LIBRARY UNIVERSITY OF CALIFORNIA. Deceived (&& ,iBgO t ^Accessions No.fov0/f. Class No. < ? ELECTRICAL ENGINEERING LEAFLETS BY PROFESSOR E. j. HOUSTON, PH. D. \( AND PROFESSOR A. E. KENNELLY, F.R.A.S. INTERMEDIATE GRADE 1895 THE ELECTRICAL ENGINEER NEW YORK Engineering Library '"FHE Electrical Engineering Leaflets have been pre- pared for the purpose of presenting, concisely but accurately, some of the fundamental principles of electrical science, as employed in engineering practice. They have been arranged under three grades ; namely, the Elementary, the Intermediate, and the Advanced. The Elementary Grade is intended for those electrical artisans, linemen, motormkh, central station workmen, or electrical mechanics generally, who may not have advanced sufficiently far in their studies to warrant their undertak- ing the other grades. Here the mathematical treatment is limited to arithmetic, and the principles are illustrated by examples taken from actual practice. The Intermediate Grade is intended for students of electricity in high schools and colleges. In this grade a certain knowledge of the subjects of electricity and physics generally is assumed, and a fuller mathematical treat- ment is adopted. These leaflets, moreover, contain such information concerning the science of electricity, as should be acquired by those desiring general mental culture. The Advanced Grade is designed for students taking special courses in electrical engineering in -colleges or universities. Here the treatment is more condensed and mathematical than in the other grades. Although the three grades have been especially pre- IV pared for the particular classes of students referred to, yet it is believed that they will all prove of value to the general reading public, as offering a ready means for ac- quiring that knowledge, which the present extended use and rapidly increasing commercial employment of elec- tricity necessitates. Laboratory of Houston & Kennelly, Philadelphia, March, 1895. CONTENTS. GRADE. No. 1. ELECTRICAL EFFECTS 1 " 2. ELECTROMOTIVE FORCE 9 " 3. ELECTRIC RESISTANCE 17 .. i ELECTRIC RESISTANCE 25 " 5. ELECTRIC RESISTANCE 33 " 6. ELECTRIC CURRENT 41 " T. OHM'S LAW .. 49 " 8. ELECTRIC CIRCUITS .. 57 " 9. THE VOLTAIC CELL .. 65 " 10. THE VOLTAIC CELL 73 " 11. THE VOLTAIC CELL 81 " 12. MAGNETOMOTIVE FORCE 89 " 13. MAGNETIC RELUCTANCE .... .. 97 " 14. MAGNETIC FLUX .. 105 " 15. ELECTROMAGNETS . . 113 " 16. INDUCED E. M. F . 121 " 17. THE DYNAMO , . . 129 " 18. THE DYNAMO .. 137 " 19. THE DYNAMO .. 145 " 20. THE REGULATION OF THE DYNAMO 153 PAGE. No. 21. ELECTRODYNAMICS 161 " 22. THE ELECTRIC MOTOR, (CONTINUOUS CUR- RENT TYPE) 169 " 23. THE ELECTRIC MOTOR, (CONTINUOUS CUR- RENT TYPE) ITT " 24. THE ELECTRIC MOTOR, (CONTINUOUS CUR- RENT TYPE) 184 " 25. ELECTRIC HEATING 193 " 26. INCANDESCENT LIGHTING 201 " 2T. INCANDESCENT LIGHTING 209 " 28. ARC LIGHTING 21T " 29. ARC LIGHTING 225 " 30. ALTERNATING CURRENTS 233 " 31. ALTERNATING CURRENTS 241 " 32. ALTERNATING CURRENTS 249 " 33. ALTERNATORS 25T " 34. ALTERNATORS 265 " 35. ALTERNATING CURRENT TRANSFORMERS. . 2T3 [Copyright, 1894, by THE ELECTRICAL ENGINEER.] WEEKLY. No 1 JIT-NT? 1fi 1804 Price, - 10 Cents. Lb > 1 Subscription, $3.00. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. A. . ..',',.,.,,. A. INTERMEDIATE GRA ELECTRICAL 1. The friction of a glass rod against silk causes both the glass and the silk to acquire a property they did not previously possess, of attracting light bodies ; *. 0., shreds of paper or cotton, in their vicinity. This is an electrical effect. In addition to this, when the glass is vigorously rubbed, crackling sounds are heard, and, in a dark room, faint gleams of bluish light accompany the sound. Moreover, if the rod while vigorously excited, be held near the face, a peculiar sensation is felt, like that caused by the passage of cobwebs over the face. Both glass and silk after being rubbed together, are said to have acquired an electric cha/rge. 2. The exact nature of the process whereby the rubbing together of two substances produces an electric charge is not known, nor is the exact nature of the charge itself understood. Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York, N. Y. It is known, however, that tlie presence of an elec- trical charge on any body or bodies is always accompanied by a strained condition in the surrounding space ; but whether this strained condition is the cause of the electrical charge or charges, or whether it is their effect is as yet unknown. All space is believed to be filled with an extremely tenuous, elastic medium, called the ether. The ether not only pervades all free space, but even exists in the inter- spaces between the ultimate particles of all solid bodies, so that it may be said in this sense to permeate all matter. Light and radiant heat are particular forms of wave- motion-disturbance in the ether; and it is generally believed that the force of gravitation is transmitted through this medium. 3. Although the exact manner in which the rub- bing together of two bodies produces the strained condition in the neighboring ether is not known, yet it is undoubtedly due to the contact of dissimilar substances. When any two dissimilar substances are brought into contact, even without friction, an electrical charge is produced at their contact surfaces, varying in amount with the nature of the substances, as well as with the character of their surfaces ; i. e., with the degree of surface dis- similarity, so to speak. 4. The charge which accompanies the contact of two dissimilar substances cannot be augmented by continuing the friction between them if both sub- stances are conductors, but it may be very greatly aug- mented by continued successive surface contact or friction, if one or both substances are non-conductors. 3 5. In a lightning flash, "which Franklin proved by his classic experiment with the kite in 1752, to be a very powerful electric spark, the crackling sounds ob- served in the experiment with the glass rod, are aug- mented to the intensity of thunder. Lightning discharges, as is well known, may fuse metal work, and rend or tear masonry. G. The discharge of a charged body by any means, as by a spark, produces momentarily what is called an electric current ; and, indeed, the establishment of a charge is also attended by a current. In nearly all such cases the current is of but momentary character. A number of successive, momentary discharges following one another with sufficient rapidity produces an approxi- mate steady electric current. 7. The dynamo electric machine is a ready source of powerful electric currents. The passage of powerful currents through conductors is attended by heating effects. After a dynamo has been generating current for some time, its coils of wire become sensibly warmed. "When passed through a metallic conductor an electric current may even melt or fuse the conductor if the area or cross-section in the latter is too small. 8. In the incandescent lamp the passage of an elec- tric current through a carbon thread or filament, raises it to a high degree of incandescence. The fila- ment is enclosed by a glass chamber, from which all the oxygen has been exhausted, and care is taken to prevent the current strength from becoming sufficiently strong to fuse or volatilize the filament. "When a powerful electric current is sent through two carbon rods which are first in contact, and are then gradually separated by about one-eighth of an inch, a powerful luminous discharge called the voltaic arc passes between the carbon points. 9. The passage of an electric current through a conductor not only produces heat in the conductor, but also invariably produces magnetic effects which are readily observed under certain circumstances. For in- stance, the passage of a powerful electric current through the wire coils on the frame of a dynamo machine, pro- duces powerful magnetic effects, and a bar of iron brought near to these magnets will be powerfully mag- netized and attracted. 10. The electric current also possesses the property of decomposing chemical solutions through which it passes ; for example, if an electric current be led, under suitable conditions, through a solution of copper sulphate it will decompose the salt in the solution, and deposit metallic copper in a coherent and adherent layer upon any conducting surface suitably connected with the lead- ing-in wires. This decomposition is called electrolysis. 11. Electric currents, therefore, produce a variety of effects which may be grouped as follows : (I) Luminous, as in sparks or in electric lamp. (#) Thermal, or heating, as in fusion of wire. Electrical Effects. (3) Mechanical, as in the disruptive effects of lightning discharge. (Jf) Physiological, as in shock to human body. (5) Magnetic, as in dynamo magnet. ,. (6) Electrolytic, as in electroplating. 12. It is well known that in order to start a body in motion or to change the direction or velocity of its motion, force must be applied. Thus a train of cars at rest requires the action of the steam en- gine to set it in motion, and when in motion, the action of the brakes to bring it to a standstill. A baseball requires a certain force to project it with a certain initial velocity and can only be stopped by the application of opposing forces. 13. "Whenever force acts through a distance, it is said to do work, and, in the cases just considered, the motion of the body tinder the force is an evidence of the performance of work. The word energy is employed in the sense of capability of doing work, or as a store of work, and when work is done, energy is expended, and some store of work has been drawn upon. The performance of any work whatever, therefore, necessitates the expenditure of energy. 14. "When a railroad train is set in motion, steam has to do work by moving the pistons to-and-f ro in the cylinders, thus exerting force through a distance. The steam derived its energy from the burning of the coal under the boiler, and the coal in its turn originally derived its energy from the sun, through the absorption of the sun's radiant energy by the vegetable matter from which the coal was formed. When a baseball is set in motion, the energy of the moving ball its power of overcoming obstacles is obtained from the muscular power of the thrower who thus exerted muscular force through a distance. This muscular energy was originally derived from the food 6 assimilated by liis body. In its turn, the food derived Us energy from the sun's radiant heat. When a dynamo is generating an electric current, its driving belt is exerting force upon the rim of the pulley, thus moving it through distance and therefore doing work. It is upon this store of work that the electric current has to draw for the accomplishment of any of its above mentioned characteristic effects. Fig. 1 shows the electrical transmission of power as contrasted with Pig. 2 which shows the mechanical trans- ELECTRICAL TRANSMISSION OF POWER FIG. I. mission of power by means of a rope. In each case the falling weight drives the generator, and a motor lifts a smaller weight. The work done by the motor is less than the work expended on the generator by an amount equal to the loss in transmission. 15. It is a well established principle in science that the total amount of energy in the universe is constant. All natural phenomena are dne to a change of form in the energy manifested when force acts on matter, and throughout all these changes whatever energy disappears in one form reappears in some other form. 16. In every transformation some energy is expended in a direction iu which it cannot be utilized; that is, in effects which are not desired ; such diverted energy is called wasted energy, but is only truly wasted from an utilitarian point of view. Since energy, like matter, is indestructible, it is evi- dent that the total work done, or the energy which appears FIG. 2. in the performance of any machine, must always be ex- actly equal to the work expended in driving it, the intake / but the amount of energy turned to useful ac- count, the output, is always less than the intake, 17. The ratio between the output and the intake, that is, the output divided by the intake, is called the efficiency of the machine, and is always less than unity ; ^ Of TH s for example, the efficiency of very large, well constructed dynamos is about 0.95. 18. From what has been said, it will be recognized that the electrical machine forms no exception to the universal rule that to produce any effect a correspond- ing expenditure of work in some form is necessary. SYLLABUS. An electric charge is accompanied by a strained con- dition in the ether surrounding the charged body. The electric charge produced by friction has its origin in the contact of dissimilar molecules. The passage of an electric current through a conductor is always attended by the production of a magnetic field. When an electric current is passed through a solution of copper sulphate under suitable conditions, a decom- position called electrolysis is effected. Work is never done or energy expended unless force acts through a distance. Energy is never created ; there- fore, when work is done some previously existing store of energy is drawn upon. The total store or quantity of energy in the universe is constant. Every electrical effect is due to energy expended, and the amount of such work can generally be calculated. The total work done by any machine must always exactly equal in amount the work expended in driving it, but the useful work done by the machine is always less than the work expended in driving it. The ratio of the output of any machine to the intake is called the effici- ency and is always less than unity. Laboratory of Houston and Kennelly, Philadelphia. [Copyright, 1894, by THE ELECTRICAL ENGINEER.! WEEKLY. No. 2. JUNE 23, 1894. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE GRADE. ELECTROMOTIVE KORCE 19. Electromotive force is the name given to the unknown force or cause whiclj produces, or tends to produce, an electric current. Whenever an electric current flows in a circuit such current is due to an electromotive force (abbreviated E. M. F.) acting on that circuit. Just as a mechanical force acting on a body produces or tends to produce motion in that body, so an electro- motive force acting on a circuit, produces or tends to produce an electric motion ; i. e., current in that circuit. An E. M. force, like all other forces, possesses a definite direction, and as all forces tend to produce motion in their direction, so an E. M. F. tends to produce current in its direction. 20. As in mechanics, two or more forces, when simultaneously acting may, when opposed, neu- tralize each other and thus produce no motion ; or, when acting in the same direction, may aid each other and thus Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York, N. Y. 10 produce increased motion, so two or more E. M. forces acting simultaneously on the same circuit, when opposed may neutralize each other, and thus produce no current ; or when acting in the same direction, may aid each other, and thus produce a stronger current. FIG. 3. Thus in Fig. 3 the man exerting a steady mechanical force moves a car weighing one ton along a level track at a rate of one mile an hour, and the single voltaic cell, symbolized as shown by two lines of unequal length and thickness, produces an E. M. r. which sends a certain current through the conducting circuit. "When, however, as in Fig. 4, the two men apply simultaneously equal mechanical forces to the car in opposite directions, a neutralization or balance is effected and no motion is produced. So the two voltaic cells I Uc.nyineer FIG. 4. connected in opposition in the same circuit neutralize or balance each other and no current is produced. Again in Fig. 5 the two equal mechanical forces ap- plied simultaneously in the same direction, move a car weighing two tons at the rate of one mile an hour, and 11 the two voltaic cells connected in series, so that their E. M. F.'S aid each other, produce a double E. M. F. that can send twice the current through the circuit. An E. M. F. may, therefore, originate a current, may increase the strength of a current already existing, or may oppose and weaken or altogether neutralize such current. 21. E. M. F. is measured in units named volts. Large E. M. F.'S are sometimes expressed in kilovolts and small E. M. F.'S in millivolts or microvolts, (i. nat'ng. An E. M. r. that has always the same direction is said to be continuous. An E. M. F. that alternately and periodically reverses its direction, is said to be alternating. As a continuous E. M. F. produces, or tends to produce, a continuous current, so an alternating E. M. F. produces, or tends to produce, an alternating or oscillating current. Continuous E. M. F.'S are further divided into steady and pulsating. Steady E. M. F.'S are, for Ibrief periods at least, practically produced by voltaic batteries and thermopiles. Continuous current dynamos (so-called), always produce in reality, fluctuating or pulsating E. M. F.'S although the fluctuations may in some dynamos be so slight and rapid as to escape notice. Alternating E. M. F.'S are either symmetr 'cat or dis- symmetrical^ according as the positive and negative waves, allowing for changes of direction, are or are not similar and equal. Alternating current dynamos (alter- nators] usually produce symmetrical E. M. F.'S, while a Ruhmkorff coil with a vibrating spring, operated by a con- tinuous E. M. F., produces a dissymmetrical alternating E.M.F. 15 The K. M. F. at breaking contact being much greater than the oppositely directed E. M. F. at making. 