40 horse-power. 
 
 14. Freight elevators ordinarily require from 2 to 10 horse- 
 power. 
 
 15. By placing several small machines which are operated inter- 
 mittingly in one group, the power of the motor required to drive the 
 group may be much less than would be required to drive all the ma- 
 chines constantly. 
 
 Before leaving this subject, the use of electricity on boats must 
 be touched upon. Fig. 292 shows one of the "electric launches" 
 which proved such a success on the lagoons at the World's Fair, and 
 which are now used in Milwaukee and other cities. These boats are 
 very much like small steam launches or naptha launches, but instead 
 of a hot steam boiler and engine, or a disagreeable naptha engine, an 
 electric motor is attached to the shaft of the screw propeller. This 
 motor, which may be put out of sight under the floor, is operated by 
 electric current from a storage battery, the cells of which are placed 
 under the seats and under the floor so as to act as ballast. The boat 
 is not as independent as a steam or naptha launch, because the stor- 
 age battery must be charged every day to keep it in good order for 
 operating, but wherever current can be obtained for charging 
 the batteries, electric launches are very convenient and popular. 
 
 Copyrighted, 1895, 
 
 255 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXVIII. 
 
 Points for Review. 1. For what purposes may electric motors be advantageously 
 used? 
 
 2. Why are electric motors particularly advantageous for use in machine shops? 
 
 3. What causes the great waste of power in manufactories as they are ordinarily 
 arranged? 
 
 4. What advantages result from removing shafts and belts from a shop? 
 
 5. What is the ideal arrangement of motors in a shop? 
 
 6. What is the commonly adopted arrangement? 
 
 7. How may the power required to operate a shop be estimated? 
 
 8. Why does wood-working machinery require more power to drive it than iron- 
 working machinery? 
 
 9. How much power will a belt drive satisfactorily when running at the speeds 
 common in manufactories? 
 
 10. Why does the width of the belt attached to a machine give uncertain evidence 
 of the amount of power which is required to drive the machine? 
 
 1 1. Why do passenger elevators ordinarily require more power in proportion than 
 freight elevators? 
 
 12. What are electric launches? 
 
 LKSSON XXIX 
 
 ELECTRIC RAILWAYS. 
 
 The application of electric motors which probably is most gen- 
 erally known and appreciated is in propelling the electric street cars 
 which are now to be found in nearly every city of fair size in this 
 country. When the first great electric railway enterprise was under- 
 taken in the year 1888 in Richmond, Va., prophecies of failure were 
 numerous and the discouragements met by the promoters of the 
 enterprise were at times sufficient to dishearten almost any one. 
 Before the equipment of that electric railway was undertaken, various 
 experimental electric railways had been laid and operated, and several 
 had been actually constructed for the regular carrying of passengers, 
 but none of them were of such magnitude as^ the railway at Rich- 
 mond and none served to prove the adaptability of electric motors to 
 the purpose of driving cars as did the equipment which was operated 
 there. 
 
 The first electric railway which was really on a commercial scale 
 was a small line built in Berlin, Germany, in 1879 by the great firm 
 of Siemens and Halske. In 1883, the first electric railway opened 
 to the public in the United States, was operated in the gallery of the 
 Chicago Railway Exposition on a track about -1,500 feet long and of 
 three feet gauge. This electric line caused a great stir in the 
 country and carried many passengers who visited the Exposition. 
 The motor car which ran on the line weighed three tons and was 
 
capable of running at a speed of nine miles an hour. It was therefore 
 quite small compared even with the smallest of electric street cars of 
 today, which often weigh eight or ten tons and run at a speed of 
 eighteen or twenty miles an hour. Even the striking though modest 
 attempts at electric railroading made in Berlin and Chicago did little 
 to bring electric cars into general use though they did serve to stir up 
 the interest of the people. The construction of the early machines, 
 as viewed today, was unmechanical and inefficient so that great 
 improvements were required before the electric cars could replace 
 horse cars or cable cars. Since 1883 the electric car has passed 
 through a period of marked development both in this country 
 and Europe. From the beginning of 1883 until 1888 many small 
 electric railways were put into operation in this country under the 
 direction of Daft, Van Depoele, Sprague and others, but until the 
 latter date, the electric car cannot be said to have proved itself a com- 
 mercial success. From 1888 to the present day, electric stret-t rail- 
 ways have grown in number and in favor with remarkable rapidity. 
 So much is this true that the street car horse has been banished from 
 the streets of many cities, and electric cars have replaced cable cars 
 even in such cities as Omaha, Neb. , Kansas City, Mo. , Grand Rapids, 
 Mich., Baltimore, Md. , San Francisco, Cal., and elsewhere. 
 
 The principle of the electric railway is very well illustrated by 
 Fig. 293. In this figure, A is a dynamo, one pole of which is con- 
 nected through a switch and fuse blocks to the street railway track, 
 and the other pole to a wire called the trolley wire which is supported 
 over the track. The motor which drives the car is placed under- 
 neath the floor, as is shown at M in the figure, and is so geared to 
 the axles that by the revolution of its armature the car is moved 
 along. In order that current may be supplied to the motor, a mov- 
 able arm extends above the car and presses a small wheel against the 
 trolley wire. This arm is called the trolley, and the current is con- 
 veyed along it and thence down to the motor. After the current has 
 passed through the motor, it completes its circuit by returning to the 
 dynamo through the rails. 
 
 The motors which are used on electric cars are series wound 
 (Lesson XX, page 160) and their speed is controlled either by means 
 of a resistance which is placed in circuit with the motor, or by some 
 equivalent device. The motors are of various forms but those which 
 are now commonly iised are completely ironclad, so that the armature 
 is protected from mechanical injury or from being splashed by water 
 from the track (Lesson XX, page 165). Nearly all of the street 
 railway motors that are now used are arranged so that the top and 
 bottom halves of the ironclad fields may be easily separated to enable 
 repairs to be made to the armature or to the field coils. This is a 
 very important point to the electric railway owner, because railway 
 
 257 
 
service is very hard on electric motors. The machines are exposed 
 to dust and dirt and are often forced to do more work than that for 
 which they were designed. On account of the cramped space which 
 is to be found under a street car, the motors must be as compact and 
 at the same time, as light as possible. These conditions combine to 
 make repairs frequent and very expensive, unless the various parts 
 are arranged so that they may be easily accessible. In Fig. 173 the 
 top part of the motor frame is shown thrown back so that the arma- 
 ture is exposed. The same thing is seen in Figs. 294 and 295 
 which show street railway motors of other types. 
 
 The axle bearings of horse-cars are usually attached directly to 
 the framework of the car floor, and the same thing is done in cars 
 that are intended to be drawn after electric motor cars as trailers 
 or tow cars. Such a construction is not sufficiently substantial in 
 electric motor cars and the axle bearings are mounted on a strong 
 iron framework which is called a truck (Fig. 296). Upon the top 
 frame of this truck is set the car -body, while the motors are usually 
 supported from the axles and the truck framework, as shown in Fig. 
 296. It is common practice to place two motors on each ordinary 
 motor car, one being slung on each axle. This is done so as to use 
 as fully as possible all the weight of the car, in order to give the 
 driving wheels a grip on the rails. When one motor is used which 
 is geared to but one axle, the wheels are likely to slip in bad weather 
 or when the car is on grades, and the speed of the car is retarded or 
 its progress may even be stopped altogether. Some inventors have 
 arranged gearing so that one motor may drive both axles (Fig. 297) 
 but such arrangements have never proved successful when put into 
 the very hard service to which the electric car is subjected. 
 
 In the operation of electric railway motors, we have for the out- 
 going electric conductor the overhead trolley wire, and for the 
 returning current the rails furnish a path. An electric railway motor 
 is therefore in an electrical position which is entirely similar to that 
 of an ordinary motor which is moved about, and the lead wires of 
 which are slid along the electric mains. Railway motors are almost 
 always connected in parallel across a constant pressure circuit. The 
 pressure used is about 500 volts. Electric railways often reach out 
 so far from the power station at which the electric current is gener- 
 ated that a lower pressure is not practical on account of the great 
 amount of copper which would be required to carry the current with 
 a reasonable loss of power. On the other hand, a pressure higher 
 than 500 or 600 volts would be unsafe to use on circuits which 
 include bare wires suspended over the streets. The pressure of 500 volts 
 is sufficient to give a severe shock but it is not dangerous to human 
 life, as has been proved by long experience, though horses and some 
 other animals which are more sensitive to electric shocks than are 
 
 258 
 
FIG. 294. 
 
 FIG. 295. 
 
 260 
 

 FIG. 297. 
 
 FIG. 299. 
 
 261 
 
FIG. 298A. 
 
 FIG. 298s 
 
i Ti 
 
 _J^_ \ \ r~\_ 
 
 n 
 
 411 X 
 
 
 
 j 
 
 
 PIG. 300. 
 
 \/ 
 
 \/ 
 
 \ / 
 
 /\ 
 
 / \ 
 
 / \ 
 
 FIG. 301. 
 
 FIG. 302. 
 
 263 
 
 ^' 
 
 XTNIVERS 
 
human beings have been killed by shocks from electric railway 
 wires. 
 
 The trolley wire which is commonly used consists of a conductor 
 of hard drawn copper No. o, B. & S. gauge in size, which is sus- 
 pended from span wires or brackets supported on poles (Figs. 298A 
 and 2986). When the distances over which current must be carried 
 are so great that a No. o wire is of insufficient conducting capacity, 
 feeders may be run from the power station to various feeding points 
 where they are connected to the trolley wire. The trolley wire then 
 serves the same purpose in the distribution for the electric railway 
 that mains do in electric light distributing systems. 
 
 The conducting capacity of the track must also be carefully 
 looked after even in the shortest lines. The rails of which the track is 
 composed are about thirty feet long and their ends are mechanically 
 connected by means of joint plates or fish plates and bolts (Fig. 299). 
 On account of the scale which is found on the rails and fish plates, 
 the joints do not conduct electricity satisfactorily and it is necessary to 
 join the rails electrically as well as mechanically. For this purpose, 
 what is, called a bond is used. A bond is a short piece of copper wire, 
 the ends of which are riveted into the adjoining ends of two rails, and 
 it thus serves to make a good electrical connection between them (Fig. 
 300). Sometimes a copper or an iron wire is placed in the ground 
 between the rails and each rail is connected to it by means of a bond 
 (Fig. 301), and the electrical connection between the rails is made by 
 means of this continuous wire. 
 
 The electric motor has also found a place in . railway service 
 which is much heavier than that of the ordinary surface street rail- 
 ways. After working its way into favor on street railways, it came 
 rapidly into use upon light suburban railways and is now looked 
 upon as an essential feature of any new system of city rapid transit. 
 Possibly one of the most striking examples of the use of electric 
 motors upon rapid transit systems is on one of the underground rail- 
 roads in the city of L,ondon, where electric locomotives are used to 
 draw the trains, to the great improvement of the atmosphere and 
 cleanliness of the tunnels. The equipment of this railway was fol- 
 lowed by the operation of the Intramural Railway at the World's 
 Fair in 1893, and that, by an elevated railway in Liverpool, England. 
 In this country, there is now in operation the great system of 
 the Metropolitan Elevated Railroad in Chicago, and several other 
 elevated and city rapid transit railways are planned in which electric 
 motors are expected to play a prominent part. The list includes the 
 great underground railroad system which is to be built to give the 
 inhabitants of New York city a satisfactory means of transportation 
 from their business places down town to homes located a number of 
 miles away to the North. 
 
 264- 
 
Even this does not set the limit to the field of the electric motor 
 when applied to railway purposes. It has been arranged that the 
 heavy trains of the Baltimore & Ohio Railroad shall be drawn by 
 means of electric locomotives through the great tunnel just com- 
 pleted under the city of Baltimore, and the locomotives for the pur- 
 poses have already been built. One is shown in Fig. 302. It is now 
 generally believed that the electric car will invade many parts of the 
 field which has heretofore been exclusively occupied by the steam 
 locomotive, and that, in many kinds of service, the electric motor 
 will as completely displace steam locomotives as it has already dis- 
 placed horse-cars and cable cars in the smaller cities. Experiments 
 have even been made with a view of placing electric locomotives in 
 service upon main trunk railway lines, and the superintendent of an 
 important English railway, it is said, believes he could quickly 
 change his whole system from one using steam locomotives to one 
 using electric locomotives if the officers of the road so directed. Be 
 this as it may, the fact is plain that the electric motor has made a 
 wonderful record for itself when used upon electric railways in the 
 past and that its record will be much more remarkable in the future. 
 