27. If as in (Fig. 0.) the water in the vessel A, is in communication with the open vertical tubes a, &, c, d, e,f 9 (7, then wlien the outlet tube B is closed, the level at which the water stands will be the same in all the tubes. But when the outlet is open, the level will be highest in the tube nearest to the reservoir, and lowest in the tube nearest to the outlet, the level in the intermediate tubes being found along the inclined dotted d' ^6' I | L +ci ft f . Engineer FIG. 6. Hydraulic and Potential Gradients, line a' ', >', <*'> ',/'> an ^ the E. M. F. which drives the current through any portion of the con- ductor such as a c may be attributed to the difference of potential between a' and c' , as represented by the differ- ence of length between the lines a a' and c c' measured in volts. As water flows from a higher to a lower level, so the electric current is assumed to flow from a higher to a lower potential ; and as differences of water level con- stitute what has been called water-motive force, so difference of electric potential constitutes electromotive force. The sum of all the differences of potential (ab- breviated P.D.'S) in a circuit is equal to the total E. M. F. in that circuit. SYLLABUS. An E. M. F. has a definite direction and tends to pro- duce an electric current in that direction. E. M. F. is measured in practical units, called volts, also in micro-, milli-, and kilovolts. The international unit of work is called the joule. The international unit of activity ; i. - 1 Subscription, $3.00. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE GRADE ELECTRIC 28. Resistance is that property of an electric con- ductor or circuit, in virtue of which an electric current or flow is limited, under any given E. M. F., to a certain value. The resistance of a water-pipe to the passage of water through it, increases directly with the length of the pipe, and diminishes with the cross sectional area of the pipe. It also varies with the nature of the material from which the pipe is made, and the smoothness of its inner sur- face. So too, the resistance of a circuit or conductor, varies directly with the length of the conductor, and in- versely as its cross-sectional area. It also varies with the nature and physical conditions of the materials of which the conductor is composed. 29. Electric resistances are compared with one an- other by reference to certain practical electrical units. The unit of resistance is called the International ohm, and is the resistance offered by a pure chemical Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York, N. Y. [Entered as second-class matter at the New York, N. Y., Post/Jfiu^Junexj, :rH * UKIVBRSIT7 18 material of definite dimensions under fixed physical con- ditions. The value of the ohm is most conveniently taken as the resistance of a column of pure mercury, one square millimetre in area of cross-section and 106.3 centimetres in length, at the temperature of melting ice. (Zero centi- grade or 32 F.) Its value, however, can be defined in terms of any conducting material, such as, approximately, one foot of No. 42 A. w. G. wire of pure copper ; and such a wire might be more conveniently maintained as a material standard than .a column of liquid mercury; but although mechanically advantageous, such a standard would possess the inconvenience that no two samples of copper wire could be obtained of exactly the same degree of purity and in the same physical condition, while mer- cury, by redistillation, can be readily obtained chemically pure, and in the same physical condition. The fundamental unit of electric -resistance, in the International c. a. s. system, is that resistance in which the unit of electric current will do work to the extent of one erg (one dyne-centimetre) in one second. This resistance is, however, so extremely small that it would be impracticable to employ it directly, and another unit, the ohm, was selected as the practical unit of electric resistance. The ohm is equal to one billion (1,000,000,< )( )( ) or 10 9 ), of the fundamental c. G. s. units of resistance. 30. Conductors are generally made in the form of wires. A circular cross-section is the commonest, though a rectangular section is sometimes employed in dynamo armature-winding for economy of space. Doubling the length of a wire doubles its resistance. Similarly, halving the length of a wire halves its re- 19 sistance ; doubling the area of cross-section also halves its resistance. Consequently, if a wire of given length be cut into two equal parts, and the two lengths be laid side by side, and so connected with the circuit that the current passes through them in parallel, their joint re- sistance, i.e., the resistance of the two in combination, will be one-fourth of the original resistance of the wire. We have seen that the resistance of a conductor varies with the nature of the material of which it is composed. As a rule, metals offer a comparatively low resistance to the passage of an electric current, and are, therefore, called electric conductors, while hard rubber, glass, gutta-percha, air, etc., offer a comparatively high resist- ance to the passage of an electric current and are, there- fore, called non-conductors or insulators. 31. For large multiples or submultiples of an ohm, or of any other unit, various prefixes are em- ployed ; as for example, the following multiples, deka . . .ten times 10. . 10 hecto. . . one hundred times 100 . . 10 2 kilo. . . .one thousand times 1,000. . 10 3 mega. . . one million times 1,000,000 . 10 6 bega.. . .one billion times 1,000,000,000. .10 9 . trega .. one trillion times .. ... 1,000,00ft, 000, 000. .10 12 quega . .one quadrillion times.. . .1,000,000,000,000,100. . 10 1 5 and the following submultiples or decimals, deci... .or one tenth 1 -*- 10 1C' 1 centi or one hundredth. ..!-*- 100 10~ 8 milli . . ..or one thousandth . . 1 * 1,000 10- :l micro. ..or one millionth. . . .1 * 1,000,000 10- 6 bicro... or one billionth 1 -* 1,000,000,000 10~ 9 tricro. . .or one trillionth. ...In- 1,000,000,000,000. . . .UM * One c. G. s. unit of resistance is. therefore, a bicrohm. One millionth of an ohm is a microhm. One million ohms is a megohm ; a billion ohms, a begohm; a trillion ohms is a tregohm; and a quadrillion ohrns, a quegohm. 32. For convenience in comparing the resistances of different kinds of material, the standard of com- parison is taken as the resistance of unit length and unit area of cross-section; namely, the resistance which would be offered by a cube of the material, one centimetre in length of edge, between opposite faces. The particular resist- ance of a body referred to unit dimensions in this way is called its specific resistance or resistivity. For example, iron has, at ordinary temperatures, a resistance about six and a half times that of copper. Thus copper of stand- ard purity (Matthiessen's standard) has a resistivity of 1594 c. G. s. units, at the melting point of ice, or 1.594 microhms, while iron at the same temperature, has a resistivity of 9.687 microhms. A study of the following table will show that, as a rule, apart from the metals, solids possess the highest re- sistivities, or are the poorest conductors. Of liquid sub- stances, oils possess the highest resistivity. Water, as measured by Kohlrausch, has a resistivity of 3.75 meg- ohms. As, however, minute traces of impurity enor- mously diminish this resistivity, it is generally believed that absolutely pure water would be almost a perfect in- sulator. Water containing 30 per cent, of its weight of nitric acid has a resistivity of 1.29 ohms. The following is a table of resistivities in International ohms. 21 Substance. Tempera- ture. Resistivity. Tem- perature Co- efficient Authority. Silver, annealed... Silver, hard drawn Copper, annealed (M a 1 1 h i e ssen's standard) 0C. 1.500 microhms 1.53 1.594 0.377 0.388 Matthiessen. ( 'opper,hard drawn Iron, annealed. . . . Nickel, annealed. . Mercury, liquid . . . ( ierni an silver .... 1.629 9.687 12.420 94.84 ab't 20.9 6'072 0.044 Graphite Sulphate of zinc, saturated solu- tio n 10 from 0.0024 to 0.042 ohms 33 6 ohms . . \ about f 0.5 j- Everett. Ewing & Macgregor. Common salt, solu- tion of minimum resistivity 10 47 " Kohlrausch &Nippoldt. Pure water ' { 20 abont 3.75 meg- ohms 84 tregohms .. ,:::: Kohlrausch. Ayrton & Gutta-percha Hard rubber Paraffin 24 46 46 449 28 quegohms . 34 :::: Perry. Latimer Claik. Ayrton & Perry. Glass, flint 16700 " Foussereau . Porcelain 540 i < The fourth column gives the temperature coefficient, that is the percentage increase in resistivity per degree centigrade increase in temperature. Thus, copper in- creases 0.388 per cent, per C., within a range of a few degrees centigrade, according to Matthiessen' s obser- vations, and, taking its resistivity as 1.594 microhms at C. at 5 C., its resistivity would be 1.594 (1 -f 5 X 0.00388) = 1.594 X 1.0194 = 1.625 microhms. 22 From this table of resistivity it is possible v to arrive at the resistance of any uniform conductor whose -resistivity is given. Thus, if the resistance of a mile of Matthies- sen's standard hard-drawn copper wire, having a cross section of one square millimetre is required, for C., we take the resistivity of 1.629 microhms, and since this would be the resistance of a bar one centimetre long and one square centimetre in cross section, the resistance of a centimetre length of wire of 1 square millimetre cross section (100 times less area) would be 100 X 1.629 = 162.9 microhms, and the resistance of a mile (160,933 cms.) of this wire would be 162.9 X 160,933 = 26,215,- 986 microhms 26.2 ohms approximately. The pro- blem of finding the resistance of any wire thus resolves itself into determining its resistivity at the required temperature, its cross sectional area in sq. cms, and its length in cms. The following table will be useful in these calculations, 1 inch = 2.54 cms. 1 sq. in. 6.4516 sq. cms. 1 foot = 30.48 cms. 1 mile = (1760 yds) = 160,933 cms. The temperature coefficient of carbon is negative; namely about 0.5 ; that is, a wire or filament of car- bon; diminishes about 0.5 per ^C. increase, for a small range of temperature. At the temperature at which glow lamps are ordinarily operated, their resistance is about half what it is when cold. The resistivity of insulating substances, diminishes like carbon, with temperature, and this in fact forms a criterion as to the class of substances (conductors or insulators) to which a body belongs. 23 33 r . Condiicta/nce is the inverse of resistance, just as conductivity is the inverse of resistivity. If, as in Fig. 7, i'our incandescent lamps be connected to supply mains as shown, and each lamp has when hot a resistance of .100 ohms, (a conductance of yj-g- or 0.01 mho), the total conductance, since the current is conveyed equally through four different paths, will be the sum of the separate conductances, i. 4 TTTT v 7 1 8Q4. Price, - 10 Cents. ULY 7 > * Subscription, $3.00. Electrical Engineering Leaflets, Prof . E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. I NTEF?JYl EDIATE GRADE. ELECTRIC 34. The conductivity of a material is the inverse of its resistivity, so that the greater the resistivity, the lower the conductivity. Thus the solution of nitric acid in water, which gives the lowest resistivity (1.^9 ohms), gives the highest conductivity T .V-g- = 0.77519, and any other mixture of nitric acid and water would conduct less perfectly, that is, would have a" lower con- ductivity. Conductivity is measured in units called mhos, a term derived from the reverse spelling of the word ohm. 35. The resistance of two or more conductors con- nected in series, that is, joined end to end, is the sum of their separate resistances. Thus three wires which have respectively 5, 10 and 15 ohms resistance, have, when connected in series, a total resistance of 30 ohms. The total resistance in a series circuit is, there- fore, the sum of all the resistances in the different parts of that circuit. Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York, N. Y. [Entered as second-class matter at the New York, N. Y., Post Office, June 14, 1894.] 26 36. It is not definitely known whether alloys are mixtures or chemical combinations of their in- gredient metals. Electrically, it might be supposed from the resistivity of alloys, that in some cases alloys are mere mixtures, and in others chemical combinations. Thus, alloys of such metals as lead, tin, zinc, and cadmium, behave electrically like bundles of wire made up in the proportions of their respective metals, while alloys of such metals as gold, silver, copper, iron, aluminium, and others, give a resistivity much higher than that which a mere bundle of such wires would lead one to expect. The temperature coefficient of an alloy is always less than would be expected from the temperature coefficients of its ingredients, and the greater the resistivity of an alloy, the lower will usually be its temperature co- efficient. Thus german silver has a resistivity of about 21 microhms (varying considerably with different samples) and a temperature coefficient of about 0.044 per cent, per degree C.; while platinoid has a resistivity of 32.7 microhms, (also varying greatly with different samples), and a temperature coefficient of about 0.021 per cent, per degree C. 37. It has recently been shown that at very low temperatures certain pure metals have exceedingly low resistivities at the lowest temperature at present experimentally attainable ( 197 C), and it has been inferred from such observations that at the assumed temperature of absolute zero ( 273.6 C.) the resistivity of such metals would be zero, so that a copper wire at such a temperature would have no resistance whatever. It would appear, however, from what has already been 27 mentioned concerning alloys and their lower temperature coefficients, that their resistivities would still be consid- able, even at the absolute zero of temperature. For the same reasons alloys are greatly to be preferred to pure metals for the construction of resistance standards, which should be as nearly constant as possible, in order that variations of temperature may make the least change in the value of their resistances. 38. The materials forming the earth's crust, such as clay, sand, gravel, marl, etc., have, when dry, very high resistivities; but since the ground below a certain depth, is almost invariably moist, even in dry weather, and since the water contains various salts in solution, the resistivity of the entire mass is greatly re- duced. When, therefore, a telegraph conductor is carried on poles between two distant points, it is not necessary to have a second or return wire to complete the circuit, since the ground between the two stations may be used as a return conductor, introducing a resist- ance much less than that which a metallic return con- ductor would possess. This is due to the enormous area of cross section of the earth, which is so great that the difference in the earth's resistance, as measured between two terminals one mile apart, or one hundred miles apart, is practically imperceptible. In order to ensure sufficient contact with the ground, the ends of the ground wires are connected with large metallic plates called ground plates, usually of copper or iron, and sometimes surrounded by charcoal, buried sufficiently deeply to meet permanently moist strata. In cities, the gas or water pipes, from their extended buried surfaces, serve as excellent telegraph ground plates. The t Of THB tJlUVBRSITY /?^ 28 resistance of the ground in a circuit may vary from a fraction of an ohm to hundreds of ohms, according to the nature of the ground connections, such high resist- ances being met with in cases where the ground is improperly made, or where the strata in which the plates are buried are permanently dry. Thus arise two varieties of circuits, one, the metallic circuit, in which the circuit is metallic throughout; and the other, the ground return circuit, in which the ground is used as the return conductor. FIG. 9. VARIETIES OF INSULATORS 39. On all pole lines the conductor, whether cover- ed or bare, is supported on suitable insulators. Fig. 9 shows a variety of insulators differing in shape and material. A, is a glass insulator; B, an oilcup in- sulator; c, a hard rubber insulator; and D, a porcelain insulator. These insulators are rigidly supported on pins placed on cross-arms. The resistivity of all these insulating materials is very high and is most con- veniently rated in quegohms. The resistance of any ordinary insulator, between the surface of the groove in which the wire is placed and the surface of the support- ing pin, would not be less than 500 begohms, but in practice the resistance an insulator offers to the escape of a current is far less, owing to the conductance of a film of moist dust and dirt on its surface. This is especially true of glass, on account of its hygroscopic nature, and these insulators are, therefore, not well adapted for use in a moist climate. The advantage of an oil insulator arises from the fact that the oil interposes a high resist- ance path to leakage over the surface, dust and moisture settling to the bottom of the oil, leaving a clean surface. The greater the number of insulators supporting a wire, the greater the number of conducting paths for the escape of current to ground, and hence the greater the leakage. Therefore, the greater the length of conduct- ing circuit, the greater the leakage, and the smaller its insulation resistance. The apparent insulation of a line measured in megohms, multiplied by its length in miles, gives its average apparent insulation per mile in megohm-miles. 