 Copyrighted, 1895, 
 
 265 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXIX. 
 
 Points for Review. 1. When was the Richmond electric railroad constructed? 
 
 2. When and where was the first electric railroad built on a commercial scale? 
 
 3. When and where was the first public electric railroad built in the United States? 
 
 4. What is a trolley wire? What is a trolley? 
 
 5. What is a truck? Why are trucks used under electric cars? 
 
 6. Why are two motors usually used on electric cars? 
 
 7. Is the electric railway current dangerous? 
 
 8. Why is it necessary to " bond" the rails of electric railways? 
 
 9. To what purposes have heavy electric locomotives been put? 
 
 LKSSON XXX. 
 
 METHODS OF HANDLING AND CONTROLLING RAIL- 
 WAY MOTORS AND GENERATORS. 
 
 The question of getting the greatest possible amount of work 
 out of his machinery and at the same time of expending the smallest 
 practicable amount of money for its safe operation, is one which weighs 
 continually on the mind of the manager of every great electric plant. 
 It is this which leads him to watch all expenditures and keep an 
 accurate account of all the supplies used in his station. The accounts 
 show him the cost of fuel, oil. water, labor, and other items for every 
 1,000 watts generated for an hour by the dynamos. By comparison 
 of these records month by month, and with the records of other plants 
 of similar size, it is possible to tell whether every possible economy 
 is practiced which will not cause oppression to the employees or 
 injury to the plant. The record of the output of a station is 
 usually made by the switchboard attendant, who, every quar- 
 ter or half hour, enters the reading of the feeder amperemeters 
 and of the voltmeters in a large book which is properly ruled. 
 Sometimes the record is made by automatic instruments. Fig. 
 323, for instance, is a reproduction of the card taken from a 
 recording voltmeter which is used in a large central sta- 
 tion, for electric lighting. The card shows the continuous 
 
 266 
 
record of the pressure which was maintained at the centers of distri- 
 bution during twenty-four hours. The distance between two suc- 
 cessive radial lines represents fifteen minutes, and the distance along 
 the radial lines included between any two circles represents two volts. 
 Recording amperemeters are not as commonly used as are recording 
 voltmeters, as the voltmeter record is a check upon the care with 
 which the pressure is kept constant, while there is no particular 
 need of keeping an extremely exact record of the current. Fig. 324 
 shows the current sent out from a certain electric light station during 
 twenty-four hours. The hours of the day and night are laid off on 
 the horizontal line and the current at any hour is equal to the length 
 of the corresponding vertical line which is included between the 
 horizontal line and the irregular line. This shows very plainly the 
 effect of the dark hours of the afternoon, in causing a great in- 
 crease in the demand for light. 
 
 The total amount of current which is required by the customers 
 of an electric light plant changes from hour to hour with compara- 
 tive slowness, as is shown by Fig. 324, and such an amount of 
 machinery can be kept running at all times as will supply the load 
 most economically. A very different condition exists in the power 
 house which supplies current to electric street cars. Fig 325 shows 
 the amount of current sent out during one hour from an electric 
 railway power house, the record being laid out in the same way as 
 that of Fig. 324. This figure shows the wonderful range and 
 rapidity of the changes in the current supplied by the station. Since 
 compound wound dynamos which keep the pressure fairly constant 
 are used in such stations, the variations of the current cause similar 
 variations of the load on the dynamos and engines. Every effort has 
 been made to reduce the range of these changes which cause shocks 
 to the machinery and so are likely to finally result in injury or break- 
 down, and which also make it impossible to keep the machinery 
 sufficiently well loaded, so that it may be operated with the greatest 
 economy. One method which has been put on trial with a view to 
 decreasing the great changes in the load on railway stations calls tor 
 the use of a storage battery. This battery has its positive terminal 
 connected directly to the positive 'bus bar and its negative terminal 
 to the negative 'bus bar; then, when a great demand for current is 
 made by the cars, part of it is supplied by the battery, and the 
 dynamos and engines are thus relieved to some extent. When the 
 current required by the cars is small, the battery takes current from 
 the dynamos, by which means it is kept charged, and thus the varia- 
 tions of the load are made much smaller than they would be without 
 the battery. This plan has not proved very successful because the 
 storage battery is too frail to stand hard service, but when a satis- 
 factory battery is developed it will fill an excellent place. Batteries 
 
 267 
 
are also used in one or two large American and several foreign 
 electric light stations to aid in supplying the current during the 
 period of greatest load, and the batteries are then re-charged during 
 the period of light load. Batteries are more likely to last a reason- 
 able length of time when used in such service, but even here they 
 have not been sufficiently successful to come into much use. 
 
 The improvements which have the greatest effect upon the loads 
 of electric railway power stations are in the street-car motors and 
 especially in the way in which they are controlled. The earlier 
 motors which were put upon street cars were wired up so that the 
 two machines were put permanently in parallel, and they were then 
 controlled by means of resistances put in series with them. A great 
 many cars are still con trolled, in this manner. When the car is to be 
 started, a controller lever is moved so that it connects the two motors 
 to the circuit in series with a resistance. To make the cars run 
 faster, the resistance is gradually cut out of the circuit, and finally a 
 certain portion of the series field coils of the motors may be cut out 
 also, if a particularly high speed is desired. The commonest form 
 of rheostat is that known as the Thomson-Houston street car con- 
 troller, which is shown in Fig. 326. It is shown connected to the 
 motors in Fig. 327. 
 
 Another way of controlling the speed of street cars is by what is 
 called the "commutated field" method. In this case, the fields of 
 the motors are wound in separate divisions, usually three in number, 
 and the speed of the motor is controlled by connecting the field coils 
 of each motor in different combinations. This is indicated in the 
 diagram of Fig. 328, where +A and A represent the armature 
 terminals of a motor, and +c, c, -fa, a, +b, b represent 
 the terminals of the field divisions. The connections of the field are 
 changed or u commutated " by means of a controller or switch which 
 consists of a wooden cylinder or barrel on which are placed brass 
 plates of various shapes. This is shown developed (rolled out flat) in 
 Fig. 329, and the forms of the plates are well shown. These plates 
 bear against spring contact buttons set in a row at the back of the 
 switch box, each one of which is connected by a wire to one of the 
 terminals at the motor. Fig. 330 shows the buttons with the wire 
 connections which run in a cable from the switch on the car platform 
 to the motors under the car. When a car is to be started, the switch 
 handle is moved and the motors are connected with their individ- 
 ual field coils in series as indicated in Fig. 331. To run the car 
 faster, the lever is moved from point to point, commutating the 
 fields into various arrangements, until on the seventh and last notch 
 the individual field coils are in parallel. The various arrangements 
 of the field coils when the switch stands at the various points are 
 shown in Fig. 332. 
 
 368 
 
u 
 
 C, C, Switches in motor circuit. D, Switch in lamp circuit. E, reversing switch. 
 P, fuse blocks in lamp circuit. G, fuse block in motor circuit. . H, H, H, incandes- 
 cent lamps. K, K, controllers. O, lightning arresters. M, rheostat. N, trolley. 
 
 IG. 327. 
 
 OF THE 
 
3 v 
 
 FIG. 330. 
 
 l\v\\\ 
 
 FIG. 329. 
 
 Trolly 
 
 Field 
 
 Armaturfe 
 
 Ground. 
 FIG. 331. 
 
FIG. 332. 
 
 I / \ 
 
 SHOWING 
 VARIATION OF CURRENT 
 
 WITM 
 
 _L SERIES-PARALLEL &RHE08TATIC j I 1 | |_ 
 
 1 1 1~ 
 
 CONTROL. 
 
 WEIBHT Or CAR. TOTAL 1*000 1 bi J 
 , u i.d wit^ TWO B-C.,aOD M.t^ 1 
 
 - +-H----I-TH- 
 
 ffragt ArppoVos for I8alc iRhdoe|2/t . ; 
 
 - ~i~ r- \~T T-f"T~ " rT~rT" 
 
 -n+t-f; 
 
 3 4 5 6 7 8 8 10 II 12 13 14 15 16. 17 18 19 21 22 232429 
 
 SECONDS 
 
 Fie. 334. 
 
 373 
 
FIG. 312. 
 
 FIG. 313. 
 
 274. 
 
FIG. 333. 
 
 275 
 
 SE LIBRA f 
 UNIVERSITY 
 
Both of the earlier forms of controllers serve very well as far as 
 handling the cars is concerned, but the use of resistances causes a 
 great waste of power, and consequently the cars require a great 
 deal of current in starting. This in turn has an effect in increasing 
 the suddenness and magnitude of the changes of load at the power 
 station. The need for a more efficient controller which would waste 
 less power and allow the cars to start with less current became so 
 pressing that various devices were designed to meet the want. All of 
 these were reduced to some form of " series parallel " controller which 
 is now used on a great majority of electric cars. With this form of 
 controller, the motors are connected in series with each other when the 
 car is started, and are then, connected in parallel when it is desired to 
 run the car rapidly. From this use of the motors both in series and in 
 parallel, came the name "series-parallel" controller. The pull or 
 torque with which a series wound motor tends to start, depends only 
 upon the current flowing through it. If two motors be connected in 
 parallel and enough current be passed through them to start a street- 
 car, the tot?l amount of current may be as much as 80 amperes. The 
 starting effort in this case is caused by forty amperes flowing through 
 each motor. Now, if the same two motors be connected in series with 
 each other and a current of forty amperes is permitted to flow through 
 them, each will exert the same starting effort as before, and the car will 
 start with the expenditure of only half the current. Having started the 
 car, the motors must be connected in parallel in order that it may run 
 at a reasonably high speed, because when the motors are in series the 
 total pressure of 500 volts is divided between them and each there- 
 fore gets only about 250 volts. The speed of a motor depends 
 directly upon the pressure at its armature terminals and therefore 
 when connected in series, the motors will run at only half speed. 
 
 The actual process of controlling a car by the series parallel 
 method consists of starting the car with a resistance and the motors 
 in series, cutting the resistance out of circuit, and then by a series of 
 commutations indicated in Fig. 333 putting the motors in parallel 
 with each other and in series with the resistance. This resistance is 
 finally cut out of the circuit, and sometimes a portion of the field 
 windings are cut out of the circuit to make the car run at a high 
 speed. 
 
 The comparative efficiency of operating cars with motors 
 equipped with rheostat and with series-parallel controllers is illus- 
 trated in Fig. 334. The time after current is admitted to the 
 motors is laid off on the horizontal line, and the distance from the 
 horizontal to the wavy lines at any point shows the amount of 
 current flowing through the motors at that instant. The upper 
 wavy line shows the current consumed when a certain pair of motors 
 were controlled by a rheostat, and the lower wavy line shows the 
 
current consumed when the same motors were controlled in the series- 
 parallel fashion. During the first ten seconds, the rheostat control 
 required twice as much current on the average as did the series- 
 parallel control, and during the first eighteen seconds the rheostat 
 required one half more current. 
 
 A similar figure might also be drawn to illustrate the difference 
 between the amounts of current used by a careful motor man in 
 starting his car and by a careless man. The former always moves his 
 controller lever from point to point with care, and permits the 
 motors to gather speed before passing from one point to a higher one. 
 By neglecting this precaution, a considerably larger current may be 
 used than is necessary. Some steam railroads pay a bonus to the 
 engineer who succeeds in making his runs each month with the 
 least coal, and it would be a paying investment for many electric 
 railroads to pay a bonus to their motor men who succeed in making 
 the runs with the least current. 
 
 The handling of railway station generators is all carried out by 
 proper switches and controlling arrangement which are placed on 
 a switchboard to which connections are run from the dynamos, and 
 from which the overhead or underground feeders run. 
 
 In Figs. 312 and 313 are shown the dynamo and engine rooms 
 of a large railroad power station. The switchboard for this station 
 is shown in the distance in Fig. 312, and on the left in Fig. 313. A 
 diagram of the connections on this switchboard is shown in Fig. 
 314. The total length of the switchboard is 96 feet, and it is of suf- 
 ficient size to carry the connections and controlling devices for two 
 hundred and fifty feeders and the generators which furnish current 
 to the feeders. 
 