40. Resistances in various forms, usually in coils of wire, are introduced in working circuits for the purpose of controlling or limiting the current strength. In other cases they -are introduced for purposes of measurement. In all cases, however, the conditions must be such that the current passing through them shall not produce excessive heating. For variable resistances, such as are employed for con- trolling the current strength in working curcuits, iron wires, strips or plates, carbon blocks or discs, or columns of liquid are employed, so arranged that different lengths can be readily introduced or removed from the circuit. Figs. 10 and 10# shows such an arrangement, where, by 30 the movement of the lever arm, the contact strip c, can be brought into contact with any of the metal buttons from /**<*/' W W Klec.l FIGS. 10 AND IQa. RESISTANCE FRAME AND DIAGRAM OF CONNECTIONS. A to B, thus varying the number of coils of wire in the circuit. The particular instrument shown is designed to FIG. 11. carry the current to supply six ordinary incandescent 16 candle-power, 110- volt lamps, without overheating. Fig. 11 shows a variable resistance or rheostat formed of iron wire, embedded in porcelain enamel, in firm con- tact with the heavy iron bed-plate. This arrangement FIG. 12. WHKATSTONE BRIDGE. ensures a rapid transference of the heat generated by the current in the wire to the iron plate, and its subsequent diffusion and radiation. 41. Where resistances are employed for the purpose of comparison or measurement, their values in ohms are calibrated with reference to a standard ohm. In this instrument a coil of platinum-silver alloy having the resistance of one ohm at some convenient definite temperature, has its terminals connected to two stout t Uc. Engineer FIG. 13. DIAGRAM OF CONSTRUCTION OF RESISTANCES IN BRIDGE. copper rods whose ends dip in mercury cups. The coil and rods within the case are embedded in paraffin wax, a central hollow core being left for the insertion of a 32 thermometer, when the instrument is submerged in water or oil. Figs. 12 and 13 represents a common form of resist- ance box called a Wheatstone bridge or Wheat-stone balance., a plan of which appears beneath together with a diagramatic view of the coils and plugs. By suitable combinations of plugs, the resistance included between the terminals x and r, can be made any integral number of ohms between zero and 10,000. SYLLABUS. The conductivity of a material is the reciprocal of its resistivity. The conductance of a conductor or circuit, is the reciprocal of its resistance, and is measured in mhos. The total resistance of a number of resistances in series, is the sum of those resistances, and the total con- ductance of a number of conductances in parallel is the sum of those conductances. The temperature coefficient of alloys is less than the temperature coefficient of pure metals. Metallic circuits are metallic throughout. Ground return circuits complete their circuit through the earth's substance. The resistance of insulators is in practice the resistance of a film of dirt or moisture upon their surfaces. The insulation of a line is expressed in megohm-miles repre- senting the average insulation resistance of a single mile. Resistances for measurement are usually in coils of wire of a suitable alloy. Resistances for controlling current strength in working circuits are frequently of iron, carbon or water. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, by THE ELECTRICAL ENGINEER.] WEEKLY. . 5. si Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. I NTERIVl EDI ATE GRADE ELECTRIC 42. The Wlieatstone bridge shown in Figs. 12 and 13, (No. 4, Intermediate Grade) is used for determining the resistance of any conducting path or circuit. The electrical connections of the bridge are shown in Fig. 14, where E, is an E. M. F., usually a battery, connected to the terminals q and r. The current from E, divides between the paths q x r and q z r, where q x, and q z, are resistances, usually called the arms of the bridge or balance, x /*, is the adjustable and known resistance under the plugs, while z r, is the unknown resistance to be measured. Calling the pressure at r, zero, and at ^, E, volts, as shown in Fig. 15, the fall of pressure through the resistances will be shown by the inclined lines p a r, and P 1) r. It is evident that if the resistances q x and q 2 are equal, and also x r and z r, then by symmetry the pressure x #, Avill be equal .to the pressure z b. Con- sequently, if these points x and z, are connected through a galvanometer, or instrument for detecting a current, Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York, N. Y. [Entered as second-class matter at the New York, N. Y., Post Office, June 14, 1894.] 34 no current will pass, and the galvanometer needle will remain at zero. The resistance in each of the arms q x and q 2, are usually 10, 100, or 1000 ohms, and in making a measurement the plugs in the branch x r, are removed or replaced until the galvanometer shows no current passing. When this is the case the unknown resistance z /*, is equal to the resistance unplugged in i09 1061 0.0001560 0.824 3 0.2294 0.1593 841 0.0001967 1.1-39 4 0.2043 0.1264 667 0.0002489 1.309 5 0.1819 0.1002 529 0.0003128 1.651 6 0. 1 620 0.07946 420 O.OOC3944 2.082 7 01443 0.06302 333 0.0004973 2.626 8 0.1285 0.04998 264 O.(00627l 3.311 9 0.1144 03963 2o9 0.0007908 4.176 10 0.1019 0.03143 166 O.OOt.9972 5.265 11 0.091)74 0.02493 132 O.(0l257 6636 12 0.08081 0.01977 104 0.001586 8.374 44. When two "conducting surfaces are placed lightly in contact, the resistance at the contact is much greater than if the surfaces are pressed firmly to- gether. This is due to the fact that under light pressure, even in the case of the smoothest surfaces, the points of contact are comparatively few, and as the pressure in- creases, these contact points are increased, thus increas- ing the available cross-section of contact. Moreover, iilms of oxide, the resistivity of which is comparatively high, and which are apt to form rapidly on nearly all metallic surfaces, increase the difficulty of obtaining perfect contact. Consequently, care must be exercised, when introducing apparatus into circuits, that the contact surfaces are free from oxide or grease, and are brought firmly together, especially in cases where the introduction of additional resistance is deleterious. An excellent form of variable resistance is based upon the preceding principle of resistance offered by light contacts ; namely, the carbon rheostat. This rheostat consists essentially of disks or plates of carbon, piled together, and provided with suitable means for varying their pressure. The telephone transmitter in common use employs the principle of variable contact pressure, to impress on the telephone conducting line, variations in the current strength corresponding to the vibrations of sound. Nearly all the telephone transmitters in common use employ either the principle of variable contact pressure, or the principle of varying the length and cross-section in conducting path through carbon particles, vibrating between relatively non-vibratory electrodes. 45. When the current strength remains constant, the resistance of any part of the circuit depends natur- ally upon the resistivity of that part, its dimensions and temperature. When, however, the current strength varies rapidly, the form or shape of the conductor also affects its resistance. For instance, a stranded conduc- tor, that is, a conductor formed of a number of separ- ate wires layed-up together, offers an apparently lower resistance to a rapidly alternating current, that is, to a current rapidly changing its direction, than- would a solid conductor of the same length and total cross-section. The cause of this increase of resistance will be con- sidered in a subsequent leaflet. -tH. It is now well recognized that lightning dis- charges partake of a rapidly alternating character. An advantage is therefore secured from the use of a stranded or strip lightning rod, as opposed to a solid rod of the same weight per unit of length. -t7. The following is a table of the resistances of various apparatus employed in the commercial applications of electricity. GALVANOMETERS. Thomson Mirror Galvanometer 1 ohm to 350,000 ohms. " " Common resistance 5,000 '* Thomson Marine Galvanometer 5,000 to 50,000 D' Arson val Galvanometers 1 ohm to 750 '* Common resistance 250 M SB 8 J V 2 R 51 T x [Entered as second-class matter at the New York, N. Y t> lW Office, June 14, 1894.] . , 50 56. Ohm's law is generally expressed as follows : W C = -= , in English speaking countries, where C = current strength in amperes ; E = E. M. F. in volts, and R, the resistance in ohms. In foreign countries the equation is usually written 7=1 /, here standing for the intensity of the current. Since, however, at the International Electrical Con- gress at Chicago in 1893, it was recommended to employ a uniform symbolic notation, in which 1 was to be used internationally in place of C, this latter symbol being adopted for another purpose, w r e shall hereafter employ /, to represent current strength. 57. From the formula, /=f, a.) we obtain, E = I R , (2.) and R = ^ (3.) In other words, having given any two of the three essential quantities, electromotive force, resistance and current strength, in a continuous current circuit, the third can always be determined by the foregoing equations. These formulae are applicable, not only to an entire cir- cuit, but also to any portion of a circuit; thus, the E. M. K. of a conductor which is required to send through it the current it conveys is usually called the " drop " in that conductor. 51 58. For example, in (Fig. 20) where a battery of E.M.F. E, and internal resistance, r^ is represented in circuit with an electromagnet of resistance r 2 and long wires of total resistance r s leading to a circuit closer. Here / = ^ = . -JL R r,+ T % + ?' S 1O OHMS -"c ' ' ^^ 2f ?i **-*! ih 85 I P = 1OOHMS r 3 =5 OHMS O.4-762 AMPERE Eltc.Enyineer FIG. 20. DISTRIBUTION OF POTENTIAL DIFFERENCE IN A CIRCUIT. in which the resistance H is equal to the sum of the separate resistances. Thus if E 10 volts, r v 6 ohms, r. z = 10 ohms, r s 5ohms, then R = 21 ohms, and /= Jf = 0.4762 ampere. This current in passing through eacli resistance r l9 r^ and r s is attended by such a distribution of E. M. F. as will satisfy equation (2). Thus in passing 52 through 7*2, it will be attended by an E. M. F. of *? 2 = 0.4762 X 10 = 4.762 volts, and a voltmeter , an in- strument for measuring electromotive forces, if suitably connected across the terminals of r z ; namely, between a and J, would indicate 4.762 volts. The same relation would be true for r z , in which there is a drop of 0.4762 X 5 = 2.381 volts. Again e v is the drop in the battery of 0.47H2 X P> 2.857 volts, so that while the current flows, the P. i>. at battery terminals 10 2.857 = 7.14o volts. Since E = e l -{-e 2 -f- <. The preceding formulae are also applicable to branch, derived or shunt circuits. Thus, in Fig. 21 A, the dynamo E, whose E. M. F. is 100 volts, and in- ternal resistance r^ is 0.5 ohm, supplies three branches /> 5 , and /' 6 , of a^lOgand ,T0ohms, respectively, through / 4 , 5 , two leads, each of one ohm. In this case we may substitute for the three resistances in parallel, their equivalent or joint resistance 72, as in Fig. 21 B. By what has been already stated in para- graph 33, the joint conductance of ?' 4 , r 5 , and r 6 , is -fa + _i_ _|_ ^_ 0.05, and, therefore, their joint resistance is Tj-.-J-g- = 20 ohms. The total resistance in the circuit is, therefore, f /\ + n -f- r$ -f- R = 22.5 ohms, and the ^ 100 current in the main circuit - = ^x-^ = 4.444 amperes. The drop in the dynamo armature due to its resist- ance will, therefore be E = I R = 4.444 X 0.5 = 2.222 volts, leaving the p. D. at dynamo terminals 97.778 volts. The drop in each lead will similarly be 4.444 volts or 8.889 volts in both, leaving 88.889 volts at the terminals c. D. O.O1 OHM O.01 OHM FIG. 22. APPLICATION OP OHM'S LAW TO A CIRCUIT CONTAINING COUNTER E. M. F. E 88.889 The current in r 4 will be -^= ^p- 1.778 amperes. " /.,. u similarly 0.889 " u a a ^, a u ^ ^ 778 " Making the total 4.444 " as before. 00. Fig, 22 represents the case of a low tension dyna- mo, E, of E.M.F. seven volts, and internal resistance /'!, 0.03 ohms, charging two storage cells in series, of 2.5 volts and 0.01 ohm each, through two leads r 2 and r 5 of 0.1 ohm each. In this case the E. M. F. of the storage cells is opposed to that of the charging battery so that the total E. M. F., E e = 7 5 = 2 volts, and the total resistance ^ + r 2 + ^ + n + ^ 0-25 ohm, E e 2 making the current / = ~~~R = rT25 = amperes. Here the drop in the dynamo armature due to its resistance will be E =. I R = 8 X 0.03 = 0.24 volt, leaving a p. D. at dynamo terminals of 7 0.24 = 6.76 volts. The drop in each lead will be 8 X 0.10 = 0.8 volts or 1.6 volts, collectively, making the pressure at storage battery terminals 5.16 volts, and the apparent drop in each cell 2.58 volts, of which 2.5 is counter E. M. F., and 0.08 p. D. due to I-R drop. 61. Ohm's law for the current strength developed by a given impressed E. M. F. in a circuft of given resistance, is not true unless the E. M. F. impressed on the circuit remains constant in strength, or in case it varies, allowance is made for its variation. In the case of alternating current circuits, where the variation in E. M. F. is periodic and obeys a definite law, and where con- sequently the current strength varies periodically, allow- ance has to be made for the effect of this variation, and the current strength in such a circuit, is not generally the simple ratio of the E. M. F. to the resistance. The necessary modification of Ohm's law as applied to alternating current circuits, will be explained in a sub- sequent leaflet. 55 SYLLABUS. In any continuous current circuit, the current strength varies directly with the E. M. F. or pressure; i. e., if the pressure be doubled, the current will be doubled, and if the pressure be halved, the current will be halved. In any continuous current circuit the current strength varies inversely as the resistance ; i. e., if the resistance be doubled the current strength will be halved, and vice-versa. Both the above relations are expressed in Ohm's law in the equation If E, the total E. M. F. be expressed in volts, and R the total resistance in ohms, then /, the current strength will be expressed in amperes. The resistance in any circuit can usually be con- veniently divided into three parts ; namely, the resistance of the source ; the resistance of the leads or conductors ; and the resistance of the receptive device ; and the formula of Ohm's law then becomes i? 1= - + fa + r* The total E. M. F. in a circuit is equal to the sum of the separate E. M. F.'S in the circuit if more than one E. M. F. is acting. The fall of pressure or " drop " in a conductor carrying a current, is the E. M. F. required to send that current through the conductor. (Hun's law for current strength is nut applicable to other than continuous current circuits, unless allowance- is nuide for variations in i. M. i. In allernat in^ current circuits tlierelore, where the i. M. r. varies peri((lic;ill\, thi> la\\ re.|iiires inodilicat inn tor the i . M. i .V that arc int i-otliiced into the circuit l>\ the \ariation of the current. l.:il..r;il..r\ "I lUuMim \ K.-iin PhiltdelphU, |< '"|M " III , 1 >(. '". 'HI' I' I '" ' Mi A I I'. Mi. IN I' Ml, | WKKKI.N. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. I NT* EF? MEDIATE GFtADE KI.KCVKMC C , ^2. The word circuit, litemllv a circle, is the path which MII electric current traverses when it lea\e.- the positive pole of Mil electric source, or hatlerv of sources, Mild passes through or iullueiices the various electro receptive or I I'M lisla t i 11^ devices placed in if-, path, iv enters the source or haltervat its ne^si.live pole, ;ind returns to its starting |>oint, M( the positive pole, after (lowing through the source. Ill actual prMctice, the. shape ol the path is seldom circular. It is evident HIM! all conducting oircBita consist essentially of three parts; namely : (/.) Of the source. (#.) Of the. conductors. (.'/.) Of the receptive or t rMiishltin^ devices. 'Flic prime ohject of all elect ric circuits is to conve\ an electric current, produced h\ Mil electric source, to more or less remote places where the electro receptive devices are located. Till, I.I-ECTRICAL ENGINKER, 303 "roadway, Nrw York, N. V. 1 I nl i' 'I .1 - >' I .'I. i-,, ni.ill. i ..I llic Nc.w V..ik, N. Y., I'osl Ollirr, June 58 63. The conditions governing the current strength in any circuit will, as already pointed out, be de- termined in accordance with Ohm's law, by the relations existing between the electromotive forces and the resist- ances. The resistance of a circuit will necessarily depend upon the separate resistances of the three parts already referred to, namely, the source, the leads, and the recep- tive devices, and, since the useful work done by the various receptive devices will depend upon the relation existing between their resistance and the resistance of the rest of the circuit; i.e., that of the leads and sources, it is necessary that the resistance of these separate parts be properly proportioned in order to obtain the desired efficiency. 64. In order to obtain the best relative resistances adapted to the conditions of different cases, a great variety of conducting paths or circuits have been devised. These, however, may readily be grouped under four leading classes ; namely : (1.) Series circuits. (2.) Multiple circuits. (3.) Multiple-series circuits. (4.} Series-multiple circuits. 65. In the series circuit of electro-receptive devices all the current passes successively through each electro-receptive device, and returns to the source. For example, in Fig. 23, which represents a Municipal Series Incandescent System, the current leaving the dynamo, E, at its positive or -f- terminal, passes through the lamps 1, '2, 3, 4, 5, 10, and returns .to the dynamo at its negative or 59 - terminal. Since in such a circuit, the extinguishment of any single lamp would break or open the circuit, and thus render all the other electro-receptive devices in- operative, some form of safety device is always em- ployed automatically to short-circuit any lamp which may become faulty, and thus permit the current to con- tinue to pass through the other devices. W5. A series circuit of electro-receptive or translat- ing devices is generally employed in the case of arc lamps, and of most telegraphic and telephonic apparatus. The resistance of a series circuit is equal to the sum of the separate resistances ; consequently, in all such cir- PIG. 23. "MUNICIPAL" SERIES CIRCUIT OF 21 INCANDESCENT LAMPS. cuits, as additional translating devices are introduced or removed from the circuit, some arrangement must be provided in the source, which will vary its electromotive force, and so ensure a proper working of the electro-re- ceptive devices by causing the current which passes through them to remain constant. The series circuit as employed for arc lamps is, for this reason, frequently called a constant current circuit. A constant current circuit of variable resistance must necessarily be a circuit of variable E. M. F. r>7. Electric sources may also be connected in series; for example, in Fig. 24, which shows two dyna- mos, K! and E 2 , connected in series. Here the positive brush of the dynamo, E 2 , is connected with the negative brush of the dynamo, E I? and their free brushes are con- nected respectively to the negative and positive leads. In the case of series connection of electric sources, the electromotive force of the single source so provided, is, of course, equal to the sum of the electromotive forces of the separate sources. Dynamos are so coupled when the electro-receptive devices they are intended to operate, require a greater electromotive force than that whicli either dvnamo alone can furnish. Etec. Engineer FIG. 24. SERIES-MULTIPLE ARRANGEMENT OF LAMPS AND SERIES ARRANGEMENT OF DYNAMOS. 68. In the multiple circuit of electro-receptive de- vices, the separate electro-receptive devices have all their positive terminals connected to a single positive conductor or lead, and all their negative terminals simi- larly connected to a single negative conductor or lead. The current from the source passes from the positive lead through as many separate branches, or derived circuits, as there are conducting paths offered to it, and, after passing through the receptive devices, returns to the dynamo at the negative lead. Thus, in Fig. 25, the dynamo D, has it positive brush or terminal connected to the positive lead, and its negative brush or terminal connected to the negative lead. The separate receptive devices, in this case a group of lamps, are connected as shown, each 61 with one of their terminals to the positive lead, and the other to the negative lead. The current leaving the dynamo, branches, as shown by the arrows, and, after passing through the receptive devices, returns to the dynamo. 69. Since in the multiple circuit, the resistance of the entire circuit decreases with every new recep- tive device added in multiple, it is evident that the cur- rent strength in the entire circuit will vary with the number of separate receptive devices; but, since neg- lecting the drop in the leads, the lamps are all subjected to the same difference of potential, it is evident that this circuit will have between its leads a constant difference INCANDESCENT LAMPS IN PARALLEL f - hi 2 AMP. 1>AMP. 1 AMP. JSAMP. Multiple Arc Circuit FIG. 25. of potential, or electrical pressure, depending upon the difference of potential furnished by the dynamo, at its brushes. Such a circuit is, therefore-, frequently called a constant potential circuit. A constant potential cii*- cuit is one in which the current strength in the circuit "necessarily varies with the number of devices operated. The electro-receptive devices, however, since their resis- tances are all equal, will be each traversed by a constant current, because they are acted on by a constant electro- motive force. TO. Electric sources may be connected in parallel. Thus, Fig. 26, shows four dynamos of equal elec- tromotive forces, such as would be employed in supply- 62 ing incandescent lamps connected in multiple. Here all the dynamos have their positive brushes connected to a single positive lead, or bus-bar, A B, and all their nega- tive terminals similarly connected to a single negative lead, or bits-bar, c D. The electromotive force of the combination is the same as that of a single dynamo, and the resistance of the combination, much less than that of a single dynamo, as would follow from the rule already stated for determining resistances in parallel. A bus-bar an abbreviation of omnibus bar receives, as its name indicates, the total current supplied by two or more dynamos. C . . . D FIG. 26. MULTIPLE CONNECTION OF DYNAMOS. The total current furnished to the bus-bars may, there- fore, be much greater than in the case of a single dyna- mo, and, when all the dynamos are exactly equal, will be just four times as great when each dynamo is operated at full load. 71. In the multiple-series circuit, a number of separate electro-receptive devices are connected in separate groups in series, and these groups subsequently connected in parallel. Fig. 27, shows nine plating baths connected in multi- ple-series. Here the baths are coupled in separate series groups of three each, and these groups subsequently con- nected in multiple. This arrangement is, however, almost entirely limited to the case of voltaic cells, and is adopted where it is necessary to obtain such relations between the current strength and the electromotive force as may be required for the best operation of receptive devices connected in the circuit. 72. In the series-multiple circuit, a number of sepa- rate electro-receptive devices are connected in separate groups in multiple, and these groups subsequently connected in series. This arrangement is employed in the case of the three-wire system for incandescent lamps, ? ? i 1 - L e 6 J;lvc. Engineer FIG. 27. ARRANGEMENT OF ELECTROPLATING BATHS IN MULTIPLE- SERIES. where, as indicated in Fig. 24, the lamps are connected in separate groups in multiple, and these groups sub- sequently connected in series. In order properly to maintain the electrical pressure at the terminals H B, and H D, of the two groups of lamps when the number of lamps in each group may vary, a third or neutral wire <; H, is carried from the point H, to the common connect- ion G, of the two dynamos, and the current through this wire automatically tends to equalize the pressure on each side of the system. SYLLABUS. * All circuits may be divided into four main classes; viz: series, multiple, multiple-series, and series-multiple. The resistance of electric devices or sources connected in series is tlie sum of their separate resistances ; the electromotive force of series-connected sources is the sum of their separate electromotive forces. In the series circuit, the current strength is constant throughout the circuit : a series circuit is, therefore, some, times called a constant current circuit. In a multiple circuit, the conductance of devices or sources connected in parallel is the sum of their separate conductances. In the multiple circuit the potential difference, dis- regarding drop in the leads, is constant. This circuit is, therefore, sometimes called a constant potential circuit. Yoltaic cells are sometimes connected in multiple- series or in series-multiple. Laboratory of Houston & Kennelly, Philadelphia. tCopyright, 1894, by TUB ELFXTRICAL ENGINEER.! WEEKLY. No. 9. AUOTIST 11, 1894. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE GRADE. VOLTAIC CELL, 73. We have already seen that the origin of the E. M. F. produced by the friction of unlike bodies is to be traced to the contact of dissimilar surfaces. Here the energy supplying the electricity is the mechanical energy required to produce the friction. We have also seen that this E. M. r. can be made to produce momentary currents. When the contact of different metallic sub- stances is produced through the agency of a liquid cap- able of conducting electricity and of being decomposed by. it, such contact produces an E. M. F. which can be uti- lized for the production of a steady current. Y4. A device for the* ready production of electro- motive force by the contact of metallic substances through the intervention of a liquid substance is called a voltaic cell. In all cases a voltaic cell consists of two dissimilar substances, generally metals, and an exciting liquid, called the electrolyte. The two metallic sub- stances form, when used in this connection, what is called Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York, N. Y. [Entered as second-class matter at the New York, N. Y., Post Office, June 14, 1894.] 66 a voltaic couple, the liquid through whose intervention their contact is continued is called the electrolyte, and each of the separate metals of the couple is called an element. Where the amount of current required for use is not excessive, a voltaic cell is one of the most convenient sources of electrical energy. It is named in honor of Alexander Yolta, who invented it. FIG. 28. SIMPLE FORM OF VOL- TAIC CELL ox OPEN CIRCUIT. Elc.Engineer FIG. 29. SIMPLE FORM OF VOL- TAIC CELL ox CLOSED CIRCUIT. 75. When as in Fig. 28, two plates of commercial zinc and copper are plunged in a dilute solution of sulphuric acid in water, the following actions take place ; namely, (!.} The acid acts on the zinc which is slowly dissolved with the formation of zinc sulphate, ZnSO^ and the lib- eration of hydrogen, //>, according to the following chemical equation, Zn .#~ 8O (2.) The hydrogen is liberated entirely at the surface of the zinc plate, where the action occurs. (3.) ~No action occurs at the copper plate. (4.) The chemical action on the zinc plate is attended by the liberation of heat which raises the temperature of the liquid. When now, the zinc is connected outside of the liquid by means of conducting wire, as shown in Fig. 29, the phenomena change and are as follows : (1.) The zinc is attacked as before with the formation of zinc sulphate, though not necessarily at the same rate as before. (2.) The hydrogen is now liberated almost entirely at the surface of the copper plate. (3.) The heat liberated during the action now appears in all parts of the circuit and is not confined to the cell. That is to say, the wire becomes warm. (4.) An electric current now traverses the conducting circuit as may be shown by its action on a magnetic needle suspended either near the wire or near the battery. 76. The combination of parts described in connection with the preceding figure constitutes a simple form of voltaic cell. The source of energy which produces the E. M. F. is clearly to be traced to the chemical potential energy, of the plates and electrolyte, liberated during the chemical combination of the positive element \\ r ith the negative radical of the electrolyte ; i.e., the SO ion or radical. The chemical equation which expresses the activity of the cell is ? therefore, Before action ; Cn -f- JL SO^ -f- Zn. After action ; Cn + IL + ZnS0 4 . The action, therefore, evidently removes an atom of zinc for every molecule of JI 2 /SO i decomposed, while the hydrogen liberated tends to collect on the surface of the copper plate. Since this hydrogen, by its contact with the copper, tends to produce an E. M. r. directed opposite to that of the cell, its presence tends to decrease the working E. M. F. and various devices are employed to avoid its presence. Polarization devices in practice provide for either pre- venting the liberation of the hydrogen or for rapidly absorbing it after formed. Both of these objects are effected by surrounding the plate by some suitable chemical substance. The tendency to the production of a counter-electromotive force by the presence of hydro- gen, is called the polarization of the cell, and the sub- stance surrounding the negative plate for the purpose of preventing such polarization is called the depolarizer. 77. Voltaic cells may be divided into the following general classes ; namely, (1.) Cells without depolarizers. (2.) Cells with depolarizers. Those of the first class are generally called single-fluid cells, and in them, on. closed circuit, polarization is apt to prove a very serious defect. The best cells of this class employ for one of the elements a carbon plate. Carbon, as is well known, possesses in a marked degree, the power of dissolving and occluding hydrogen gas. When an exciting liquid like chromic acid is em- ployed, which, besides acting on the zinc, also possesses the power of combining v/ith and dissolving hydrogen, the polarization, which would otherwise exist, is reduced. 69 Strictly speaking, then, the fluid in such single-fluid cells, acts both as the exciting and depolarizing fluid. 78. Voltaic cells with depolarizers may be divided into two well defined classes ; namely, (1.) Cells with a single fluid and a solid depolarizer, and (&) Cells with two separate fluids, one exciting, and one depolarizing, with a porous partition between them. In the simple or voltaic cell shown in Fig. 29, the current produced on the closing of the circuit is conven- tionally assumed to flow in the direction shown by the arrows ; namely, through the electrolyte from the zinc plate to the copper plate, and, outside the electrolyte, from the copper terminal, through the conducting path, to the zinc terminal. Since, according to convention, that pole of the source out from which the electricity flows is the positive pole, and that pole into which it flows, the negative pole, it will be seen that the positive terminal, or electrode, is the terminal connected with the copper plate, while the negative terminal or electrode will be that connected with the zinc plate. Strictly speaking this convention will make the polarity of the plates, where covered by the electrolyte, exactly opposite to the polarity in the parts above the liquid ; namely, the zinc plate will be positive in the liquid and the copper plate negative. It is evident that the above is a mere convention, that the zinc plate cannot be both positive and negative. In- deed, if tested by an electrometer, the zinc plate can be shown to be negative throughout its mass, and the cop- per plate can be similarly shown to be positive. Still it 70 is convenient to refer to the copper plate as the negative plate because the current enters it from the liquid, and the zinc plate as the positive plate, because the current issues from it into the liquid ; and, moreover, since these terms are sanctioned by general usage we shall employ them in future. Y9. It may be interesting, in connection with the preceding, to state the manner in which the con- vention as to the direction in which the electric current is assumed to flow (namely from the positive to negative ) arose. It was originally arbitrarily assumed that the character of the electrification produced by say, catskin against glass, was negative on the catskin and positive on the glass. The glass was, therefore, regarded as having a positive charge, and the catskin, a negative charge, and, when these charges neutralized each other, it was as- sumed that the charge passed in a momentary current or discharge from the glass to the catskin ; ?'. 1 1 A un IT QT 9^ 1SQ4- Price, - 10 Cents. 1 25, 1 M. Subscription, $3.00. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE GRADE. VOLTAIC CELL 90. No matter what form be given to the voltaic cell, it can never reasonably be expected to compete, in point of economy, with a well designed dynamo-electric machine, where any considerable output of electric energy is demanded. The source of energy in the voltaic cell is the chemical potential energy of the plates and of the electrolyte. In the dynamo electric machine the energy liberated from coal, burned under a boiler, is eventually converted into electrical energy by causing the conductors on the armature to cut magnetic flux paths. In the vol- taic cell a comparatively expensive metal, zinc, is burned in an exciting liquid, and the output of the cell, is neces- sarily much higher in price than in the case of the dynamo. Thus, assume the cost of a pound of zinc, sufficiently good to be employed in a battery, to be $0.07. This pound of zinc, as we have seen, would be consumed by a delivery of 1,347,500 coulombs; therefore, if the E. M. F. of the cell is as high as 2 volts, the energy delivered by the Published by THEKELECTRICAL ENGINEER, 203 Broadway, New York N. Y. Entered as second-class matter at the New York, N. Y., Post Office, June 14 1894.] 82 pound of. zinc will be 2,695,000 volt-coulombs or joules, and dividing this by the number of seconds in an hour, we obtain 748.6 watt-hours, = 0.7486 K. w. hours. The price of a K. w. hour of electric energy produced from this voltaic cell would, therefore, be o-.^rib = 9-35 cents in zinc consumed alone, regardless of the cost of the electrolyte, interest, depreciation and attendance. On the other hand, it is well known that large engines require about 1.8 Ibs. of coal to be burned under their boilers for every indicated horse-power-hour delivered to the dynamos, so that with coal costing, say, $3.00 per ton of 2,240 Ibs., the cost of a horse-power-hour in coal is 0.241 cent, regardless of water, oil, waste, attendance, interest and depreciation. This represents 0.323 cent per indicated K. w. hour, and with dynamos converting 90 per cent, of the indicated horse power into electrical energy, the cost of coal per K. w. hour is, therefore 0.359 cent. It will, consequently, be seen that the cost of a K. w. hour produced by a voltaic cell, in zinc consumed, is about 26 times the cost of the same amount of energy (3600 joules) produced by a steam dynamo on a large scale. 