 On the lower panels of the switchboard are the amperemeters, 
 automatic circuit breakers, switches, voltmeters and rheostats for 
 the dynamos, while on the upper panels are the feeder switches, am- 
 peremeters, circuit breakers and lightning arresters. Below the 
 platform, between the two panels, may be seen the positive and neg- 
 ative bus bars and the equalizer bar. The point of contact between 
 the compound coil of the series field and the dynamo brush in each 
 machine is connected with this equalizer bar in order to prevent any 
 of the compound wound dynamos taking more than their share of 
 the load. The three (positive, negative and equalizer) terminals 
 from each generator run to a 3-pole switch on one of the lower 
 panels of the switchboard, so that when the generator is discon- 
 nected from the bus bars all three of the wires may be opened at the 
 same time by one switch. 
 
 The main positive bus bars which are shown just below the 
 platform are connected through eight 4,000 ampere switches and 
 amperemeters to the feeder bus bars which are shown back of the 
 upper part of the upper panels. From these feeder bus bars, taps 
 
are made through switches, amperemeters and circuit breakers to 
 the individual feeders. The bus bars are nearly all of them 3-inch 
 round copper, 21 feet long, supported on suitable insulation, and 
 taps are connected to them by means of split collars which are 
 clamped on. 
 
 The present boiler capacity of this plant is 3,750 horse power, 
 and the engines and generators have a capacity of 6,000 horse power. 
 Two hundred and seventy cars are now operated from this station. 
 About 60 tons of anthracite pea coal are consumed every day, and 
 three firemen and two cleaners are kept at work continuously. As 
 will be seen in Figs. 312 and 313, the armatures of the dynamos are 
 each built up on the main shaft of a double Corliss engine, which is 
 the most economical arrangement for driving large dynamos which 
 is possible. The dynamos run at a speed of about 80 revolutions a 
 minute, and each one has 10 pole pieces placed radially on its circu- 
 lar yoke which has an outside diameter of 12 feet 6 inches. Bach 
 armature is 90 inches and its commutator is 60 inches in diameter. 
 
 Copyrighted, 1895, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXX. 
 
 Points for Review. 1. How are output records kept in large electric stations? 
 
 2. Why are recording voltmeters particularly useful? 
 
 3. In what special respect does the load of an electric railway power plant differ 
 from that of an electric light plant? 
 
 4. How may a storage battery be used to smooth the ' ' load curve " of an electric 
 station? 
 
 5. What methods are used for controlling street-car motors? 
 
 6. Why is the series-parallel method more economical than the rheostat or corn- 
 mutated fields method? 
 
 7. What is the essential point in the series-parallel method of control for street-car 
 motors? 
 
 8. In what way can a motor-man economize in the current used by his car? 
 
 9. What instruments are placed on a station switchboard? 
 
 10. What is the object of the third bus bar, or "equalizer," when compound dyna- 
 mos are connected in parallel? 
 
 XXXI. 
 
 MODEL ELECTRIC PLANTS. 
 
 When Mr. J. E. H. Gordon wrote a Practical Treatise on Electric 
 Lighting in 1884, he filled the rather large book with descriptions 
 of dynamos and electric lamps made in forms which are now nearly 
 all discarded, but at that time there was little else to write about in 
 respect to the question of electric lighting. There were at that time 
 no great electric lighting plants such as we have today, nor were 
 there any even to be compared with those in existence only five 
 years later than the date of the book. With the same courage and 
 optimism which led him to say in 1881, "the day will come when 
 gas-light will be as obsolete as wooden torches, and when in every 
 house the incandescent lamp will have replaced the gas jet," Mr. 
 Gordon left space in his book for a chapter called Central Station 
 Lighting. Under the heading was only the single paragraph, U I 
 had intended to write a long chapter with the above heading, but, 
 for various rersons, I am not yet prepared to do so. I have, how- 
 ever, left in the heading for the convenience of inserting such a 
 chapter in a future edition of this book, should one ever be required." 
 
 At the present time, or ten years later than the time when Mr. 
 Gordon wrote, we have numbers of books upon the subjects of elec- 
 tric lighting and electric plants, and the progress of the decade has 
 been so enormous that many of the descriptions in Gordon's books 
 seem to belong to another age. We may say, indeed, that they do 
 belong to another age, for ten years constitutes an epoch in the 
 history of the modern development of electricity. 
 
 279 
 
It is instructive and interesting to see the way in which electric 
 plants have developed since 1884. The development is best shown 
 by figures representing plants which were built at different periods. 
 Fig. 303 shows one of the earliest electric light plants of the world, 
 the first Edison central station for the public supply of electric cur- 
 rent, which was located at Appleton, Wis., in 1881. At the left 
 hand of the figure is shown the exterior of a small frame shanty in 
 which this plant was located, while at the right hand of the figure 
 the shanty is shown with one side removed so that the plant with its 
 dynamo, pulleys and belts is exposed to view. This plant was operated 
 by water power and the gears on the water wheel shaft used to drive 
 the counter shafts to which the dynamo was belted, are shown in the 
 center of the figure. This plant was put in operation before the day 
 of the three-wire system, and it therefore has only one dynamo. 
 Behind the dynamo in the figure, the regulating and indicating 
 apparatus are vaguely seen. A peculiar and interesting point in the 
 figure is the dynamo which, it will be noticed, looks quite different 
 from those illustrated in preceding lessons. This dynamo has a 
 spindling, lean appearance which forms a decided contrast to the 
 chunky, substantial appearance of the modern dynamos. The field 
 magnets of the dynamo, which is bipolar, are divided into several 
 legs as though there were several separate horse-shoe electromagnets 
 attached to the poles. At the time these machines were built, this 
 was supposed to be the best way of constructing dynamos, but the 
 modern construction with a single short horse-shoe has been proved 
 to be the best form for bipolar dynamos with salient poles. One of 
 these old so-called u spindle shank" dynamos which was used by 
 Mr. Edison in his first public exhibition of incandescent electric 
 lights at Menlo Park in 1880, is now in the dynamo collection of the 
 University of Wisconsin, where it makes a striking contrast to the 
 appearance of the substantial later dynamos of equal capacity which 
 stand by its side. Notwithstanding its peculiar appearance, the old 
 dynamo is still good for any reasonable service, and, indeed, it had 
 been doing almost daily work from 1880 up to the time of the 
 World's Fair, where it was exhibited, and from whence it was for- 
 warded to its present place. One of the old spindle shanks with a 
 modern dynamo of the same capacity beside, it, is shown in Fig. 304. 
 
 The plant which is now located at Appleton, Wis. , is as great 
 a contrast to the original one as the old dynamo is to modern 
 machines. It now contains several fine dynamos with excellent 
 regulating devices, housed in a substantial building, which are used 
 to furnish current to incandescent and arc lights, stationary electric 
 motors, and to electric cars. 
 
 The great landmark in electric central stations, the Pearl Street 
 station of New York city, was operated continuously from the fall oi 
 
 380 
 
FIG. 303. 
 
 FIG. 304. 
 
 OT 
 
FIG. 305. 
 
 FIG. 306. 
 
(U: 
 
 FIG. 307. 
 
 283 
 
FIG. 308. 
 
 FIG 309. 
 
 284, 
 
FIG. 310. 
 
 FIG. 311. 
 
 285 
 
 t 
 
 OF THE 
 
 UNIVERSITY^ 
 
FIG. 315. 
 
 FIG. 316. 
 
 286 
 
1 882 until a short time ago, when it was destroyed by fire. It has 
 now been replaced by a magnificent station to which reference will 
 be made later. Fig. 305 shows one of the great "Jumbo" dynamos 
 which were used in this station, each directly coiipled to its own 
 engine. Each one of these dynamos had a capacity of i,5OOsixteen- 
 candle power incandescent lamps and occupied not less than 175 square 
 feet of floor space. It is interesting to compare the Jumbo machine with 
 one of the latest triumphs of electrical engineering, the great "steam 
 dynamo" shown in Fig 306, which has a capacity of 3,600 sixteen- 
 candle power incandescent lamps and occupies but little more floor 
 space than the Jumbo. The Jumbo dynamos were wonderful 
 machines in their day and a few are still running in European elec- 
 tric light stations, but most of them were soon superseded by faster 
 running central station dynamos driven by belts instead of being 
 directly coupled to engines. 
 
 This move in the line of construction changed the arrangements 
 of city central stations so that several great plants built in New York, 
 Chicago, Philadelphia and Boston were constructed after the general 
 plan shown in Fig. 307. This figure shows a cross section of one of 
 the central stations of the Edison Electric Illuminating Company of 
 New York city. Here we see high speed steam engines located in 
 the basement so that they may be on a solid foundation, and from 
 their fly wheels belts run to dynamos located upon the floor above. 
 The two floors above the dynamos are occupied by boilers which 
 furnish steam to the engines located in the basement, and by arrange- 
 ments for handling the ashes which come from the boiler furnaces. 
 Above the boilers is a floor wholly given over to bins for holding coal 
 for the boilers, which is hoisted from the street by an elevator. The 
 top floor is given to repair shops, store rooms, etc. Fig. 308 shows 
 the front of the Central Station building. This Central Station 
 fairly represents the type which has been used for a number of years 
 in great cities where, on account of the expense of land, it is desira- 
 ble to occupy as little ground space as possible. In the great stations 
 which have been built in Chicago, Boston and New York within 
 three or four years, the arrangement is made still more economical. 
 This will be referred to later. 
 
 In the smaller cities and towns where land is not so valuable, it 
 is usual to place the boilers on the ground floor with the engines, and 
 the dynamos are then placed either upon the same floor or on the 
 floor above. One arrangement of a central station, with the boilers, 
 engines and dynamos all on the same floor, is well shown in Fig. 309. 
 Two engines are shown in this with a dynamo driven by a' belt from 
 each fly wheel, and between the engines a shaft is coupled so that 
 additional dynamos may be belted from its pulleys. A station with 
 boilers and engines on one floor and the dynamos on the floor above 
 
 287 
 
is very well shown in Fig. 310 ^hich is a cross section of a large 
 plant. Fig. 311 shows the boiler room of another great plant simi- 
 larly arranged. These figures are taken from actual plants which 
 are in successful operation and their countei parts may be seen in a 
 great many cities and towns in this country. Each plant illustrated 
 is a model of its kind and from that stand point will bear the 
 closest comparison which the classes may make between it and 
 plants which they may have the opportunity of examining. 
 
 After several years, during which small dynamos were used in 
 electric plants belted to counter shafts or directly to the fly wheels of 
 engines, the manufacturers of dynamos began again to make dyna- 
 mos, which, like the "Jumbo ? ' machines, should be directly connected 
 to engines, and the largest central stations are now built with such 
 machines (Fig. 306). The greatest machines of the kind ever built, 
 and indeed the largest dynamos of any kind, are the great dynamos 
 which are now being erected in the power house of the Niagara Falls 
 Power Company at Niagara Falls. 
 