91. The above has reference only to the case where a voltaic battery is required to produce a large amount of electrical energy. In the case of the stean: engine, the necessary expense for attendance, as well as the inconveniences attending the delivery of a very small amount of power, renders the battery a very convenient source of energy for such small powers as driving sewing machines, fans or other similar apparatus. The amount of power delivered to the moving air by a fan of nine inches in diameter, running at 1,000 revolutions 83 per minute, is about three watts, increasing very nearly as the cube of the velocity up to a critical speed. The amount of energy absorbed by an ordinary sewing machine making about 480 stitches per minute, is approxi- mately 12 watts. The efficiency of the small motors re- quired to drive the fan or sewing machine being usually no more than 0.5, about twice the mechanical energy must be supplied electrically in every case. 92. A ready measure of the amount of power which a cell can furnish, neglecting the consequences of polarization, is the square of its E. M. F., divided by its resistance. This may be called the electrical capability of the cell and is equal to the number of watts which would be expended by the cell if placed on short circuit, being expressed thus : P = ^ r It is evident, therefore, from the above expression that the electrical capability of a cell increases as the square of its E. M. F., and is inversely as its resistance. In the case of any given cell the E. M. F. is beyond control. In any battery the E. M. F. can be increased by adding cells in series. The internal resistance of a cell can be decreased by decreasing the distance between the plates or by in- creasing their area. The latter is usually the most prac- tical method. It will, therefore, be seen that all methods for increasing the electrical capability of a voltaic source are practically limited to coupling a number of cells together so as to increase the E. M. F. or to decrease the resistance, or both. The electrical capability of a battery of N^ cells is JY times the electrical capability of a single cell, and is inde- pendent of the grouping of the cells, provided that the cells are all similar, and are symmetrically grouped. For example, if a cell has an E. M. F. of 2 volts, and an internal resistance of 0.25 ohm, its electrical capability will be 2 X 2 = 16 watts. A battery of 24 such cells U.^o would have 16 X 24 = 384 watts. For if arranged in one series the capability of the battery would be /2 \/ 24"\ 2 ^ ^ ^- = 384 ; or if arranged in 2 series of 12 each U.^o X 2i^. in multiple, the capability of the battery would be (2 X 12) 2 25 -\ = 384, and so on for any symmetrical /0.25 -j\ grouping of rows and series. 93 When a given small amount of activity is to be furnished by a voltaic battery, the first considera- tion is, of course, to obtain this activity with a maximum economy. The maximum economy may be either the maximum economy of operating the battery, or the maximum economy of installing it, which, of course, are entirely different. The maximum economy of operation is obtained when the number of cells is such that the materials they con- sume, together with the interest, depreciation and attend- ance, on the whole plant, is a minimum. The maximum economy of installation is obtained when the number of cells employed that will satisfactorily perform the re- quired activity is a minimum. If P, be the amount of this activity expressed in watts, then the minimum num- ber of cells is 4P, divided by the capability of each cell. Or the number of cells, N = . In other words, the maximum economy of first installation is reached when the capability of the battery is four times the activity. Thus, if a battery were required to operate a sewing machine taking 12 watts, through a motor of 0.5 effi- ciency, requiring 24 watts, and if each cell of the battery to be used had an E. M. F. of 1 volt and an internal resistance of 0.125 ohm, its capability would be 8 watts, and the minimum number of cells required would be 4X24 = 12 8 This number of cells would be independent of the grouping adopted, for the same results would be ob- tained with 12 cells in a single series, two rows of six, three rows of four, four rows of three, six rows of two, or by all twelve cells in parallel. Practically, however, the arrangement would depend on the winding of the motor. If the motor were wound for six volts, then all the cells would be connected in one series. The E. M. F. of the battery would be 12 volts, its internal resistance 12 X 12 1.5 ohms, and its electrical capability ^ = 96 1.5 watts. If the battery were placed on short-circuit it would therefore do work through its own resistance at the rate of 96 watts, which is four times the external activity or output required. On connecting with the motor, the current delivered at the required output would be 4 amperes. The drop in the battery would be 1R 4 X 1-5 = 6 volts, or half the E. M. F., and the pressure remaining at terminals would be 6 volts. The external activity would be 24 watts, the internal activity 24 watts, the total activity in the circuit 48 watts or half rgffZ ! 55 ^OR-A. 0V THB IUHITBRSITT; 86 the capability of the battery, and the efficiency of the battery would be 0.5. 94. Minimum cost in battery installation satisfies the following six conditions : (1.) The capability of the battery is four times the output. (2.) The capability of the battery is twice its total activity. (3.) The internal and external activities are equal. (^.) The pressure at battery terminals is equal to the drop in the battery. (5.) The efficiency of the battery is 0.5. (6.) The current strength through each cell is half that which it would deliver on short-circuit. 95. The voltaic cell ranks high as an electric source for the production of comparatively constant E. M. F.'S. Ordinarily the E. M. F. of a voltaic cell, as usu- ally constructed, is variable, depending as it does upon a number of circumstances such as temperature, strength of solution, purity of plates and exciting liquid, atmos- pheric pressure, and activity of the circuit. Yet if certain precautions are taken in the preparation and use of voltaic cells, they can be made to produce a very uniform E. M. F. The E. M. F. of a dynamo machine can only remain uniform when the speed of rotation is maintained constant and the field magnets retain a con- stant strength, conditions which are frequently difficult or expensive to maintain, when only a small amount of power is desired. On the contrary, in the case of the voltaic cell, if fairly uniform conditions are assured, the value of the E. M. F. will remain very nearly constant. So true is this that the legal value of the unit of E. M. F., 87 the volt, lias been decided as being a definite fraction of that furnished by a particular form of standard cell known as a Clark standard, at a definite temperature, (y.J^-th part at 15 C.) 96. The following fields of usefulness exist at present for the voltaic battery : (1.) As a limited source of power. This we have seen is limited by the expense of materials and of operation. The limit is apparently reached in practice at the power required to drive a sewing machine, which at ordinary moderate speeds, as already mentioned, is about twelve watts. Allowing an efficiency of 0.5 in the small driving motor, the delivery of power from the battery becomes about 24 watts. Beyond this amount of power, the ex- penses of installing and maintaining a battery is, even with the best existing types, generally regarded as prohibitory. (#.) For signalling purposes, as in telegraphy, tele- phony, annunciators, etc. Here the amount of work required is usually very small. The amount of energy delivered by a battery of 100 cells to a telegraph line be- ing usually about three watts. In all large telegraph stations, dynamos, or storage cells charged by dynamos, are being generally employed. (&) For testing purposes, as, for example, in furnishing a uniform E. M. F. (4>) For electroplating ; although here also, on all but the smallest scale of operation, the dynamo has come into use. (5.) For electro-therapeutic purposes^ owing to the portability of a voltaic battery. It will be seen, therefore, that the principal uses for voltaic cells are for testing, and for domestic service 88 where a current from dynamos cannot readily be obtained. This statement refers to the existing batteries which burn zinc. Should it become practically possible to consume carbon in a commercial voltaic battery and obtain an output approaching the theoretical amount, it might readily become much cheaper to produce electrical energy from batteries than from dynamos, and the use of the steam engine, as the prime source of electric power, might be superseded. SYLLABUS. The source of energy in a voltaic cell is the chemical potential energy of the plates of the electrolyte. Voltaic batteries, as at present constructed, can never compete with steam-driven dynamos for the delivery of large electric currents. For small powers, however, vol- taic batteries possess many advantages over dynamos. The electrical capability of a cell, or the amount of power the cell is capable of furnishing when placed on short circuit, is equal to the square of the E. M. F. of the cell divided by its resistance. The electric cell, when suitably constructed, ranks high as a source of extremely uniform E. M. F. In this respect it far surpasses the dynamo. The legal value of the volt, the unit of E. M. F., is taken as that furnished by a particular form of Clark standard cell, at a fixed temperature. The following are the principal fields of usefulness for the voltaic cell, viz.: (1.) As a limited source of power. (2.) For signalling purposes. (3.) For testing purposes. (4.) For electro- plating. (.) For electro-therapeutics. Laboratory of Houston & Keimeily. Philadelphia. [Copyright, 1894, by THE EI.FCTKICAI. ENGINEER.] WEEKLY. No. 12. SEPTEMBER 1, 1804. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE Gf^ADE. IV! as tie to motive Korce 97. Surrounding every magnet there is a region of magnetic influence, technically known as the mag- netic field. The region is permeated with what are fre- quently called lines of magnetic force, but which may be more accurately described as magnetic flux-paths. Magnetic flux possesses the following properties, namely : (1.) A bar of iron when introduced into the flux becomes magnetized. (2.) A freely suspended magnetic needle brought into the flux, comes to rest in a definite position. (3.) An electric conductor moved across the flux paths has an electromotive force developed in it. The exact nature of magnetic flux is not understood, but it appears to be attended by a stress in the ether. 98. A convention is employed as to the assumed direction of the magnetic flux, similar to that em- ployed in the case of electric flux ; viz., the magnetic flux is assumed to issue from the magnet, as shown in Fig. 36, at its positive or north, i. e., north-seeking pole N, Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York N. Y. [Entered as second-class matter at the New York, N. Y., Post Office, June 14 1894.] 90 (that which tends to point northwards if the needle be free to move), and, after passing through the region around the FIG. 36. DIAGRAM OF ASSUMED DIRECTION OF FLUX-PATHS ix A MAGNETIC CIRCUIT. magnet, t;> re-enter it at its negative or south-seeking pole, s, thus corresponding with the direction of the electric flux, which is assumed to leave a-n electric source FIG. 37. PERMANENT MAGNET AND FLUX-PATHS SURROUNDING IT, AS INDICATED BY IRON FILINGS ON PLATE LAID FLAT ON MAGNET. at its positive pole and re-enter it after having passed through the circuit at its negative pole. 91 99. Figs. 37 and 38 represent the assumed direction of the magnetic flux in the case of a magnet of the form shown, placed in Fig. 37, with its greatest length hori- zontal to the plate, and in Fig. 38, with its greatest length vertical to the plate. A careful inspection of these figures will show that the poles are not by any means concentrated at points situated at the extremities of the har, but are distributed over a considerable area. FIG. 38. FLUX-PATHS SURROUNDING MAGNET POLES AS INDICATED BY IRON FILINGS ON PLATE LAID UPON THE POLAR EXTREMITIES. Magnetic figures may be obtained by suitably support- ing a sheet of paper over a magnet, sprinkling iron filings upon the paper, and then gently tapping it so as to enable the filings to arrange themselves under the influence of the magnetic forces. 100. The preceding figures show the flux-paths only in the immediate vicinity of the magnet, being lim- ited by the size of the sheet of paper employed. In reality the magnetic flux exists for indefinitely great distances 92 around the magnet, but at distances exceeding a few inches becomes so weakened that its detection requires the use of comparatively delicate apparatus. The magnetic flux-paths, around any bar magnet, as they may be traced either by iron filings, or by an exploring 'magnetic needle, (i.e., a suspended magnetic needle which assumes the direction of the flux at the point it occupies) show that the flux paths coincide with the stream-lines which would be produced by a tube filled with and surrounded by an incompressible liquid, such as water, if a force pump within the tube, drove the liquid out at one end of the tube and sucked it in at the other end. In the case of a magnet, the magnetic stream-lines, as already remarked, are assumed to pass out from the north pole and re-enter at the south pole. The force which causes this flux, corresponding to the force driving the pump producing the liquid flux, is called the magnetomotive force, usually abbreviated M. M. F. The M. M. F., in the case of the magnetic circuit, corresponds to the E. M. F. in the case of an electric circuit. We have seen that in the electric circuit, no flux, i.e., no current can exist without the establishment of an E. M. F. in the circuit. Similarly, in the magnetic cir- cuit, no flux can exist without the establishment of a M. M. F. in the circuit. The unit of magnetomotive force is called the gilbert, from Dr. Gilbert of Colchester, a famous early authority on magnetism, (1600 A.D ) 101. There are two distinct varieties of M. M. F.: namely, the permanent, or that naturally existing in certain kinds of matter, notably in iron, nickel, cobalt ; and the transient, or that produced in the neighborhood 93 of a conductor by the passage through it of an electric current. When an electric current circulates through a conductor, a certain distribution of flux is produced in the region surrounding the conductor. If, however, this region is occupied by iron, the amount of flux produced is enormously increased, and the only explanation con- sistent with the facts is that there is a source of M. M. F. in the magnetized iron as well as in the electric current ; FIG. 39. SECTION OF A COMMON TYPE OF DYNAMO WITH MAGNETIC CIRCUIT INDICATED. for, if the iron be hard, its magnetic condition will in a great measure persist after the magnetizing current has ceased to flow, in which case the iron must be regarded as the seat of a permanent M. M. F. 102. In nearly all practical magnetic circuits, the magnetic flux passes, for the greater part of its path, through iron. Thus in Fig. 39 is shown an ordi- nary bi-polar dynamo in which the magnetic circuit is 94 indicated by the dotted lines. Here the path through the field magnets is entirely of iron ; through the arma- ture largely of iron, and through the interpolar spaces between the armature and the surrounding pole faces, A A, through air. In the multipolar dynamo shown in Fig. 40, the mag- netic circuits passing through each pole, divide, passing as before through circuits, indicated by the dotted lines, FIG. 40. DIAGRAMMATIC SECTION OF A SEXTIPOLAR DYNAMO, SHOWING THE MAGNETIC CIRCUITS AND GENERAL ARRANGEMENT OF FLUX DISTRIBUTION UNDER THE EXCITING-COIL M. M. F.'S. lying mainly through iron, as at F, but partially through air, as at B. The M. M. F. which drives the magnetic flux through any circuit is dependent on two factors; namely, the current strength passing through the magnetizing coils, and the number of turns of wire in these coils. This product is generally expressed in ampere-turns. 95 103. The unit of M. M. F. is the gilbert. It is the M. jyi. F. produced by or approximately 0.7958 T: 7[ ampere-turn. It is only necessary, therefore, to multiply the number of ampere-turns on the field magnets of a dynamo machine by 1.257, to obtain then. M. F. 0.7958 expressed in gilberts. For example, a particular 10 K. w. dynamo of the bi-polar type has two field coils, one 011 eaclx magnet. The total number of turns on these two coils is 2,100, and the current, which circulates through these coils at full load, is 2 amperes. The M. M. F. in ampere-turns is, therefore, 4,200, and in gilberts, 5,279. 104. "While magnetomotive forces may be conve- niently and accurately expressed in ampere-turns, the c. G. s. system of International measures requires that the unit of M. M. F. should differ by a numerical factor. In dealing with M. M. F.'S, it is commonly convenient to express their values in ampere-turns, but for purposes of computation, and for simplicity of reasoning, it is usually advantageous to employ the more fundamental and scien- tific unit, the gilbert. 105. Magnetomotive force, like electromotive force, possesses direction. That is, several M. M. F.'S may oppose or aid one another, the resultant M. M. F. being their geometrical sum, precisely like the case of various E. M. F.'S acting in an electric circuit. Thus the M. M. F.'S produced in the magnetic circuit of a dynamo, by two separate magnetizing coils, as shown in Fig. 39, will be additive, if the exciting coils are magnetized by currents in the same direction, and sub tractive if the coils are magnetized in opposite directions. 96 The M. M. F. produced by a current of 100 amperes, passing through a single loop of conducting wire, would be 100 ampere-turns, or 125.7 gilberts, whether that turn of wire were alone or whether it were associated with other turns of wire in a coil, and whether it surrounded iron or not ; but the flux, which that M. M. F. would pro- duce, would vary very greatly in these different cases. SYLLABUS. A magnetic field, or a region permeated by magnetic flux, accompanies every magnet or every conductor con- veying an electric current. A magnetic field produces, or is accompanied by, a stress in the ether, which may manifest its presence in a variety of ways. The density of the magnetic flux in any field is greater near the magnet than at distances from the magnet, and is usually at its maximum value in the neighborhood of the magnetic poles. The unit of M. M. F. is termed the gilbert, and is equal to the M. M. F. produced by 0.7958 ampere-turn. All magnetic flux, i.e., all magnetism, is produced by M. M. F., just as all electric flux or current is produced by E. M. F. M. M. F.'S, like E. M. F.'S, possess direction, so that several M. M. F.'S may oppose or aid one an other ; that is, their general effect is either subtractive or additive. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, by THE ELECTRICAL ENGINEER.! WEEKLY. No. 13. SEPTEMBER 8, 1894. Electrical Engineering Leaflets, BY Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. IWTTEI^JYIEDIATE GJ*AI>E. Magnetic Reluctance 106. The magnetic flux produced in any magnetic circuit by a given M. M. F. depends upon the mag- netic resistance of the circuit. In this respect magnetic resistance is similar to the resistance which an electric circuit offers to the passage of an electric flux under the influence of a given E. M. F. Magnetic resistance is called reluctance. In order to increase the magnetic flux under given M. M. F. it is only necessary to decrease the reluct- ance of the circuit. The following differences exist between magnetic re- luctance and electric resistance ; viz., (1.) Unlike the resistance in an electric circuit, the reluctance of masses of similar dimensions of nearly all materials, except iron and the magnetic metals, is practi- cally the same as that of air. (&) The electric flux can be confined to a definite path, usually a wire, while the magnetic flux, in general, can- not. The reason is that an electric conductor can be Published by THE ELECTRICAL ENGINEER, 203 Broadway, New Vork N. Y. [Entered as second-class matter at the New York, N. Y., Post Office, June 14, 1894.] 98 readily insulated, whereas there is no known insulator for the .magnetic flux. The magnetic flux which pro- ceeds from the north-seeking pole of a magnetic source passes through numerous diverging paths, re-entering the magnet at its south-seeking pole. (3.) In the case of an electric circuit, where a long single wire sustains a steady current, the current density is the same at all points in any cross-section of the wire ; in the case of a magnetic circuit, the flux density, in general, varies at different points in the cross- section of the circuit, and decreases as we recede from the poles. 107. As the specific resistance of a conductor is best defined under the term resistivity; namely, the resis- tance offered by a unit volume, or a unit cube of a material taken between its opposite faces, so the specific magnetic reluctance of a substance is best defined under the term reluctivity, or the magnetic reluctance of a unit cube, i. e., of a cubic centimetre, taken between parallel faces. The magnetic reluctivity of vacuum is taken as unity, and the reluctivity of air, copper, wood and nearly all substances except the magnetic metals, does not differ appreciably from the reluctivity of vacuum. The reluctivity of the magnetic metals varies with the density of the flux tra- versing them. Fig. 41 shows curves of reluctivity in various samples of iron and steel for different flux densities. Thus the lowest curve No. VII. , representing soft annealed Nor- way iron, shows, for example, a reluctivity of 0.7 thou- sandth, i. e.y -j-^-yth that of air, at a flux density of 10 kilogausses. In other words each cubic centimetre of this iron subjected to a flux intensity of (B = 10,000 gausses, 99 ABSCISSAE: FLUX DENSITY, GAUSSES (e) TOTAL MAGNETIC INTENSITY OR FLUX DENSITY (S) GAUSSES FIG. 41. Curves of reluctivity in iron and steel in relation to flux density, from measurements by Kennelly. 100 offers, between opposed faces, a reluctance of 0.0007 oersted. 108. There are three varieties of magnetic circuits ; viz., (1.) The non-ferric circuit, where the magnetic circuit is completed through air or other non-magnetic mate- rials. Such would be the magnetic circuit of a hollow coil of wire. (.) The ferric circuit, where the magnetic circuit is entirely completed through iron, as in the case of an FIG. 42. Non-ferric magnetic circuit. Coils of insulated copper wire on rubber cylinders distributing a magnetic circuit through air, wood and hard rubber. FIG. 43. Ferric magnetic circuit, an alternating current transformer. With the exception of "leakage" all the flux passes through iron. iron ring wrapped with wire, or an electro-magnet with the keeper pressed upon its poles. (3.) The aero-ferric circuit, in which the circuit lies partly through air and partly through iron. To this class of circuit belong the great majority of dynamos and electro-magnetic apparatus. Fig. 42 represents a type of non-ferric circuit. Fig. 101 43 represents a type of ferric circuit ; and, Fig. 44, a type of aero-ferric circuit. 109. The reluctances of practical magnetic circuits are very difficult to compute, owing to the varia- tion of the cross-section of the magnetic circuit at differ- ent points. Fig. 45, however, shows a particular case of non-ferric magnetic circuit, which is amenable to very simple treatment. If in the anchor ring of wood, copper, or other non- magnetic material, uniformly wrapped with wire, as FIG. 44. Aero-feme circuit ; a telegraph relay. Magnetic circuit through cores, yoke and keeper of magnets and air gaps at poles. shown in Fig. 45, the mean circumference of the coil is 50 cms., and the cross-section of the interior of the coil is five square centimetres, then the reluctance of the mag- net circuit, which will be confined entirely to the space within the winding, will be approximately - = 10 5 oersteds. All the flux paths in this case will be circular, and there will be no magnetic flux outside the winding. A compass-needle, therefore, held near the ring, provided the ring be uniformly wrapped, will fail to show whether the current is flowing through the winding or not. This 102 is the only known ease in which magnetic flux can he readily confined to ;i determinate path. 110. If the core of the preceding ring be replaced by soft iron, then the reluctance of the circuit may be 1,000 times less, and, consequently, the magnetic flux in the circuit a thousand times greater. Such a ring, although carrying a powerful magnetic flux, would still evidence no external magnetism, hut if a saw-cut he made through the ring at any point, as in Fig. 4(5, the opposite faces of this gap would show opposite polarity, and the magnetic circuit would then become of the aero- FIG. 45. Principal sections of closed circular coil and its mag- netic circuit. Core of wood or iron. i. 46. Diagram of aero-ferric magnetic circuit. Anchor ring iron core with air-gap. Fio. 47. Diagram of aero-ferric magnetic circuit. Anchor ring iron core with wider air-gap. ferric type, with flux lines preceding through the sur- rounding air. At the same time, the magnetic reluct- ance of the circuit is markedly increased ; thus if the saw-cut is one millimetre in width, and its area of cross- section five square centimetres, the increase of reluctance thus added to the previous ferric circuit would be ~ = 0.02 oersted approximately. The true value of the re- luctance would be somewhat less than this owing to the 103 slight diffusion of the flux beyond the limits of the air gap as shown, thereby sensibly increasing the effective cross-section of the air gap. 111. If the width of the air gap be increased, as shown in Fig. 47, then the increase in the reluct- ance of the circuit will produce a still more marked variation in the amount of magnetic flux, and the diffu- sion of the lines will be more marked. Owing to this diffusion, the reluctance of the air gap will be increased, FIG. 48. Diagram of aero-ferric circuit of half ring. but not in proportion to its length, the average cross- section being greater than five square centimetres. If the air gap be still further widened, as in Fig. 48, the same effects are still more markedly produced until the total flux in the magnetic circuit may be only a small fraction of that existing in the original case. But this weaker magnetic flux may have far more powerful influence upon neighboring magnets, owing to the exter- nal diffusion of the flux paths, as shown. 104 It is a curious fact that although the reluctivity of all non-magnetic substances is practically the same as that of the air-pump vacuum, yet the reluctivity of different specimens of iron is subject to marked varia- tions. An exceedingly small percentage of carbon in iron may greatly increase its reluctivity. As a rule the softer and purer the iron, the lower its reluctivity. Nickel is, perhaps, the only ingredient which forms an exception to this rule. SYLLABUS. The reluctivity of a magnetic circuit is the resistance it offers to the passage of the magnetic flux through it under a given M. M. F, Specific reluctance, or reluctivity of a substance, is the reluctance offered by a cubic centimetre of the sub- stance between opposite faces. Reluctance is measured in units called oersteds. An oersted is the reluctance offered by a cubic centimetre of air-pump vacuum. Magnetic circuits are of three kinds ; non-ferric, ferric, and aero-ferric. The reluctivity of iron is much less than that of air, but varies with the flux density; at first diminishing and afterwards increasing with the density. A closed circular coil is the only form of magnetic circuit in which the flux is strictly limited to a definite path. In aero-ferric circuits, the diffusion of the mag- netic flux will be greater as the portion of the circuit occupied by air is increased. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, by THE EI.FCTRICAL ENGINEER.] WEEKLY. No. 14. SEPTEMBER 15, 1894. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE GRADE. MAQNETIC FLUX. 113. The magnetic flux in any magnetic circuit is directly proportional to the M. M. F. acting on that circuit and inversely proportional to its reluctance ; or, since the unit of magnetic flux is the weber, the unit of M. M. F. the gilbert, and the unit of magnetic reluctance the oersted, we have the general expression, webers = or, 9 = *-, oersteds (R this corresponding with the expression given by Ohm's electric circuit law, volts / E amperes = - or, 1 = . ohms R Here SF, is the existing international symbol for Mag- netomotive Force, similarly, (R, denotes Eeiuctance, and the is the symbol for Flux. In either of the above equations, any two of the three Published by THE ELECTRICAL ENGINEER, 203 Broadway, New Vork N. Y. [Entered as second-class matter at the New York, N. Y., Post Office, June 14, 1894.] 106 independent quantities being known, the remaining one can be calculated. Thus, gilberts = webers X oersteds ; SF = (R. Or, oeistedg=8 aberte ; = g. webers (P 114. In practical magnetic circuits it is often a matter of considerable importance to be able to calculate the magnetic flux. In order to do this, in accordance with the preceding principles, it is only necessary to de- termine the values of the M. M. r. and the reluctance ; for, as is evident, there are but two ways in which the value of the magnetic flux in any circuit can be varied ; namely, by altering the value of either of these quan- tities. The value of the M. M. F. is most readily increased by increasing the strength of the exciting current. We will now show, by some practical examples, how the preceding equations may be applied in the determination of magnetic flux in a circuit. 115. Case 1 the simple case of an anchor ring, of soft Norway iron, wound with insulated wire : We commence with this case, because, as we have already pointed out, this type of circuit, if properly con- structed, possesses no magnetic leakage. The area of cross-section of the iron ring, of dimensions shown in Fig. 49, is 3.1416 square inches = 20.268 square centi- metres, and the mean length of the circuit, or the mean circumference of the ring, is 37.7 inches 95.74 centi- metres. If the total flux, which it is desired to send through this circuit, be 350 kilowebers, it is required to 107 determine the M. M. F. which must be applied to the ring in order to produce this flux. The reluctance of the circuit is obtained as follows : When 350,000 webers are transmitted uniformly through a circuit, the cross-section of which is 20.268 sq. cms., the flux density will be - 3 /o A / = 1?>270 gausses = 17.27 kilogausses ; i.e., 17.27 kilowebers to the square centi- metre. Referring to the diagram, Fig. 41, it will be seen on Curve No. VII., that, at this density, the reluctance of a cubic centimetre of Norway iron is by measurement, 19.2 FIG. 49. Ferric Magnetic Circuit. Norway iron ring uniformly wound with wire in turns. Mean circumference 37.7 ia. = 95.74 cms. Cross-section of ring 3.1416 sq. in. = 20.268 sq. cms. millioersteds. The reluctance of the circuit is, therefore, X 19.2 = 90.7 millioersteds = 0.0907 oersted. Inserting this reluctance in the equation F = <# (R, we have the required M. M. F. = 350,000 X 0.0907 = 31,745 gilberts. Since one gilbert is 0.8 ampere-turn approxi- mately, the required number of ampere-turns is 31,745 X 0.8 = 25,396, or more nearly 31,745 X 0.7958 = 25,250 ampere-turns. If now the winding of the ring be composed of 1,500 turns of insulated wire, the cur- 0? THJS UFIVBRSIT7 108 rent in each turn must be *,-- = 1 T.I 6 7 amperes, and tliis is the required exciting current. 116. Case 2. Taking now the case of an aero-ferric circuit, in which part of the flux-paths lie through air, say, an electromagnet, as shown in Fig. 50, let us first assume that the leakage is sufficiently small to be negligible ; that the air-gap in the circuit is fixed ; i.e. 9 that the keeper cannot move up to the poles of the mag- net ; and that the iron in the electromagnet and keeper is ordinary, soft, wrought iron. Then if a total flux of 01-3 J? lec.Enffi*etr FIG. 50. Electromagnet of wrought iron, aero-ferric circuit. Air gaps % in. = 1.27 cm. Mean length of magnetic circuit 140.24 cms. Cross-section of magnetic circuit 25 sq. cms. 300 kilowebers is to be sent through the circuit, it is required to find the excitation necessary for the magnet to produce this flux. Here, as before, we have to find the total reluctance of the circuit. Taking first the air reluctance, each air-gap, #, , has a length of half an inch = 1.27 cms.; and a cross-section of 3.875 square inches = 25 square cms. The reluctance of each air- gap is, therefore, J^f-? X 1 = 0.0508 oersted, since the reluctivity of air is unity. Since they are placed in series, the reluctance of both air gaps is, therefore, 0.1016 oersted. 100 If the cross-section of the cores, yoke and keeper is uniformly 25 square cms., the flux density will be uni- formly $- = 12 kilogausses. Referring to Fig. 41, the reluctance of wrought iron at this density may be taken as approximately 1.316 millioersteds in a cubic centimetre. With this reluctivity the reluctance of the iron in the circuit will be i||^ X 1.316 = 7.248 millioersteds, or 0.007248 oersted, since the mean path in the iron has a length of 137.7 cms. Adding the reluctance of the air, FIG. 51. Electric Circuit Analogue of JEro- Ferric Circuit in Fig. 50. Without Leakage. we have, for the total reluctance in the circuit, 0.108848 oersted. Consequently, the M. M. F. required will be 300,000 X 0.108848 = 32,654 gilberts = 25,980 am- pere turns, and, should the number of turns on both coils together be 5,000, the required exciting current is 5.196 amperes. The electric circuit corresponding to this case is shown in Fig. 51. Here two equal E. M. F.'S in series, each of 110 16,277 volts, act on a circuit of 0.108848 ohm resistance. The drop of magnetic potential in each air-gap is 15,240 gilberts, while the drop of potential in each core is 453.7 gilberts. Owing to the fact that the air round the magnet is not a magnetic insulator, the preceding calculation cannot be regarded as strictly correct, since we have left all ex- ternal, or leakage flux out of consideration. It is evident that with ferric circuits, in which the flux density is not excessive, that is say, in which the reluctivity of the cir- cuit is far less than that of the external air, the leakage will be small, even though the arrangement of the cir- cuit differs materially from the anchor ring type with uniform winding. As the air-gaps in a circuit become wider and more numerous, the leakage flux bears a larger proportion to the total, and the circuit becomes less amenable to simple numerical treatment, owing to the complexity of the various branch circuits, and the difficulty of computing their local reluctances. This condition renders the calculation of practical magnetic circuits much more tedious and difficult than that of ordinary electric circuits. Most dynamo or motor mag- netic circuits can, however, be computed with a degree of approximation sufficient for practical purposes in design. The following case illustrates the method of procedure. 117. Case 3. Let the magnet, shown in Fig. 50. possess a leakage flux of 20 per cent, through the path 5, 6, 7, 8. That is to say, for every 100 webers passing through the cores and yoke, 20 pass through the air between the poles, and only 80 pass through the keeper. Eequired the M. M. F. to send 300 kilowebers through the keeper, as before. Ill The flux through the cores and yoke will now be 375 kilowebers, at a mean density of ^-f- =15 kilo- gausses. By reference to Fig. 41, the reluctance of a cubic centimetre of wrought iron for this density is 3.077 milli- oersteds. Of the 25 square cms. of cross-section in the cores and yoke, only 20 can now be considered as carrying the keeper flux, the remainingfive square cms. being allot- ted to the leakage flux. Since the mean length of circuit 15.240 Volts 0.0608 Ohm 300,000 Amp.-, FIG. 52. Electric Circuit Analogue of ^Ero-Ferric Circuit in Fig. 50. With Leakage. through cores and yoke is 98.85 cms., their reluctance in the main or keeper circuit is &-^$-8- X 3.077 = 15.21 milli- oersteds = >. . . . 0.01521 oersted. The reluctivity of the keeper remains at 1.316 millioersteds, as its flux den- sity is 12 kilogausses. The keeper reluctance is, therefore, 3 ||3 X 1.316 = 2.045 millioersteds = 0.002045 " The two air-gap reluctances remain as before at . . . . 0.1016 " So that total reluctance in keeper, etc. = 0.118855 oersted. 112 The M. M. F. necessary to force 300 kilowebers through this circuit is 300,000 X 0.118855 = 35,656.5 gilberts = 28,370 ampere-turns, or 14,185 ampere-turns to each spool, requiring a current strength of 5.674 amperes. The corresponding electric circuit is shown in Fig. 52. It will be observed that while the drop of pressure in the air-gaps is 15,240 gilberts, as before, the drop in the cores and yoke has been increased by the introduction of leakage from 1560.9 gilberts to 4562 gilberts. The reluctance of the leakage path between the poles is observed to be 0.4146 oersted. It is evident, there- fore, that, whenever the reluctance of leakage paths can be computed, the distribution and amount of leakage flux can be determined. SYLLABUS. In any magnetic circuit the webers = the gilberts divided by the oersteds. Corresponding to Ohm's law in the electric circuit, the amperes the volts divided by the ohms Given, any two of the three quantities in either oi' the above formulae, the value of the other quantities- may be calculated. The value of the magnetic flux in any circuit may be increased either by increasing the M. M. F. or by decreas- ing the reluctance. The M. M. F. may be increased by increasing the number of ampere-turns in the magnetiz- ing circuit. In the design of electromagnets, the reluctance can be varied either by varying the dimensions of the iron cir- cuit, or by varying the character of the iron employed. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, by THE ELECTRICAL ENGINEER.] WEEKLY. No. 15. SEPTEMBER 22, 1894. Electrical Engineering Leaflets, BY Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE GRADE. ELKCTROMAQNBTS, 118. An electromagnet is a magnet produced by the passage of an electric current through a coil of r o D wire linked with a magnetic circuit. The name electro- magnet is practically limited, however, to cases where the core, placed inside the helix, is made of soft iron. Under these circumstances the core acquires the proper- ties of an electromagnet, and, disregarding residual mag- netism, loses these properties when the current ceases. 119. When a bar of hard or soft iron is brought into a magnetic flux, an alignment of its molecules, or ultimate particles, is supposed to take place. This align- ment is more readily produced in soft iron than in hard- ened iron, as, indeed, would be supposed, bearing in mind, the characteristic property of hard iron which opposes any deformation or change of shape. When the prime M. M. r. ceases to act on the iron, as would occur either by withdrawing the iron core from the prime flux, or by causing the magnetizing current to cease, the freedom of Published by THE ELECTRICAL ENGI 203 Broadway, New Vork N [Entered as second-class matter at the New York, N. Y., 114 movement naturally possessed by the molecules of soft iron, permit them readily to lose their new alignment, and the structural M. M. F. is dissipated with the resump- tion of practically its previous condition. In the case of hardened steel, however, the resistance which the mole- cules offer to change of position, enables the structural M. M. F. to be largely retained. For this reason the mag- netism produced in soft iron is sometimes called tempor- ary magnetism, and that in hard steel, permanent magnetism. IV V VI FIG. 53. Indicating the direction of magnetization in an iron bar as dependent on the direction of winding and of current. 120. When an electric current passes through a wire, an M. M. F. is established around the wire. This M. M. F. produces a distribution of flux in cylinders con- centric to the wire, the intensity diminishing directly with the distance from the axis of the wire. The di- rection of this flux, relative to the direction of the current in the wire, being as shown in Fig. 53. When the wire is bent into a circular loop, it is evident that the M. M. F. produced by the loop is directed either all upwards or all downwards through the loop, so that the direction of the flux depends on the direction of the current. The 115 M. M. F. from a helix, which lias a succession of turns, is also directed through the helix in one direction or the other, both according to the direction of the current and to the direction of the winding. 121. Magnets may be divided into different classes according to the character of the work they are called upon to perform ; namely, (1.) Tractive magnets ; and, (2.) Portative magnets. A tractive magnet is one designed to exert a pull at a distance. A portative magnet is one designed to hold or support heavy weights attached to its armature, when the latter is at rest upon the poles. 122. An electromagnet is designed to produce a cer- tain traction or pull on its keeper. This pull may be exerted either when the keeper is at a distance ; that is, separated by an air-gap ; or, when the keeper is actually brought into contact with the polar surfaces. In most practical cases, however, the attractive force is brought to bear between two parallel surfaces, usually called the polcvr surfaces across which the flux passes perpendicularly, as shown in Fig. 54. Under these cir- cumstances, every square centimetre of opposed polar (B 2 surfaces attracts the other with a force of dynes, where (E is expressed in gausses, so that if the intensity is everywhere the same across the surfaces A, B, c and D, E, F, the total force of attraction between the surfaces (& 2 will be 8 X - - dynes. S being the area of polar sur- 8 7T face in square cms. 116 123. If the flax does not possess the same value at different portions of the polar surfaces, then the active surface must be divided into a sufficient number of elements to permit the flux density to be considered uniform over each element, when the separate forces of each element can be determined, and their sum will be the total force on the whole surface. So far as the at- tractive power of a magnet is concerned, the total value of the flux is of secondary importance ; it is the distri- FIG. 54. Flux normal to opposed plane parallel polar surfaces. bution of the intensity of the flux at the active surfaces, in gausses, which is of primary importance. In soft Norway iron, the flux intensity can hardly be maintained above 19 kilogausses. The attraction between opposed surfaces of one square cm. in area traversed perpendi- cularly by 19 kilowebers, will be, therefore, 19,000 X 19,000 = d 25.133 and, since one dyne equals 1.0203 milligrammes weight, 11 at Washington this force represents 14,658 grammes, or 32.31 pounds weight, at "Washington, per square centi- metre of active polar surface (208.4 pounds per square inch). 124. In Fig. 40 on page 94, a sextipolar dynamo is represented in cross-section. Since flux passes into, or out of, the armature beneath each polar surface, each magnet core may be said to attract the armature, with a force that can be readily computed, to at least a fair degree of approximation, when the elements of the magnetic circuit are known. For example, suppose that in each of the six magnetic circuits shown, the useful flux, i.e., the flux passing through the armature core, is two megawebers. Then four megawebers will enter or leave the armature under each pole-piece. If the surface of each pole-piece is 200 square inches, i.e., 1,293 square cms., the mean intensity in the air-gap and polar surfaces will be 4 > QQO > QQO = 3,094 gausses. 1,293 Assuming that this intensity is Uniform over the sur- faces, the tractive force, per square centimetre, exerted between pole and armature will be 3,094 X3,094_ 38( ^ 90() dvnes=388>5 gramme s weight, .25. loo and, since there are 1,293 square cms. opposed and active, the total force will be 388.5 X 1,293 = 502,300 gms. weight = 1,108 Ibs. weight. As the armature revolves, therefore, its iron core will be pulled upon outwards opposite each pole, with a force of about half a ton's weight, and the framework sup- porting the armature must be sufficiently strong to safely 118 support these forces, in addition to the ordinary centri- fugal forces of rotation. 125. When a current passes through the armature, whether it be acting as a generator or motor, the effect of the M. M. F., set up by the current in the arma- ture windings, is to superpose its flux upon that previously established by the field magnet M. M. F.'S. The combina- tion of the two magnetic circuits is to destroy the sym- metry of the flux distribution. For example, if the machine is receiving current as a motor, the effect of introducing the armature M. M. F.'S will be to so modify the flux distribution, shown in Fig. 40, as to increase the intensity in the air-gaps underneath all the left-hand edges of the polepieces (as each pole is regarded from the armature), and reduce the intensity at the opposite or right-hand edges. At the same time, the flux will be deflected from the perpendicular, and drawn through the air more or less obliquely. Under these circum- stances tangential pulls will be exerted upon the arma- ture, and each square centimetre on the left-hand edge will exert an increased pull in proportion to the square of the intensity, while the right-hand polar edge will exert a pull which is relaxed in corresponding measure. The resultant, or preponderating forces on the left-hand polar edges, will draw the armature round clockwise. This effect of the M. M. F. in the armature is called armature reaction. It is by reason of armature reaction that a motor pulls, and that a generator has to be pulled, while the pull is in all cases a distribution of dynes per square cm. over the opposed polar surfaces, under distortion from 10 the original symmetry of flux distribution. The funda- mental law of tractive force in the electromagnet is con- sequently the fundamental law of rotary force in all electric dynamos and motors. 126. The portative force, which a magnet can exert, may readily reach 200 Ibs. weight per square inch of active polar surface when the poles are of soft iron. It should, therefore, be the object, when designing a ferric magnetic circuit for simply portative purposes, to magneti- cally saturate the polar surfaces as nearly as possible, and not allow the iron to become equally saturated at any other part of the circuit. For this reason it is usual to con- strict the section of the iron at the poles. The length of the circuit is then reduced as far as possible, so as to only allow just room for the exciting coil. In this way a very small electro-magnet weighing a decigramme can be made to support 2,500 times its own weight, a magnet weighing 100 grammes, 600 times its own weight, and a multipolar magnet, weighing a ton, should be able to support about 500 times its own weight. When a tractive magnet has to exert a definite pull, under a given M.M. F. upon its armature, across an air-gap, the design- of the magnetic circuit has to be altered. It is found that the best area of polar surface to employ is that which makes the reluctance of the air equal to the reluctance of the iron. This rule ensures the best in- tensity in the polar surfaces assuming no leakage to exist. The influence of leakage calls for a reduction in the air reluctance. 127. When a tractive magnet has to alternately attract and release its armature in rapid succes- sion, as, for example, in the case of a telegraph relay, the 120 armature lias to be made very light, in order that its in- ertia may not unduly increase the magnetic forces to be exerted. An ordinary Western Union, neutral relay of 140 ohms resistance, has about 7,500 turns of wire, and, when excited by a steady current of 25 milliamperes, i.e., to a M. M. F. of 1ST. 5 ampere-turns, or 235.8 gilberts, exerts a total pull upon the face of its armature amounting to about 78 grammes weight when the distance between poles and armature is 0.1 cm. The reluctance of its circuit is then about 0.25 oersted, and the flux, con- sequently, about 943 webers. SYLLABUS. When a bar of soft iron is submitted to the permeat- ing action of a magnetic flux, its ultimate particles be- come aligned, and a structural M. M. F. is established in the bar. On the withdrawal of the permeating mag- netic flux, the structural M. M. F. disappears, through in- instability. Hard iron or steel retains the structural M. M. F. in a greater or less degree. The tractive force exerted between opposed plane /n2 parallel polar faces is - - dynes per square cm. of either, 8 7T or 0.2567 & 2 dynes per square inch, or 5.771&X 10~ 7 Ibs. per square inch, (B being expressed in gausses. The electromagnetic rotary pull of a dynamo or motor is due to tractive forces set up between the armature and (& 2 field poles represented locally by - - dynes per square 8 7T cm. from point to point. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, by THE ELECTRICAL ENGINEER.] WEEKLY. No. 16. SEPTEMBER 29, 1894. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE GRADE INDUCED EX JVL 128. Whenever a conductor moves across a magnetic flux, or a magnetic flux moves across a conductor, an E. M. F. is generated in the conductor ; or, generally, whenever relative motion exists between a conductor and magnetic flux whereby either crosses the other, an E. M. F. is generated in the conductor. The amount of this E. M. F. is dependent on the rate of- cutting of flux, and will evidently vary both with the rapidity of mo- tion of either the flux or the conductor and with the intensity of 'the flux. 129. The following varieties of induced E. M. F. come under the above general head ; namely, (a.) Dynamo-electric induction, where a conductor is moved across magnetic flux. (b.) Magneto-electric induction, where magnetic flux is moved across a conductor by the motion of a magnet. (n direction oblique to flux. to flux. to flux. general case. Fig. 55 shows the first case, where a straight wire A B, of length I cms. moves through a uni- form flux with a velocity of v cms. per second, which would (-firry it in oru- second to the position A' u'. Tin-. flux cutthrough in this time would be that panning through the rectangle A B B' A'. Here the total flux cut will be the area of this rectangle in sq. cmn. multiplied l>\ the intensity in gausses, and will vary with three <|n;intiti-.- : viz., the length of the wire, the velocity of the mot inn. and the intensity of the flux. 123 131. In the second Ctte, as shown in Fig. 50, where a conductor, oblique to the flux, is moved in a direction at right angles to it, the j;. M. i -. generated will depend on the amount of flux cut per second, and this will be equal to the flux passing through the rectangle A It t/ A', where the side A >, is the virtual length of A B, that is, its length projected at right angles to the flux, and the projected length will evidently be smaller, the greater the angle /9, or the greater the obliquity to the flux. In the third case, shown in Fig. 57, both conductor and motion are oblique to the flux, and the E. M. F. in the conductor is proportional to the amount of flux con- tained in the rectangle A b V of, where A J, is the virtual or projected length of the conductor, and A a', its virtual velocity. The direction of the E. M. F. produced by the movement of a conductor across magnetic flux is, perhaps, most readily determined by Fleming's hand rule, in which, if the right hand be held as shown in Fig. 58, then if a conductor be moved in the direction in which the thumb points, and at right angtes to the flux in the direction pointed out by the fore-finger, the E. M. F. generated will flow in the direction pointed out by the middle finger. For example, if a straight wire 10 cms. long, make an angle of 30 with the flux, whose intensity is 500 gausses, and if it moves at a rate of 30 cms. per second, in a direction making an angle of 30 with the flux paths, the virtual length of the wire considered as lying across the flux would be 0.5 X 10 = 5 cms., and the virtual velocity of cutting the flux would be 0.5 X 30 = 15 cms. per second, so that the E. M. F. induced in the wire would be 5 X 15 X 500 = 37,500 c. o. s. units, = 375 micro- 124 volts. This E. M. F. would be produced during the motion of the wire, but would cease the moment the wire came to rest. 132. In order that an induced E. M. F. may set up a current in a conductor, the circuit of that con- ductor must be closed ; i. e., it must form a conduct- ing loop. If portions of this loop are cutting across mag- FIG. 58. netic flux, and thereby generating E. M. F. around the wire, the loop must either be enclosing more, or less, flux. If, therefore, in a conducting loop all the elementary por- tions be cutting through flux at an aggregate rate of 100 millions of webers per second, the flux added to, or sub- tracted from, the loop, will be 100 million webers per second, and the E. M. F. around the loop will be one volt. 125 In practice, when a dynamo armature is revolving in the magnetic flux established by its field magnets, the loops of conductors on the armature are having flux poured into them and then poured out of them successively ; that is, are having E. M. F.'S induced in them in one direction as the flux is pouring in, and in the opposite direction as the flux is pouring out. For example, if the flux through a dynamo armature be 100 megawebers, and the revolving armature be so constructed that it poured all this flux through a loop upon the armature surface at a uniform rate during one-tenth of a second, the rate of pouring flux into the loop during that period, would be 1,000 megawebers per second, and the E. M. F. existing in the loop during the same period would be 10 volts. If then, during revolution, the armature con- tinued emptying the loop at the same rate in the- next tenth of a second, the E. M. F. in the loop during that period would be still 10 volts, but in the reverse direction. 