 The works of this company constitute the greatest industrial 
 power plant ever constructed. A general view of the plant is 
 shown in Fig. 317. Taking water from, the Niagara river above the 
 falls, a canal built for the power company by the Cataract Con- 
 struction Company conducts the water about 1,500 feet, to where the 
 water wheels are located. These wheels are located at the bottom of 
 an enormous wheel pit 179 feet deep, 21 feet wide, and of sufficient 
 length to permit the location of many very powerful turbine water 
 wheels. The water is conveyed from the canal on the surface of 
 the ground down to the wheels at the bottom of the pit, through 
 great steel tubes or " penstocks " seven and a half feet in diameter. 
 After the water has passed through the wheels, delivering up to them 
 its power, it is carried away through a tunnel a mile and a quarter 
 long, to be discharged into the river below the falls. The canals 
 and tunnels of the Niagara Falls Power Company have been con- 
 structed on such a scale that the amount of water which will pass 
 through them is capable of delivering 125,000 horse-power to the 
 water wheels, and the charter of the company permits it to take as 
 much water as will give 200,000 horse-power. The amount of power 
 represented by this is as much as one-tenth of the power which can 
 be developed by all the water wheels in the United States, and is 
 greater than the water power of the following great power and 
 manufacturing centres, all added together: Lawrence, Lowell and 
 Holyoke, Mass.; Manchester, N. H.; Lewiston, Me.; Bellows Falls, 
 Vt; Rochester, Cohoes, Oswego and Lockport, N. Y. ; Paterson, N. 
 J. ; Augusta, Ga. ; and Minneapolis. Even this enormous amount of 
 power which the Niagara Falls Power Company proposes to supply 
 to its customers is very small compared with the power which is 
 contained by all the water in the falls. If all the power represented 
 
 388 
 
by the water as it flows from the upper rapids over the falls BO the 
 lower rapids were utilized, it would make about eight and a quarter 
 million horse power, or more than four times as much as the 
 of all the water wheels in the United States, and considerably moi 
 than the combined power of all the steam engines and water wheels N 
 which are used in this country. The Niagara Falls Power Company 
 were not able to take advantage of the total height down which the 
 water flows, but if the power of all the water in the falls were as 
 fully utilized as the power company propose to utilize that of {he 
 water which they pass through their wheels, it would still yield four 
 million horse-power, or much more than half of all the power 
 now used in the country. It is seen from this that the great 
 plans of the Niagara Falls Power Company, when fully carried 
 out, will divert only about one- twentieth of all the water from 
 the falls, and plenty will remain for the purposes of other power 
 companies, if the organization of others becomes desirable, and yet 
 leave sufficient water so that the grandeur and beauty of the falls 
 shall not be injured. 
 
 In Figs. 318 and 319 are shown two views of the wheel pit 
 and power house of the Niagara Falls Company. The first figure 
 shows a vertical section taken crosswise through the wheel pit 
 and house and the second shows a vertical section taken 
 lengthwise through the pit and shows the positions of two of the 
 water wheels and dynamos which are now being erected. In the 
 lower left hand corner of the latter figure is seen the tail race tunnel 
 by which the water is discharged into the river. In the figures, W 
 W are the water wheels, which are twin wheels having the enormous 
 capacity of 5,000 horse-power, and P P are the penstocks. S S are 
 great hollow steel shafts no less than thirty-eight inches in diameter, 
 except at the bearings where they are solid and eleven inches in dia- 
 meter. Each shaft conveys the 5,000 thousand horse-power developed 
 by the wheel, to which it is attached, to a great dynamo fastened to 
 its upper end. At C in the figure, the canal which brings water to 
 the penstocks is shown, and at T is shown the electric traveling 
 crane, capable of lifting fifty tons, which is placed in the power 
 house to be used in placing the machinery in position and in case the 
 machinery must be taken to pieces at any time for the purpose of 
 repairs. Three of these ( ' generating units ' ' will soon be ready to 
 deliver power to such mills as are located within a short distance of 
 the great power-house. The 5,000 horse-power water wheels which 
 are over five feet in diameter and revolve at a speed of 250 revolu- 
 tions per minute are marvels of engineering and constructive skill, 
 but we cannot stop to consider their details or the remarkable bear- 
 ings upon which are supported the enormous weights of the dynamo 
 and shaft which are connected to each wheel and which amount to 
 a total of some 80 tons. The revolving parts of each dynamo 
 
 289 
 
alone weigh 40 tons and are of the most massive character. These 
 dynamos, which were designed and built by the Westinghouse Elec- 
 tric Company, generate a two-phase alternating current at 2,000 volts 
 pressure, having the quite low frequency of 25 periods per second, 
 and it is expected to use the currents for operating either motors or 
 lights. As the plant is primarily designed for the transmission of 
 power to factories and mills, it is expected that the greater part of 
 the current will be used in operating motors. Thus far, only three 
 generating units of 5,000 horse-power each have been ordered for 
 the electric power-house, although some additional water power is 
 now being furnished directly to paper mills. The present unfinished 
 condition of the electric power-house is plainly shown by Fig. 320, 
 which is from a photograph lately taken at Niagara. The figure 
 shows plainly where the great dynamos are being erected. The 
 frame of the dynamo field magnets, which compose the revolving 
 part, is a ring of forged steel made by the Bethlehem Steel Company, 
 by the same process which is used by them in making armor plate for 
 the Government men-of-war. The constructive details of the pole 
 pieces and the armature have not yet been made public by the man- 
 ufacturers and therefore cannot be described. Fig. 321 shows the 
 way in which the dynamos will appear when entirely completed. 
 
 The first customer to which electric power will be delivered 
 when the great dynamos have been put into service will be the 
 Pittsburg Reduction Co., whose works for the production of aluminum 
 by electro-metallurgy are to be moved from Pittsburg, Pa. to Niagara, 
 in order to take advantage of cheap electric power. It is expected 
 that other manufactories will follow and that quite a colony of large 
 mills and factories will in time be gathered about the Niagara electric 
 power house. To these mills it is proposed to distribute the current 
 at 2,000 volts pressure. It is also proposed to distribute power at an 
 early date to factories at considerable distances, and even to power 
 users in the city of Buffalo, thirteen miles away. After a time it is 
 even possible that power will be furnished, as has been proposed, 
 from the Niagara plant for the purpose of propelling canal boats on 
 the Erie canal, and for manufacturing purposes in cities as far from 
 Niagara as Rochester, Syracuse and Albany. For the transmission 
 of power over these long distances, the pressure at which the current 
 is supplied to the lines will be raised by means of transformers from 
 2,ooo volts to 10,000 or 20,000 volts or even higher, and will be 
 reduced to a safe value by transformers before entering the consum- 
 ers' premises. Many of the proposals that have been made in the 
 newspapers in regard to the transmission of power from Niagara are 
 manifestly impractical, but many of its possibilities may yet be unap- 
 preciated, and it is impossible to tell what developments may occur. 
 
 Before leaving the question of central stations, it is well to 
 examine the common methods of handling dynamos in a plant 
 
 290 
 
FIG. 317. 
 
 291 
 
FIG. 319. 
 
 FIG. 320. 
 
 292 
 
FIG. 321. 
 
 VJUL2JLJLJUL 
 FIG. 322. 
 
 293 
 
designed to furnish electricity for lights and power. As has already 
 been explained, the current from the dynamos is led to the switch- 
 board by conducting cables of the proper size, which are connected 
 to the bus bars through proper indicating instruments and switches. 
 In continuous current low pressure stations, where shunt-wound 
 dynamos are used, one dynamo terminal is usually connected directly 
 to the proper bus bar without the intervention of a switch, while the 
 other dynamo terminal is connected to its bus bar through a single 
 pole switch. In alternating current' stations, where the dynamos 
 furnish a pressure of 1,000 volts or more, a double pole switch to 
 which both cables from the dynamo are connected is deemed essen- 
 tial. It is usual to operate continuous current dynamos in parallel on 
 one set of bus bars, but alternators are almost always operated on 
 separate circuits in this country, on account of the difficulty of keep- 
 ing them in step (L,esson XXVII, page 237). This makes quite a 
 difference in the arrangement of the switchboards in the two kinds of 
 stations. In continuous current stations all feeders are connected 
 directly to the main bus bars, but in alternating current stations the 
 feeder switches are usually arranged so that any feeder may be indi- 
 vidually connected to any dynamo as desired. 
 
 Fig. 315 shows the switch board of a 3 -wire Edison incandescent 
 lighting station. The dynamo regulators are shown on the lower 
 part of the board and are numbered i, 2, 3 and 4 to correspond with 
 the numbers of the four dynamos used. Directly above these are the 
 positive, neutral and negative bus . bars. Still higher on the board 
 are seen four ammeters which are connected to the four dynamos and 
 which, at all times, tell exactly the amount of current being 
 supplied by each machine to the bus bars. On each side of these 
 ammeters may be seen two sets of feeders, each set having three 
 wires which are connected through switches and cutouts with the 
 three bus bars. Between the feeder switches on the left and the am- 
 meters there are three incandescent lamps arranged in the form of a 
 triangle. These constitute the ground indicator which has already 
 been described in Lesson XXIV, page 207. 
 
 At the extreme left and also at the extreme right are pressure 
 indicators and multiple arcing galvanometers. The latter are 
 used when it is desired to cut a fresh dynamo into the circuit and by 
 means of them it is determined when the pressure of the dynamo is 
 the same as the pressure of the bus bars. If a dynamo were cut into 
 the circuit at a time when its pressure was not equal to the pressure 
 of the circuit there would be a flicker or jump in the lamps at the 
 moment of closing the switch and the dynamo would take either a 
 large load or else would have current forced into it so that it would 
 tend to run as a motor, depending upon whether its pressure were 
 higher or lower than that of the bus bars. 
 
 One of the pressure indicators is connected to one side of the 
 
three-wire system and the other to the other side by means ot a series 
 of switches located beside the pressure indicators. The switches are 
 so arranged that the indicators may be connected with any of the 
 centers of distribution on the line or with the bus bars. 
 
 The switches for cutting the dynamos in or out of the circuit 
 are not shown in the figure but are located upon the head boards of 
 the dynamos as shown in Fig. 304. 
 
 We will suppose, for an example, a large continuous current 
 station in which one or two engines with their dynamos have been 
 running all day to supply the demand for current in the daytime, 
 and, as evening approaches, additional engines and dynamos must 
 be put into service to provide for the greater demand for current dur- 
 ing the hours of dusk. A short time before additional machines 
 are likely to be needed one or more engines with their dynamos are 
 made ready for running, and are then started at a slow speed to warm 
 them up. After a time one of the sets is brought to full speed and 
 the dynamo attendant at the switchboard changes the resistance in 
 the field circuit by means of the dynamo regulator, which is placed 
 on the board, until the lamps mounted on top of the dynamo burn 
 with approximately normal candle-power. The dynamo is then 
 ready to be put into circuit whenever it is needed. When this time 
 comes, the switchboard attendant connects the free terminal of the 
 dynamo to the dynamo galvanometer (Fig. 322) and moves the dynamo 
 regulator until the galvanometer needle comes to zero. The pressure 
 developed by the fresh dynamo is then exactly equal to the bus bar 
 pressure. The attendant now closes the dynamo switch, thus putting 
 the machine into circuit, and then moves the regulator until the 
 amperemeter shows that the dynamo is taking its proper proportion 
 of the load. While this is being done, another generating set is 
 brought to speed and made ready to go into circuit whenever it is 
 required. The operation is repeated until all the dynamo capacity 
 that is required during the period of heavy load is in service. Some 
 cities are subject to sudden periods of darkness caused by clouds 
 or smoke, and at such times it often requires very prompt action on 
 the part of station attendants to get the dynamos into circuit as 
 quickly as they are needed. 
 
 After a period of heavy load is over, the dynamos are withdrawn 
 from the circuit and the engines shut down. When a dynamo is to 
 be withdrawn from the circuit, its regulator is moved until the 
 amperemeter shows that it carries very little load and the switch is 
 then opened. 
 
 The process of getting extra dynamos into service in an alter- 
 nating current station is quite similar to the preceding, but after the 
 dynamo is made ready to receive its load, it is not put in parallel 
 with another machine but one or more feeders are transferred to it 
 from another alternator by means of the feeder switches. 
 
 29o 
 
The arrangement of the connections in an electric railway power 
 station, where compound dynamos are used, has been indicated in 
 the preceding lesson. The method of getting compound machines 
 into and out of circuit is much the same as when shunt-wound 
 machines are used. 
 
 Fig. 316 shows the electric light station at Tokio, Japan. We 
 see by the appearance of this that that peculiar nation, the Japanese, 
 have adopted the comforts of civilized life as well as the methods of 
 war developed by civilized nations. 
 
 Copyrighted, 1895, 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXXI. 
 
 Points for Review. \. How long has it been since electric lighting plants became 
 common ? 
 
 2. When was the first central station put in operation, and where ? 
 
 3. What changes have been made in the general mechanical construction of 
 dynamos in the past ten years ? 
 
 4. Where was the first large central station of the world located ? How long ago 
 was it started ? 
 
 5. How do the great steam dynamos of the Pearl street station compare with those 
 built now ? 
 
 6. What kind of electric stations were built after the ' ' Jumbo " type of dynamos 
 were abandoned by dynamo manufacturers ? 
 
 7. What instruments are placed on a station switchboard ? 
 
 8. What is the object of the third bus bar, or "equalizer," when compound dyna- 
 mos are connected in parallel ? 
 
 9. What is the object of " multiple arcing galvanometers " or "dynamo galvano- 
 meters," as they are sometimes called ? 
 
 10. Why are double pole switches always used on the switchboards of alternating 
 current power stations where 1,000 volts are used, while single pole switches are used in 
 stations where current is supplied at low pressure? 
 
 11. Why is it of advantage to operate dynamos in parallel? 
 
 12. Why are alternators not usually operated in parallel? 
 
 13. How are the feeders usually connected in a continuous current station? How 
 in an alternating station? 
 
 14. What is the object in an electric station of starting a spare engine before it is 
 actually needed for service? 
 
 15. What is the process of putting a dynamo into circuit in a constant pressure 
 electric power station? 
 
 16. Why is it necessary to have the pressure of the incoming dynamo exactly equaJ 
 to that of the bus bars before the new dynamo is connected to the circuit? 
 
 17. What is the process of cutting a dynamo out of circuit? 
 
 XXXII. 
 
 UNDERWRITERS' RULES, ETC. 
 
 The importance of using the utmost care in laying out and 
 putting in place the electric light wires which go into houses has 
 already been explained in Lesson XXIII. It now comes to an 
 explanation of the more important rules for this work which have 
 been issued by various associations of underwriters or fire insurance 
 companies. These associations issue rules for carrying on electrical 
 wiring, and in the large cities supervise or inspect the work in order 
 that danger from fire may not be introduced into buildings insured 
 by them. Many of the chances for danger which exist in electric 
 plants are caused through carelessness or lack of knowledge and 
 
 297 
 
experience on the part of wiremen who may be employed on account 
 of the false economy of the owner of the plant. In electrical work, 
 as in much else, the cheapest is by no means always the be'st, but it 
 is often difficult to make this fact seen, so that a carefully enforced 
 set of rules for wiring is the best safeguard which the owners of 
 buildings and the underwriters have against dangers caused by care- 
 less workmen and poor workmanship. 
 
 The following points require to be specially looked after: 
 
 1. That the general workmanship be good, and especially that 
 joints be well made and well insulated. 
 
 2. That the conductors have ample cross-section, and contain 
 the fewest possible joints. 
 
 3. That the insulating material on the conductors be of the 
 very best, and that the insulation resistance of the completed wiring 
 be sufficiently high. 
 
 4. That the insulation resistance of the wiring be tested from 
 year to year to ascertain whether or not it is deteriorating. 
 
 5. That all constant pressure circuits be properly protected bv 
 safety fuses. 
 
 By insulation resistance, is meant the resistance as measured 
 from either conductor of the plant to the ground, or from one con- 
 ductor to the other. Practical methods for making insulation tests 
 have already been explained in L,esson XXIV. The actual resist- 
 ance of the insulation of the wiring in any particular building, will 
 always depend upon the length of wire, number of lamps, and char- 
 acter of the fixtures used in the installation. Thus, for instance, 
 if wire is used which has an insulation resistance of 1,500 megohms 
 per mile and ten miles are used the total insulation of the wire cannot 
 be expected to be more than 150 megohms, while if only two or three 
 miles of wire were used, the total insulation resistance might be ex- 
 pected to be much greater. As a general rule, leakage at joints, 
 lamp sockets, fuse blocks, and fixtures of all kinds, has a much more 
 marked effect on the insulation resistance of new wiring than does 
 the leakage through the covering of the wire itself, so that the under- 
 writers require these points to be specially well looked after. It is 
 usual to expect a much higher insulation in wiring, before the sock- 
 ets and fixtures are connected up than afterward, and in some places 
 the insulation resistance which is required in any plant is allowed to 
 depend upon the number of lamps which are connected to the wires. 
 
 Unless the best of materials and workmanship are used for the 
 wiring put in a building, the insulation resistance will begin to fall 
 within a few months, even though it was very high when the wiring 
 was first put it. This fall in the quality of the insulation is due to 
 several causes, chief among which are poorly insulated joints and 
 inferior rubber in the covering of the wires. Portions of wiring which 
 had been in service from a few months to a few years have often 
 
 298 
 
been removed, and which in the meantime had so deteriorated, 
 that in certain spots the rubber covering on the wire had practically 
 all rotted away. It is sufficient to say that good rubber-covered 
 wire does not act in this way. On account of the deterioration of 
 poor material, an inspection is made of wiring from time to time in 
 some cities, and, if any u tap " falls below 100,000 ohms in insula- 
 tion measured between the wires and the ground, or between the 
 wires themselves, it is required to be repaired. 
 
 It is necessary to use safety fuses on all constant pressure cir- 
 cuits. Safety fuses must be of such a capacity that they will blow 
 or melt just above the rated carrying capacity of the smallest wire 
 which they protect. It is customary to place fuses at every point 
 where a change is made in the size of wire, excepting where small 
 fixtures or drop cords are attached to tap lines. L,esson XXIII. 
 
 The rule governing the minimum number of lamps ultimately 
 dependent upon one cut-out varies in different cities. The New 
 York rules at one time required that each fixture, even if it were 
 only a single lamp drop cord, must be connected to the tap line by 
 safety fuses. The Chicago rules now allow groups of lamps requir- 
 ing five amperes to be operated through one set of fuses. All motors 
 must be protected by double-pole cut-outs and controlled by double- 
 pole switches. All cut-outs must be so placed that they can be 
 readily seen and reached. 
 
 In general the size of a fuse depends upon the size of the 
 smallest conductor it protects, and not upon the amount of current 
 to be used in the circuit. Below is a table showing the safe carrying 
 capacity of copper conductors of different sizes in Brown & Sharpe 
 gauge as given in the rules of the Chicago Fire Department: 
 
 TABLE A. TABLE B. 
 
 Concealed Work. Open Work. 
 
 B. & S. G. Amperes. Amperes. 
 
 0000 
 
 ... 218 
 
 312 
 
 000 
 
 181 
 
 262 
 
 00 
 
 150 
 
 220 
 
 
 
 125 
 
 185 
 
 I . 
 
 105 
 
 156 
 
 2 
 
 88 
 
 I3 1 
 
 3 
 
 75 
 
 no 
 
 4 
 
 63 
 
 92 
 
 5 
 
 53 
 
 . ... 77 
 
 6 
 
 45 
 
 65 
 
 8 
 
 33 - 
 
 46 
 
 10 
 
 25 
 
 32 
 
 12 
 
 17 
 
 23 
 
 14........ 
 
 12 
 
 16 
 
 16 
 
 6 
 
 ... 8 
 
 18 
 
 3 
 
 ......... 5 
 
 299 
 
The safe capacities given here are greater than those given in 
 some rules, but experience has shown that they are amply small for 
 real safety, provided the wiring is well done and proper sized fuses 
 are used. 
 
 Diagram shotting circuit for twio Push Buttons for & single Bell. 
 
 X 
 
 Diagram anotuiog circuit tor ringing twuo Bella from one Push Button, 
 FIG. 334. 
 
 FIG. 336. 
 
 FIG. 335. 
 
 300 
 
By ' * open work ' ' is meant construction which admits of all 
 parts of the surface of the insulating covering of the wire being sur- 
 rounded by free air. The carrying capacity of 16 and 18 wire is 
 given, but no wire smaller than 14 is to be used, except for fixture 
 work. 
 
 Until comparatively lately, no uniformity existed in the rules 
 which were in force in different parts of the country, but the associa- 
 tions of the underwriters located in each city or district made their 
 own rules. This resulted in much annoyance, and did not tend to 
 produce the best workmanship; and it was found to be of advantage 
 to formulate a satisfactory set of rules for general adoption, which 
 was done. These rules were not only approved and adopted by the 
 various associations of underwriters, but were also approved by the 
 two large associations composed of the officers of electric light com- 
 panies the National Electric L,ight Association and the Association 
 of Edison Illuminating Companies. 
 
 The set of rules thus approved are now generally printed by the 
 local Boards of Underwriters of the various cities, and can be ob- 
 tained from their inspectors. In some cities, the city authorities con- 
 trol this matter and either furnish their inspectors with the generally 
 adopted rules or with some equivalent. The approved rules divide 
 electric light and power circuits into six classes, i. The circuits 
 inside of central stations and the dynamo rooms of isolated plants. 
 2. L,ines constructed for the purpose of operating arc lamps in 
 series. 3. High pressure alternating current lines. 4. L,ow 
 pressure continuous current lines, and low pressure inside wiring 
 (low pressure circuits being taken to include all circuits on which the 
 pressure does not exceed 300 volts). 5. Electric railway circuits. 
 6. Primary and storage battery circuits. For each of these classes 
 of circuits, special rules are directed towards the perfect safety of the 
 systems, which especially emphasize the very essential points ex- 
 plained above. Every rule has a good reason for its existence, and 
 experience has shown its propriety. An excellent statement of the 
 reasons why each rule should be carefully carried out is given by 
 Mr. C. C. Haskins, the electrical inspector of the city of Chicago, in 
 a series of articles published during 1895 ^ n tne Electrical Journal. 
 
 The only classes of wiring with which the underwriter's rules 
 do not deal directly are connected with telephone, district messenger 
 call, burglar alarm, electric bell, and similar systems which are oper- 
 ated by electric batteries. Even in regard to these wires the rules 
 enjoin proper precautions to prevent electric light and power wires 
 from becoming crossed with the poorly insulated battery circuit 
 wires. 
 
 The wires used for these battery circuits have a very different 
 
 301 
 
insulation from that of the electric light wires. The wire com- 
 monly used inside of buildings for such circuits is called " annuncia- 
 tor wire," which is a copper wire with an insulation consisting of 
 two heavy cotton wrappings wound in opposite directions, which are 
 thoroughly waxed and paraffined. These wires are made of various 
 colors and are frequently striped in different colors. Sometimes what 
 is known as "office wire" is used for telephone and messenger call 
 connections. The insulation of office wire ordinarily consists of two 
 braidings of cotton well soaked in paraffine. 
 
 While no danger can arise from the use of these poorly insulated 
 wires for such circuits, provided they are not in a position to become 
 crossed with electric light wires, yet a great deal of inconvenience 
 may be caused by them. For instance, in Fig. 334 are diagrams which 
 show the arrangement of electric bell circuits. The battery consists of 
 one or two open circuit cells, which are connected in series with the 
 bell and "push button" by wires, which may run through the walls 
 of a house. When the button is pushed it closes the circuit and the 
 bell rings. When the button is not being pushed the circuit should 
 be open and the battery be at rest. If a leak occurs from wire to 
 wire the battery remains in action all the time, and the depolarizer 
 (if the battery has one) soon becomes exhausted and the battery 
 becomes polarized or " run down. " The bell then refuses to ring 
 when the button is pushed. If the battery has no depolarizer (L,esson 
 IV, page 24), the process of running down occurs in exactly the same 
 way, but more rapidly. This is the condition of numberless elec- 
 tric bell circuits in houses all over the country where the front door 
 bell fails to ring when its button is pushed. The trouble is caused 
 by the current leaking from poorly insulated wires where they come 
 in contact with dampness or at some point where they are both 
 placed under one metal staple, and the difficulty in a great majority 
 of the cases would never have appeared had wire with good rubber 
 insulation been used. As No. 18 B. & S. wire is commonly used for 
 bell circuits, the extra cost caused by using rubber covered or 
 weather proof wire is not very great, while the inconvenience 
 avoided by its use may be considerable. 
 
 It must not be assumed that all the trouble to which bell circuits, 
 and similar circuits are heir, comes from poor insulation. Battery 
 zincs become used up or the water evaporates and the battery may 
 not work on that account. The mechanism of bells and push buttons 
 is very simple and not likely to get out of order, but trouble may 
 occur even in them. The contacts in push buttons gradually become 
 corroded and when the button is pushed it does not complete the 
 circuit. This is easily remedied by taking the cover off the 
 button and scraping the contact points. When a bell gets out of 
 order a little testing will quickly locate the trouble. The mechan- 
 ism of a bell consists of a stationary electromagnet (Fig. 335) with a 
 
vibrating armature A which is fastened at one end to a spring hinge 
 S and carries at the other end the bell clapper H. When an 
 electric current is passed through the electromagnet of a bell, the 
 armature is attracted and moves forward so that the clapper strikes 
 the gong. At the same time the electric circuit is broken by a spring 
 contact C at the back of the armature, the magnet loses its mag- 
 nesism, and the armature flies back to its original position. When 
 the armature flies back, the circuit is again completed at the spring 
 contact, the armature flies forward, the clapper strikes the gong, and 
 the whole process is rapidly repeated over and over again as long as 
 the electric circuit is complete at the push button. The back and 
 forth motion of the armature causes the clapper to strike a succes- 
 sion of blows on the gong and thus causes the ringing of the bell. 
 Wheti a bell gets out of order, the trouble is usually to be found in 
 the spring contact, which may be dirty or out of adjustment, or the 
 electromagnets may be short circuited. Fig. 336 is a diagram of a 
 push button, the simplicity of which may be seen at a glance. 
 
 It is an interesting fact that the use of electric bells was the 
 first application of electricity to household purposes, and that the 
 principle of the electric bell was first made use of by Prof. Joseph 
 Henry about 1830. 
 
 Copyrighted, 1895, 
 
 303 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXXII. 
 
 Points for Review: 1. Why are " Underwriters' Rules" necessary? 
 
 2. What points must be specially looked after in electric wiring? 
 
 3. Why are sockets, fuses and fixtures more likely to allow electricity to leak than 
 is insulated wire? 
 
 4. What rules are now generally used to govern electric wiring? 
 
 5. Why do the rules separate wiring into different classes? 
 
 6. What classes of wiring do hot fall directly under the protection of the Under- 
 writers' Rules? 
 
 7. What is annunciator wire? What is office wire? 
 
 8. Why do leaky wires cause an open circuit battery to run down? 
 
 9. Where can copies of and the reasons for the different rules adopted by the 
 Underwriters be found? 
 
 LESSON XXXIII. 
 
 ELECTRIC WELDING, FORGING, ETC. ELECTRICITY 
 APPLIED TO THE KITCHEN. 
 
 The use of the electric current for heating and working metals 
 is not new. As early as 1865 patents were issued relating to the 
 subject, but on account of the great expense of the current generated 
 by batteries, these early endeavors came to naught, and not until 
 within a very few years has electric metal working been made an 
 actual success. It was as late as 1888 before electric welding was 
 applied to commercial uses, but immediately upon its introduction it 
 came rapidly into favor and even created much excitement among 
 some manufacturers. 
 
 Electrical methods are now used for welding, brazing, heating, 
 shaping and tempering metals. For most of these purposes the 
 method in common use is to pass an electrical current of very great 
 volume through the metal to be worked. This great current gener- 
 ates sufficient heat as it passes through the resistance of the metal to 
 quickly raise the temperature to a welding or bending heat or even 
 to melt the metal. This method of heating has an advantage over 
 the ordinary method of heating in the forge fire which heats a piece 
 of metal from the outside, while the electrical method heats all parts 
 of the metal equally and at the same time the heated metal remains 
 perfectly clean. The apparatus which is used for heating usually 
 consists of an alternating current transformer which reduces the 
 pressure of an alternating current from 300 volts to less than two 
 volts, and increases its volume proportionally (Lesson XXVII, 
 page 235). A welder transformer is shown in Fig. 337. ^ The grooved 
 copper casting shown in the figure is the secondary coil of the trans- 
 
former which has only one turn. The primary winding made up of 
 numerous turns of wire is intended to lie in the groove of the sec- 
 ondary, while the core which is seen enclosing one side of the second- 
 ary casting embraces both coils. At the top of the secondary casting 
 are sliding clamps in which the metal to be heated is fastened. 
 
 Electric welding, as ordinarily carried on, consists of heating, 
 by the process above described, nthe pieces of metal to be welded 
 while they are firmly butted against each other. When the metals 
 have been heated till they are soft at the points in contact they are 
 squeezed together a certain amount, the current is shut off, and the 
 weld is complete. This is the process developed so usefully by Prof. 
 Elihu Thomson. The apparatus which is generally used in the 
 Thomson welding process is: i. An alternator usually giving a 
 frequency of from 4x3 to 60; 2. A welding transformer with clamps 
 and arrangements for automatically making the welds; 3. Apparatus 
 for controlling the amount of current supplied to the transformer. 
 Fig. 338 shows a complete Thomson welder. The transformer is 
 seen in the center of the case and the clamps on top. The weights 
 at the left are for squeezing together the heated rods held in the 
 clamps, and the relay shown at the right hand side is for cutting off 
 the current when the weld is completed. In welding heavy work, 
 hydraulic pressure is used to squeeze the weld, as shown in Fig. 339, 
 which is a welder intended for electrically welding carriage axles. 
 In Fig. 340 is shown an arrangement to be used for heating pipe 
 which it is desired to bend. The transformer is seen at the bottom 
 of the figure, and the clamps, which are stationary in this case, are 
 shown holding a piece of pipe. 
 
 Many metals may be welded by the electrical method which 
 cannot be coaxed into a weld by the ordinary methods. The metals 
 which have been welded by the Thomson process are shown in the 
 accompanying table: 
 
 Wrought Iron 
 
 Wrought Copper 
 
 Tin 
 
 Cobalt 
 
 Aluminum 
 
 Gold (Pure) 
 
 Cast Iron 
 
 Cast Copper 
 
 Zinc 
 
 Nickel 
 
 Silver 
 
 Manganese 
 
 Malleable Iron 
 
 Lead 
 
 Antimony 
 
 Bismuth 
 
 Platinum 
 
 Magnesium 
 
 Various Grades 
 of Tool Steel 
 
 Musshet Steel 
 
 Wrought Brass 
 
 Fuse Metal 
 
 Aluminum Al- 
 loyed with Iron 
 
 Silicon Bronze 
 
 Various Grade? 
 of Mild Steel 
 
 Stub Steel 
 
 Cast Brass 
 
 Type Metal 
 Solder Metal 
 
 Aluminum 
 Brass 
 
 Coin Silver 
 
 Steel Castings. 
 
 Crescent Steel 
 
 Gun Metal 
 
 Aluminum 
 Bronze 
 
 Various 
 Grades Gold 
 
 Chrome Steel 
 
 Bessemer Steel 
 
 Brass 
 Composition 
 
 German Silver 
 
 Phos. Bronze 
 
 
Again, many of these metals may be welded to each other in 
 combination. The combinations which have been made are shown 
 in the table below. In each of the cases where a weld can be made 
 at all it becomes practically as strong as the metal itself. 
 
 COMBINATIONS. 
 
 Copper to 
 Brass 
 
 Brass to 
 Wrought Iron 
 
 Brass to Tin 
 
 Wrought Iron 
 to Tool Steel 
 
 Wrought Iron to 
 Musshet Steel 
 
 Wrought Iron 
 to Nickel 
 
 Copper to 
 Wrought Iron 
 
 Brass to Cast 
 Iron 
 
 Brass to 
 Mild Steel 
 
 Gold to German 
 Silver 
 
 Wrought Iron 
 to Stub Steel 
 
 Tin to Lead 
 
 Copper to 
 German Silver 
 
 Tin to Zinc 
 
 Wrought Iron 
 to Cast Iron 
 
 Gold to Silver 
 
 Wrought Iron to 
 Crescent Steel 
 
 
 Copper to 
 Gold 
 
 Tin to Brass 
 
 Wrought Iron 
 to Cast Steel x 
 
 Gold to Platinum 
 
 Wrought Iron 
 to Cast Brass 
 
 
 Copper to 
 Silver 
 
 Brass to 
 German Silver 
 
 Wrought Iron 
 to Mild Steel 
 
 Silver to 
 Platinum 
 
 Wrought Iron to 
 German Silver 
 
 
 A very striking application of electric welding has been adopted 
 by at least one manufacturer for welding together the parts of street 
 railway track material, such as switches, frogs, etc., which are ordin- 
 arily made up by bolting together pieces of rails cut to proper shape. 
 By the welding process bolts may be replaced, and the work, there- 
 fore, is made much more substantial. A process has even been 
 developed for welding the rails of a street railway track together, 
 thus doing away with the usual bolted joints which cause so much 
 roughness in the track and require such a large expense for repairs. 
 Fig. 341 shows a track-welding outfit. The forward car is equipped 
 with a great welder shown in operation in Fig. 342. This welder is 
 arranged to work on track which is in place in the street. The cur- 
 rent is supplied to it by a rotary transformer which transforms the 
 500-volt continuous current taken from the trolley wire into an alter- 
 nating current at a pressure of about 350 volts. Fig. 343 shows a 
 complete weld at a rail joint. As much as 250 horse-power is 
 required for a few seconds in making such a large weld. 
 
 One of the striking things about Thomson electric welders, is 
 their ability to weld up rings, so that they may be used in welding 
 wagon tires, chain links, etc. In this case the question occurs, why 
 does the current not flow around through the solid metal from clamp 
 to clamp, instead of through the path where the ends of the ring 
 butt against each other. This is simply a question of electrical re- 
 sistance. In the case of a wagon tire, the electric current would 
 have to flow through a path several feet long in going from clamp to 
 clamp through the solid metal, while the path through the point to 
 be welded is only a few inches in length, so that the latter path 
 is of much the least resistance, and nearly all of the current follows 
 it. In very small rings, enough current may pass between the clamps 
 
FIG. 337. 
 
 FIG. 340. 
 
 307 
 
 OF THE 
 
 XTNIVERSIT 
 
FIG. 338. 
 
 FIG. 339. 
 
 3O8 
 
FIG. 341. 
 
 FIG. 342. 
 
 309 
 
FIG. 343. 
 
 Sheet iron shield to prelect workrnans hands 
 
 FIG. 344. 
 
 310 
 
FIG. 345. 
 
 FIG. 346 A. 
 
 OF THE 
 XTNIVERSI 
 
 311 
 

 FIG. 347. 
 
 312 
 
through the solid part of the ring to heat it red hot, but that does 
 not: interfere with the* welding. An interesting application of the 
 Thomson process has been lately made to softening, at points where 
 bolt holes must be drilled, the very hard nickel steel armor plates 
 which are made for United States men of war. These plates are so 
 hard that it is almost impossible to drill them as they come from the 
 steel works, but by means of an electric heating arrangement, they 
 are softened at the spots where the bolt holes must be made without 
 injuring the temper of the othea parts of the plates. 
 
 Another method of utilizing the heating effect of electricity ior 
 the purpose of welding and working metals, is that known as the 
 arc process. This was first used by De Meritens, a Frenchman, and 
 was later more fully developed by a Russian named Bernardos. In 
 this process, a continuous current is used at a pressure of about 150 
 volts, one terminal of the electric generator being connected to the 
 metal which it is desired to heat, and the other terminal being 
 attached by a flexible conductor to a portable carbon rod (Fig. 344). 
 When the carbon rod is brought against the work, an electric arc is 
 set up and the metal is heated. This device has been used in the 
 process of filling with metal blow holes which occur in valuable 
 castings. It has also been used for welding the seams in small iron 
 boilers, receivers for compressed air, etc. It is of special advantage 
 for the latter work since the arc can be slowly drawn along the edges 
 of the plate to be welded, thus bringing them to a welding heat, and 
 the weld is then completed by pressing or hammering the plates 
 together. 
 
 In each of the methods of electric welding, it is to be noticed 
 that the electric current is used only for the purpose of heating the 
 product previous to welding, and that the pressure required to com- 
 plete the weld is applied by mechanical means of some kind. 
 There is another striking and even startling method of electrically 
 heating metals for purposes of working them. If a pail of water in 
 which is dissolved common washing sdda, have immersed in it a lead 
 plate which is connected to the positive terminal of a 150 or 200 volt 
 electric circuit, it gives all the apparatus necessary for quickly heat- 
 ing iron. The metal to be heated is grasped in tongs which are 
 electrically connected to the negative terminal of the electric cir- 
 cuit, the handles of the tongs being insulated. When the metal is 
 plunged into the pail of water, it is quickly brought to a white heat 
 and may then be withdrawn and worked on the anvil or welded to 
 another piece of heated iron^by the ordinary blacksmith's method. 
 Any metal may be heated by this process, but welding can be per- 
 formed only on those metals which, like iron, may be welded by the 
 blacksmith. The heating of the metal when it is plunged into the 
 water, is apparently caused by an electric arc which is set up around 
 
 313 
 
die submerged metal on account of its becoming surrounded by a 
 coating of hydrogen gas. The amount of current used varies from 
 a few amperes to many hundreds, depending upon the size of the 
 metal to be heated. It gives one a remarkable sensation to see a 
 piece of metal which is dipped into a pail of water come quickly to 
 a blinding white heat, and, when held in another pair of tongs (not 
 connected to the electric circuit), to see it again dipped in the same 
 water for the purpose of cooling it. 
 
 The direct heating effect of an electric current as it passes 
 through resistance coils, is now applied to the ordinary purposes of 
 warming and also to cooking. Fig. 345 shows one of the various 
 forms of electric heaters, all of which simply consist of resistance 
 wires embedded in an insulating material. These heaters have been 
 used considerably for warming electric street cars, and are coming 
 into more or less use in other situations. Electric teakettles, which 
 are kettles with an electric heater in their base, are not uncommon. 
 Electric flat-irons, curling irons, soldering irons and similar devices, 
 are slowly making their way into common use in towns where incan- 
 descent electric light circuits are at hand to supply the necessary 
 current. In Figs. 346, A and B are shown an electric saucepan and 
 an electric curling-iron. 
 
 Whole electric kitchen outfits may be obtained, including an 
 electric range, and they are sure to come into quite common use 
 when their cost has been reduced to about that of coal ranges. The 
 arrangement of a complete electric kitchen outfit is shown in 
 
 Fig. 347- 
 
 While electric cooking can be said to be commercially satisfac- 
 tory on account of its convenience, cleanliness, and adaptability, 
 electric heating for general purposes can never replace the direct use 
 of coal or the use of steam heating, until electricity is directly gen- 
 erated from the fuel without the intervention of steam engines in 
 which enormous losses of heat are absolutely unpreventable. The 
 nature of steam engines makes it impossible, even with the best of 
 them, to convert into useful power more than ten or fifteen per cent 
 of the heat energy contained in coal which is shoveled into the 
 boiler furnace. When the steam generated by the boiler is directly 
 used for heating, a very much greater proportion of the heat in the 
 coal is converted to a useful purpose. The electric heater can never 
 entirely replace direct heating by stoves, or by steam pipes, as long 
 as the generation of electric currents is dependent upon steam 
 engines. 
 
 Another electrical device jvhich is in quite common use is the 
 electrically heated flat-iron. This is a flat-iron with insulated resist- 
 ance wire imbedded in its interior, very much in the same manner 
 as the resistance coil is imbedded in electric heaters. These irons 
 are much used in laundries because they can be kept in continuous 
 
 214 
 
use and no time is lost by the ironers in changing irons. The elec- 
 trical irons are connected to the electrical circuit by means of a 
 double flexible conductor, so that the current can reach them in 
 whatever position they stand. Electrical irons are also used a great 
 deal in private houses, because they can be so conveniently heated 
 up, and they can be kept heated without requiring the presence of a 
 hot stove. The irons must be carefully used, and when ironing is 
 not being done with them the current must be turned off or the 
 resistance wire is likely to be burrled out. 
 
 Copyrighted. 1895, 
 
 315 
 
The National School of Electricity. 
 
 REVIEW OF LESSON XXXIII. 
 
 Points for Review. 1. For what kinds of metal working are electrical methods 
 nsed? 
 
 2. What is the most commonly used method and what are its advantages? 
 
 3^ Of what does the Thomson apparatus for heating metals by electricity con- 
 sist? 
 
 4. What is ordinarily meant by electric welding? 
 
 5. What metals may be welded by the Thomson method? 
 
 6. How is it that metal rings may be welded on a Thomson welder? 
 
 7. What is the Bernardos process of working metals, and for what purposes has it 
 been used? 
 
 8. How may a piece of metal be brought to a high temperature by dipping it into 
 a pail of water? 
 
 9. How is the heating effect of the electric current used for warming and for cook- 
 ing? 
 
 10. Why is it not possible at present for electric heating to replace the use of 
 stoves for heating? 
 
 LESSON XXXIV. 
 
 ELECTRO THERAPEUTICS. 
 
 One of the first practical uses ever made of electricity was in the 
 treatment of disease. Its application at the beginning, however, 
 was entirely experimental and the cures achieved through its agency 
 were largely matters of accident, as it was used indiscriminately and 
 for all pathological conditions. Static electricity or Franklinization 
 was almost exclusively employed, not especially because it was be- 
 lieved to be better than any other variety of the current but for the 
 reason that there was no other form of controllable electricity in 
 existence at that time. Electricity in this field followed the course 
 pursued by nearly all other therapeutic agents upon their introduc- 
 tion. It was used for every disease under any circumstance by the 
 most ignorant people; indeed nothing was known of its physiological 
 action and long years ago its use would have been prohibited by 
 popular opinion but for the fact that since the ancient Phoenicians 
 generated electricity by rubbing a piece of amber, calling the pro- 
 duct a reanimated soul, the mysteries of electricity have been some- 
 thing to conjure with. The result of the first few years' use of elec- 
 tricity in medicine demonstrated its popular approval and the result 
 
was that large numbers of ignorant and incompetent people set 
 themselves up as electrical doctors and a most natural consequence was 
 that in the legitimate profession of medicine electricity came into 
 disrepute. A further sequence was that these medical quacks were 
 given free and undisturbed occupation of the field to the exclusion 
 of scientific men who would have got out of electricity all that there 
 was good in it, and this unhappy condition obtained until a very few 
 years ago when a few members of the profession became brave enough 
 to risk the condemnation of their fellows by making exploratory 
 incursions into the so-called mysteries of electricity. Such investi- 
 gators are becoming more numerous day by day and it is safe to say 
 definitely that electricity is rehabiliated in the profession of medicine 
 and that it has taken its place with other powerful agents when 
 applied in the hands of scientific men. One of the principal reasons 
 why electricity has not been better understood in the profession of 
 medicine, is that medical men as a rule are not physicists and they 
 have not mastered the physical laws of electricity and hence are 
 not able to investigate as to its physiological action under circum- 
 stances likely to be met with in medical and surgical practice. The 
 few brave men mentioned above have recognized this weak position 
 and they have gone to work earnestly to master electro-physics and 
 their work brings us down to the present day and to a clear judg- 
 ment as to what electricity will do in medicine and surgery. 
 
 According to the circumstances of its application, its physiolog- 
 ical action or its purpose in the treatment of disease, electricity is 
 divided into the direct or continuous or galvanic current, the alter- 
 nating, or to and fro, orfaradic current, and static electricity or elec- 
 tricity at rest. 
 
 The Direct Current in Therapeutics. The action of the direct 
 current in its application in therapeutics may be arranged in four 
 principal divisions according to the use to which it is put; first, elec- 
 trolysis; second, cataphoresis ; third, the electro-cautery, and fourth, 
 the incandescent lamp for purposes of diagnosis in the exploration of 
 cavities. 
 
 Electrolysis, as applied in therapeutics, does not differ from the 
 ordinary chemical electrolysis of which we have learned far back in 
 the course, but there are physiological changes that must be under- 
 stood in order to appreciate the conditions under which electrolysis 
 can be applied in the healing of disease. We have learned that in 
 the electrolytic decomposition of water, hydrogen collects at the 
 negative or cathodal electrode, and oxygen at the positive or anodal 
 electrode. This is also true when electrolysis is applied upon the 
 human tissues, but further than this, the acids of the tissues are 
 attracted to the anode or positive electrode, and alkalies to the oppo- 
 site pole. The simple statement of these facts carries with it appre- 
 
ciation of the conditions calling for the use of these two forms of 
 electrolysis. In anodal electrolysis the first effect is exactly the 
 effect of acid applications, that is, first astringent, second styptic, 
 third caustic. The cathodal or negative effects, on the contrary are 
 similar to those occurring upon the presence of alkalies in the 
 tissues, that is, there is a softening of all the tissues, or a disposition 
 on the part of the tissues affected to become pulpy and consequently 
 yielding. T f we take a piece of raw meat, for instance, large enough 
 for clear demonstration, and insert a positive needle electrode at one 
 point and the negative at an opposite point, and apply a few milliam- 
 peres of current, and if this action be allowed to operate for a few 
 minutes, we will find upon investigation that the meat at and about 
 the anode has become hardened and dry and contracted, due to acid 
 influence, while the tissues at and about the cathode have become 
 soft and more pulpy than they were before. The anodal effect is 
 produced by a coagulation of the albuminoids in the tissues, due to 
 acid reaction. In therapeutics these two forms of electrolysis are 
 clearly defined and will be applied under opposite conditions; cathodal 
 or negative electrolysis will be indicated for the softening of connec- 
 tive tissue or scars, for instance. More specifically, it may be used 
 for example, to soften scar tissue resulting from an attempt on the 
 part of a person to swallow some active caustic such as lye, where 
 the throat and oesophagus have been badly burned and where scar 
 tissue has subsequently formed. During the application of the cur- 
 rent, in which the cathode will be applied as nearly as possible to the 
 part affected and the anode remotely, the scar tissue will become 
 softened in a marked degree, and at the end of a few minutes' appli- 
 cation, a bougie or dilator may be pasred that it was impossible to 
 pass before the application, and if this course of treatment is con- 
 tinued at intervals of once a week it will be found that on each 
 application the calibre of the canal will be materially increased in 
 size, and a larger sound can be passed at the end of each electrical 
 application ; so that a course of treatment, extending over some weeks, 
 will result in the restoration of the oesophagus to nearly its normal 
 size. The' principle involved in this treatment can be applied indefi- 
 nitely under a variety of circumstances where the pathological condi- 
 tions are like those in the case cited. 
 
 Naturally anodal or positive electrolysis would be applicable in 
 the contrary conditions such as the naevi or birth marks that so dis- 
 figure the face and that occur frequently in other parts of the body, 
 or other forms of abnormal growth possessing much vascularity. 
 In the destruction of these growths the thermo-cautery used to be 
 applied and invariably left scars sometimes more unsightly than the 
 original growths. The knife has also been employed with equally 
 disfiguring effect. In employing anodal electrolysis the desire is, of 
 
 313 
 
course, to cut off the circulation that supplies the growth or that 
 reddens the naevus mark, and by using fine needle electrodes either 
 one at a time or a number together, with the anode or positive elec- 
 trode at the point of attack, and the negative remotely, the albumin- 
 oids in the blood are coagulated and the blood supply practically cut 
 off, but it is done so gracefully and with so little destruction of tissue 
 that no scars result and the naevus part becomes the color of the 
 surrounding skin, and in the case pf the polypi and fungous growths 
 the circulation is cut off and they disappear, leaving no scar or at 
 most a small point. 
 
 Cataphoresis or Transfusion or Electric Osmosis. If two fluids 
 are separated by a membrane or consistent partition, 'and the direct 
 current of a few milliamperes is passed from one fluid to the other, 
 the fluid in that compartment connected with the positive electrode 
 or anode, will be found to decrease in quantity, and that connected 
 with the cathode or negative electrode, to increase. In other words, 
 the current has the effect of carrying one fluid through the partition 
 to the other, the direction of conveyance being from positive to 
 negative. Reverse the current and the bulk of the fluids is changed 
 again. This is simply the principle of osmosis as applied in physics. 
 We have a collection of fluid in the synovial sac of the knee joint 
 and- we desire to be rid of it. Pressure and other means have failed 
 but we will find after experiment that the dissipation of the fluid will 
 be materially hastened if not actually brought about by this form of 
 current intelligently applied; or we have an accumulation in the 
 pleural sac, or a pericardial effusion, that may be dissipated in the same 
 way. It may not be said positively that the application of this princi- 
 ple will invariably remove these fluids, but conditions will be very 
 favorable to their removal, and nature and other agents may be greatly 
 aided. This principle is reversible and may be applied from without 
 internally. We wish to administer medicines that it is not expedient 
 to give by the mouth or by means of the hypodermic needle. Solu- 
 tions of the proper agent on a sponge electrode, the particular elec- 
 trode being intelligently chosen according to the reaction of the 
 agent, and applied at a point on the skin nearest that desired to be 
 reached, with the opposite electrode remotely attached, is a thoroughly 
 practicable and serviceable method of administration. L,ocal anaes- 
 thesia may be brought about in the same way and this method of 
 administration will be found an extremely desirable one where it is 
 required to operate within confined localities or in very small com- 
 pass, such as medicinal applications for intra-uterine inflammations 
 or inflammatory conditions of the nasal fossae, or the throat. 
 
 In electrolysis and cataphoresis we have been considering the 
 applications of electricity in the treatment of disease where the cir- 
 cuit; was partly made up of human tissue, and where the effect de- 
 
 319 
 
sired to be obtained was brought about by changes effected in the 
 tissues. We come now to the application of electricity where the 
 tissues of the body are not involved in so far as the circuit is con- 
 cerned. These applications are the electro-cautery and the electric 
 light, the latter used mostly for the illumination of cavities desired 
 to be explored for purposes of diagnosis. In neither of these forms 
 of the application of the electrical current are we confined to the 
 use of the direct or continuous current, for the alternating current 
 may be used instead wherever it is convenient or obtainable, but as 
 the alternating current has so recently come into use for these pur- 
 poses, and as physicists and manufacturers have hardly perfected the 
 necessary apparatus for its use in these connections, it may be well 
 to consider this work only in relation to the direct current. 
 
 Electro Cautery. We have seen that when a direct current is 
 flowing through a circuit, the amount of current at every point in 
 the circuit is the same, and the heating effect of a current at any 
 point in its circuit is dependent upon the resistance at that point. 
 If the circuit, the main part of which offers but little or no resistance, 
 is partly composed of a substance having a comparatively high resis- 
 tance, the amount of heat that will be generated in this part, will be 
 in direct proportion to the square of the quantity of current that passes 
 through it in a given unit of time. This quantity may be so great as to 
 generate sufficient heat to render the resistant part of the circuit incan- 
 descent, and when this is the case we have the essential necessary to 
 the heating of a knife or burner for cautery work. The sources from 
 which current may be derived for cautery work are the primary bat- 
 tery, the secondary battery and the dynamo; but it will hardly be 
 profitable for us to recapitulate the details incident to the use of these 
 forms of current as we have already been over that ground suf- 
 ficiently to be familiar with it. It may be said that the electric cautery 
 has marked an evolution 'or distinct epoch in surgical procedure. 
 The thermo-cautery formerly in use, had the disadvantage of ex- 
 treme irregularity of temperature, and it was almost impossibe to 
 regulate the temperature of a thermo-cautery knife, so that one could 
 always be sure of obtaining the grade of cautery action desired. If 
 we require a cutting effect, and do not fear bleeding, we may, with 
 the electro-cautery, have the knife at a white heat. If we prefer 
 to create an eschar as we progress, so that bleeding will not take 
 place, we may have the cautery at a dull red heat, and if we do not 
 require a cutting action at all, but simply a styptic action, the heat 
 may be still more moderate; moreover the electro-cautery has the 
 added advantage over the ordinary thermo-cautery, in that the wire 
 electrode or knife or burner may be introduced while cold into a 
 cavity arid directly down upon the point of attack, and be heated to 
 the required intensity while in position. It will not be necessary to 
 
 320 
 
detail cases where the electro-cautery is indicated. These will be 
 apparent to the medical man without discussion. 
 
 The Electric Light. The extreme daintiness of manufacture 
 and gracefulness of the mechanism now turned out by manufacturers 
 makes it possible to achieve some beautiful results with sounds in 
 combination with mirrors placed at proper angles, and little lamps 
 to light the cavity at the end of the sound. These instruments are 
 made for optical inspection of the cavities of the body such as the 
 stomach, bladder, throat, rectum. As a rule the cavity intended to 
 be explored is partially or wholly filled with distilled water in order 
 to protect the membranes from the heat of the lamp. By the careful 
 and scientific use of such mechanism as this we are able to discover 
 and locate stone in the bladder or ulcerations or disease of any 
 character upon the internal mucous membrane of the cavities or 
 canals. 
 
 Indiiced Currents and Static Electricity . These forms of electric 
 action are now also extensively used in therapeutic applications and 
 produce effects differing according to the nature of the mechanism 
 used to produce them. Induction coils can be so wound as to vary 
 the E.M.F. over very wide ranges, and the interruptions of the pri- 
 mary current may likewise be varied, and, according as these 
 are fast or slow, the physiological and therapeutic effects differ. The 
 medical induction coil is a very convenient and serviceable instrument 
 for arousing torpid physiological action and bringing into play latent 
 energy. But, as its currents are small in quantity, as compared 
 with direct currents, and are either intermittent or alternating, the elec- 
 trolytic and cataphoric effects are very inconsiderable in comparison. 
 The high E.M.F. of these currents render them very stimulating, but 
 the secondary current may, by great frequency 6f interruptions, be 
 made to act as a sedative to irritated sensory nerves. 
 
 Static electricity possesses still higher E.M.F. and much less 
 current than that derived from induction coils, but it has been found 
 to exercise a marked influence upon molecular action in the tissues 
 of the body and is a powerful modifier of nutrition. It is used to 
 the best advantage in cases of malnutrition and functional disorders, 
 especially those affecting the nervous system. 
 
 The instruments and apparatus for the application of electricity 
 in its varieties of form, and for the various therapeutic uses, are so 
 great in number and of such variety that it would not be expedient 
 to enter upon their discussion, and it need be said only that the 
 mechanism involved in this sort of apparatus does not differ in any 
 degree in principle from the instruments and apparatus used in the 
 ordinary commercial applications of electricity, and the only differ- 
 ence will be found in the direction of mechanical construction. 
 
 Copyrighted, 1895, 
 
 321 
 
INDEX. 
 
 PAGE. 
 Acid radical 92 
 
 Air gap 158 
 
 Alternating currents 151, -220 
 
 Ammeters 75 
 
 Ampere 12, 44 
 
 Ampere hour 86 
 
 Ampere's rule 37 
 
 Ampere turn 39 
 
 Anode 63 
 
 Apparent resistance 227 
 
 Arc lights 167 
 
 Arc switchboards 176 
 
 Armatures 152 
 
 B 
 
 Batteries 15 
 
 Primary 15 
 
 Storage 25 
 
 Bipolar machines 165 
 
 Brushes 152 
 
 Brush holders 166 
 
 Built-in system of wiring 193 
 
 Bunsen's photometer 210 
 
 Candle power 210 
 
 Candle foot 211 
 
 Carrying capacity of wires 299 
 
 Cathode 63 
 
 Calorimeter 53 
 
 Circular mils 184 
 
 Coercive force 31 
 
 Collecting rings 152 
 
 Commutators 152 
 
 Compound winding 160 
 
 Condensers 87 
 
 Conduits... ,. 132 
 
 PAGE. 
 Conductivity 4 
 
 Conductivity tests 138 
 
 Conductors 4 
 
 Counter electromotive force 160 
 
 Controllers 276 
 
 Coulomb 7 
 
 Coulombmeters 86 
 
 Coupling box 194 
 
 Crosstalk 140 
 
 D 
 
 Dieletric 87 
 
 Differential magnets 172 
 
 Divided circuits 46 
 
 Divided wire bridge 71 
 
 Drop, method of finding 185 
 
 Earth currents 142 
 
 Edison tubing., 194 
 
 Eddy currents 157 
 
 Effective current 223 
 
 Effective pressure 223 
 
 Electricity 1 
 
 Statical 2 
 
 Current 2 
 
 Electric arc 167 
 
 Cooking 314 
 
 Launches 292 
 
 Locomotives 265 
 
 Plants 279 
 
 Railways , 256 
 
 Smelting 101 
 
 Welding 305 
 
 Electrical capacity 86 
 
 Potential 13 
 
 Electro-chemical equivalent 24, 65 
 
 Electro-dynamometers 78 
 
PAGE. 
 
 Electrolytic copper 99 
 
 Electrolysis 63, 100 
 
 Electrolytes 63 
 
 Electromagnet 40 
 
 Electrometers 8 
 
 Electromagnetic induction 122, 213 
 
 Electromagnetism 36 
 
 Electrophorus 10 
 
 Electroplating 91 
 
 Electroscope 5 
 
 Electro therapeutics 316 
 
 Electrotyping 98 
 
 Fan motors, etc 242 
 
 Farad 86 
 
 Feeders 196 
 
 Fish plates 264 
 
 Five-wire system 189 
 
 Frequency 224 
 
 G 
 
 Galvanometers 58 
 
 Ballistic 88 
 
 Tangent 59 
 
 Reflecting 59 
 
 Sine 59 
 
 Astatic 61 
 
 D'Arsonval 62 
 
 Constant of 62 
 
 Geissler pump 178 
 
 Ground detector 208 
 
 H 
 
 Heat loss 53 
 
 Holtz machine 10 
 
 Hot wire instruments 78 
 
 Hysteresis 157 
 
 PAGE. 
 
 Joule 51 
 
 Junction boxes 196 
 
 K 
 
 Kelvin balance 219 
 
 sockets 181 
 
 Leaks and Breaks 138 
 
 Lenz's law , 217 
 
 Leyden jar 90 
 
 Lines of force 34 
 
 M 
 
 Magnetic field 32 
 
 Magnetic pole 32 
 
 Magnetic conductivity 41 
 
 Magnetic induction 29 147 
 
 Magnetic storms 142 
 
 Magnetism 27 
 
 Magnetos 147 
 
 Magneto motive force 41 
 
 Magnets 28 
 
 Mclntyre joint 128 
 
 Measurement of light 205 
 
 Megohm 72 
 
 Microamperes. 75 
 
 Microphone 121 
 
 Milliamperes 75 
 
 Morse alphabet , 106 
 
 Multiple arcing galvanometers 294 
 
 Multipolar machines 165 
 
 N 
 
 Neutral wire 187 
 
 Nickel solutions 97 
 
 Incandescent lamps 178 
 
 Induced currents 214 
 
 Induction 4 
 
 Induction coil 214 
 
 Induction motors 238 
 
 Inductive capacity 88 
 
 Inside wiring 198 
 
 Insulators 4 
 
 Insulation tests 138 
 
 Ions 63 
 
 Ironclad motors..., .... 165 
 
 Oersted's experiment 35 
 
 Ohm 44 
 
 Ohm's Law... , 42 
 
 Paramagnetic 30 
 
 Permeability 40 
 
 Polarization 19 
 
 Polyphase machines 238 
 
 Poor connections. ... ,. 138 
 
PAGE. 
 
 Potentiometer 83 
 
 Power loss in transmission 253 
 
 Pressure indicators 80 
 
 Pressure wires 196 
 
 Primary coil 214 
 
 Rail bonds 264 
 
 Recording voltmeter 266 
 
 Regulators 161 
 
 Rheostats 66 
 
 Reluctance 41 
 
 Residual Magnetism "... 40 
 
 Secondary coil 214 
 
 Self induction 218-226 
 
 Series winding 160 
 
 Shunt winding 160 
 
 Silver solutions 93 
 
 Sludge 100 
 
 Solenoids 39 
 
 Span wires 264 
 
 Sparking 166 
 
 Spark coils 218 
 
 Specific heat 55 
 
 Sprengle pump 180 
 
 Spring jack 134 
 
 Squirrel cage armature 239 
 
 vStarting boxes 190 
 
 Statical electrictity 2 
 
 Law of attraction 3 
 
 Synchronism 236 
 
 Telegraph 
 
 Relays 108 
 
 Duplex 108 
 
 PAGE. 
 
 Diplex 108 
 
 Multiple HO 
 
 Automatic 117 
 
 Autographic 117 
 
 Sounder 103 
 
 Line 103 
 
 Telephone 118 
 
 Receiver 118 
 
 Transmitter 118 
 
 Line 125 
 
 Switchboard 134 
 
 Temperature coefficient 49 
 
 Testing power circuits 191 
 
 Testings sets 70 
 
 Testing submarine cables :.. 145 
 
 Three-phase machines.... 237 
 
 Three- wire system. 187 
 
 Trailers 258 
 
 Transformers 216-231 
 
 Capacity of 235 
 
 Trolley wire 257 
 
 Two-phase machines 237 
 
 U 
 
 Underwriters' rules. 
 
 297 
 
 V 
 
 Volt 13 
 
 Voltameters 62 
 
 Voltmeters 80 
 
 Electrostatic 82 
 
 W 
 
 Wattmeters . 84 
 
 Watt hour 85 
 
 Weather-proof wire 192 
 
 Wheatstone bridge 68 
 
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