133. All forms of dynamo electric machinery, that is, all forms of machinery for the generation or modi- fication of E. M. F., are devices whereby magnetic flux is poured into and emptied out of conducting loops. The underlying principles are always the same, although the methods adopted are very varied in detail. 134. It is important to point out that the magnitude of the E. M. F. induced in a conducting loop does not depend upon the total flux which may be poured into the loop, but upon the rate at which the entry and exit are made. A large total flux, entering slowly into a loop, may produce less E. M. F. in magnitude than a small total flux entering rapidly. For this, reason, the E. M. F. 126 generated by a dynamo increases with an increase in the speed of revolution of its armature. 135. A resultant E. M. F. is never produced in a con- ducting loop, by its passage through flux, unless the amount of flux entering the loop is different from that leaving it. Thus, if the conducting ring A B c, in Fig. 59, with its plane at right angles to the uniform flux, be moved at right angles to the flux, then it will have no resultant E. M. F. generated, since the flux it en- Elec.Engineer FIG. 59. FIG. 60. Conducting ring normal to flux, mov- Rotation of a loop in a magnetic flux, ing in plane normal to uniform flux. closes at any instant is always the same. Or> since the amount of flux cut by the advancing edge and there- fore entering the loop, is equal to the amount of flux cut by the following edge and therefore leaving the loop, the equal and opposite E. M. F. generated in these two halves of the ring exactly neutralize. 136. If, however, a conducting loop be rotated in a magnetic field, a resultant E. M. F. will be gene- rated in it. If, for example, the loop A B c, be rotated 127 round the axis H K, Fig. 60, it will, in different positions, have an amount of flux poured into it differing from that poured out from it. If at any instant, the rate at which flux is being emptied or introduced were con- tinued unchanged for one second, the amount of flux (webers) leaving or entering in that time, would be the resultant E. M. F., in c. G. s. units, round the loop at that instant. Fig. 61 shows a device whereby an E. M. F. can FIG. 61. E. M. F. Produced by Rotation of Coil in Earth's Flux. be obtained from the earth's magnetic flux, by revolving a coil of many turns, in a supporting frame. 137. When the circuit of a voltaic cell is closed through a long coil of insulated wire, an electro- motive force is developed in the wire by the flux linked with the magnetic circuit established under the M. M. F. of the active conductor, and this induced E. M. F. has a direction opposed to the E. M. F. of the battery. When, however, the circuit of the battery is broken, owing to 128 the disappearance of the flux from the loops of the cir- cuit, which has the same effect as the linking of flux with the loop in the opposite direction, an induced E. M. F. is produced in the wire, whose direction is the same as that of the E. M. F. of the battery. These phenomena are called the phenomena of self-induction. The ten- dency of the E. M. F. so produced is to oppose the change of current which sets it up. 138. When two conducting circuits are placed in each other's vicinity, and an electric current is established in one of them, during the time the current is increasing in strength, the flux it produces links with the neighboring circuit and develops in it an E. M. F. which is called an E. M. F. of mutual induction. SYLLABUS. "When a relative motion takes place between a con- ducting circuit and a magnetic flux, an E. M. F. is pro- duced in the circuit, varying in direction with the direction of the motion. The E. M. F. so developed is called in- duced E. M. F. and is equal, in c. G. s. units, to the total amount of flux that is, or would be, cut in one second of time. The E. M. F. of induction may, therefore, be in- creased by increasing the velocity of the movement or the intensity of the flux. If the total flux linked with a circuit, including all the loops it may have, be expressed in webers, the E. M. F. in volts, induced in that circuit, at any moment, will be the rate at which the flux is changing, divided by 100,000,000. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, by THE ELECTRICAL ENGINRKR,") WEEKLY. No. 17. OCTOBER 6, 1894. Electrical Engineering Leaflets, BY Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE GRADE DYNAMO. 139. A dynamo-electric machine is a device for pro- ducing E. M. F. by successively filling and empty- ing loops of wire with magnetic flux. A dynamo-electric machine consists essentially of two parts ; namely, (1) That called the field, which produces the magnetic flux; and (2) That called the armature, which bears the loops which are successively filled and emptied with flux. Numerous machine designs have been produced by which these objects can be accomplished, thus giving rise to numerous classes of dynamo-electric machines. The function of the field magnets is to provide the magnetic flux in its magnetic circuit, and the loops of wire on the armature are either carried through this flux, or the flux carried through them, so that they are suc- cessively filled and emptied. The rate at which each loop is filled and emptied determines the jalue of the E. M. F. generated in. it, and the number of such loops Published by THE ELECTRICAL ENGINEER, 203 Broadway, New Vork N. Y. [Entered as second-class matter at the New York, N. Y., Post ok*, J^ei* jf8g| ,' f 1 A. 130 with their grouping or connection, the total amount of E. M. F. generated by the machine. 140. In the United States, continuous current dyna- mos have their armatures either of the drum- wound or ring- wound type. An example of the drum- wound type of armature is represented in cross-section at Fig. 39, page 93, and in perspective in Fig. 62. Let us consider two loops, c D E F, G H j K, Fig. 03 (a\ in a drum-wound armature, at right angles to eack other, and without being connected to a commutator. These FIG. 62. Drum Armature. loops are supposed to be supported on the axis A B, in a bipolar flux represented as uniform by the arrows. It will be seen from an inspection of the figure, that the two loops being at right angles, one is filled with flux, and the other is completely empty. It will be seen from Fig. 63 (5), that, considering the armature to be revolving at a speed of 10 revolutions per second, then in the ^J^th part of a second, the loop c D E F will be advanced to the position c' D' E' F', represented by the dotted line, and the amount of flux then emptied out of it will be that included in the two narrow parallelograms c c'f'fsmdd d' e' e, which are included between the pro- jections of the sides of the loop in the two positions. If 131 the density of the flux be three kilogausses and the areaof the two parallelograms 34 square cms., the flux emptied out in this yJrj-th part of a second will be 3,000 X 34 = 102 kilowebers, representing a rate of filling of - 240 24,480,000 webers per second 0.2448 volts generated in the loop. On the contrary, the horizontal loop G H j K, repre- sented separately in Fig. 63 (c\ will have advanced dur- ing the same -^th of a second, to the position repre- sented by the dotted line G' H' j' K', and will only have a. i FIG. 63. introduced into it the flux included in the parallelogram g' h' j' k'. With the same density of flux the area of this parallelogram will be 259 square cms., making a total loss of flux in the ^^th of a second = 777,000 webers, and the rate of emptying 777,000 X 240 webers per second = 186,480,000, an E. M. F. in the direction shown by the arrows of 1.8648 volts. It is easy to see that if the interval of time had been taken sufficiently small, the E. M. F. in the horizontal loop would amount to 1.885 volts, while in the vertical loop it would be zero. For loops that may lie upon the surface of the armature, between the vertical and horizontal positions, the value of the E. M. F. generated will be intermediate between zero and 1.885. 132 The rule for determining the direction of E. M. F. induced in a loop, is as follows. When a watch is held in front of an observer, the light by which he sees the dial, passes directly from the dial to his eye. If now the watch-face be regarded as a loop, then when flux is poured through it in the same direction as the light (i.e., entering at the back and passing towards the observer), the E. M. F. around the loop will be in the same direction as the motion of the hands of the watch. If, on the contrary, the motion of the flux be against the motion of the light the direction of the E. M. F. will be against the direction of the hands of the watch. Decreasing, or pouring-out flux, produces an E. M. F. in the opposite direction to increasing or pouring-in flux. Thus, if the loop c D E F, Fig. 63 (a\ be rotated about its axis A B, in the flux indicated by the arrows, into the position represented by the dotted lines, it will be pouring flux out, equivalent to having flux poured into it from the opposite side, and the E. M. F. induced around the loop, during the passage shown, will be in the direction of the arrows. The application of this rule to the loop, shown in Fig. 63 (c\ will show that the rotation of the loop, in the direction indicated by the arrow, will produce an E. M. F. which will be directed during the motion of the loop from G to H, and from j to K. 142. A diagram representing a ring-wound armature, commonly called a Gramme-ring armature, from the name of the French electrician, Gramme, who first employed it in practice, is shown in Fig. 64 (a). The flux is supposed to enter the ring on the side B c D E F, and to leave it at the side M L K j H. It will be observed that the flux fills the loops at the 133 top and bottom of the ring, while the loops on the hori- zontal line are empty ; the intermediate loops, having intermediate quantities of flux passing through them. As before, the E. M. F. in the horizontal loops will be a maximum, since, at this position, the rate of filling is a maximum, and the loops on the vertical line have no E. M. F. in them. 143. Fig. 64 (5), shows the connection of the loops on the Gramme ring in a single series to form a com- plete circuit. When the ring is set in rotation, the FIG. 64. E. M. F. in the various turns unite on one side of the vertical line to produce a total E. M. F. which is equal and opposite to that produced by the loops connected in series on the other side of the vertical line, so that, pro- vided the winding be uniform, and riot dissymmetrical, no current will be produced in the ring, the two oppos- ing E. M. F.'S balancing, for otherwise wasteful currents would probably be set up in the armature. If two brushes, z, z', were placed at points on the vertical line, so as to maintain contact with the armature wires during its rotation as they come round, then the E. M. F.'S gene- rated in the opposite sides of the armature would tend 134 to pour a continuous current through the circuit con- nected with the brushes. In practice, since the armature is wound with insulated wire, often laid on in several layers, it is found convenient to carry out connections at regular intervals to insulated conducting segments of a device called a commutator. The brushes are rested on the surface of the commutator at the proper points. The voltaic analogue of the E. M. F.'S in the armature are shown in Fig. 64 (c). 144. The amount of E. M. F. produced in any case may be determined by the following rule. Mul- tiply the total flux in webers, passing through each pole into the armature, by the number of revolutions of the armature per second, and by the number of wires counted once round the surface of the armature. The quotient divided by 100,000,000 will give the volts. The total flux may be determined when the total reluctance of the magnetic circuit, or circuits and the M. M. F. of the field magnets are known. 145. The output of a dynamo machine is most con- veniently given in kilowatts, and is found by multiplying the pressure in volts which the machine sustains at its terminals, by the current in amperes it maintains at full load. Thus, a railway generator producing a current of 952.4 amperes at 525 volts ter- minal pressure, will develop in the external circuit an activity of 952.4 X 525 = 500,000 watts, and the ma- chine will be a 500 K. w. machine. In practice a certain relation always exists between the output of a machine, its E. M. F., and internal resistance. It is evident that if the resistance of the machine exceeds a certain value, the current passing through that resistance at a continuous K. M. F. and output, would produce an excessive and dan- gerous amount of heat in the machine. Indeed, in prac- tice, the only electrical difference existing between a 3,000 K. w. machine, say, of 500 volts, and a one K. w. machine, at the same pressure, lies in the resistance of its armature. 146. Were it practicable to operate a dynamo on short circuit, the maximum possible activity would T? TT* be obtained, and would be equal to J?/ = J^X = r r watts, where E, is the E. M. F. of the machine in volte, and /*, its internal resistance in ohms. This theoretical maximum output may be called the electrical capability of the machine, and, in practice, a certain fraction of this electrical capability may be taken as the output. This fraction varies with the character and size of the machine, from 0.1 in small machines to, say, 0.02 in machines of 200 K, w. 147. The electrical capability of a machine is not altered by the size of the wire employed in the winding, provided the volume of the winding space on the armature be maintained constant, and that the pro- portion of space allotted to insulation remains constant. Thus, if the diameter of the wire employed be halved, there will be room for four times as many wires, and the total resistance of the armature will be increased 16 times, since the length has been quadrupled, but the cross-section of each turn reduced to one-fourth. The ratio of E* to r, will be ^| ; 'i.e. 9 will remain the same. This is equivalent to the statement, that, provided the 136 insulation space of the armature remains constant, the output of the machine remains the same at any voltage, so that if the same machine be wound for 30 or 60 or 100 volts, its output will remain unchanged. SYLLABUS. When the flux is being poured into a loop in the same direction as the light passing from a watch face towards the observer, the direction of the induced E. M. F. is the same as the motion of the hands of a watch. The electrical capability of a machine is equal to square of its E. M. F. in volts, divided by its resistance in ohms. The electrical capability bears a ratio to the out- put varying according to the type and size of the ma- chine. This ratio is within wide limits independent of the E. M. F. for which the machine is wound. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, by THE ELECTRICAL ENGINEER.") WEEKLY. No 1 8 OPTOT* 131 8Q4- Price " 10 Cents - Ld, 1 te. Subscription, $3.00. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE CF?ADE DYNAMO. 148. By the electrical efficiency of a dynamo is meant the ratio between the electrical activity in the external circuit of the machine, and the total elec- trical activity it produces both in its internal and exter- nal circuits. Thus, if a dynamo develops an activity of 100 kilowatts in its external circuit (say, 1,000 amperes at 100 volts, as measured at its terminals), and expends, electrically, four kilowatts in its armature and field mag. nets, i.e., internally, then the total electrical activity in the circuit will be 104 K. w., and the electrical efficiency of the machine will be expressed by External Activity __ 100 Internal Activity + External Activity 104 The commercial efficiency of the machine differs from this, and is the ratio existing between the output of the machine and its intake. Thus, if the same machine ex- pended, besides the four KW. in its field and armature, say, five KW. in mechanical friction and other losses, Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York N. Y. [Entered as second-class matter at the New York, N. Y., Post Office, June 14, 1894.] 138 then the total activity expended in the machine will be nine KW., and its commercial efficiency will be External Activity or Output _ 100 _ ~ Q-^A Intake ~ 109 ~ 149. In any generator the following losses prevent the output from being equal to the intake ; namely. (1.) Mechanical losses, including friction of all kinds. (2.) Electrical losses. These are of two kinds ; that in the conducting circuit on the armature due to the passage of an outgoing current through the resistance of the armature, and that due to small local or eddy currents in the substance of the wire on the armature, or in the iron of the armature core and pole-pieces. (3.) Magnetic losses, or those due to the reversal of the magnetism in the iron. 150. The mechanical losses may be classed as follows, viz., air friction, brush friction and journal fric- tion. If the armature resistance of a 100 KW. dynamo be 0.005 ohm, and the current it delivers 800 amperes, the activity expended in heating the armature wire will be #r= 800 X 800 X 0.005 = 3,200 watts ; i.e., 3,200 joules per second. The loss in the armature winding of a generator of one KW. capacity is often 12 per cent, of the output, while in the armature winding of a 200 KW. generator it is usually about two per cent, of the output. Similarly, . if the field magnets require a current of eight amperes to excite them and are supplied direct from the brushes, the energy expended in heating their 139 circuit will be E I = 125 X 8 = 1,000 watts. This amount will vary considerably with the type and size of machine, say, from 10 per cent, of the output in a 2 KW. generator to 1.5 per cent, of the output in a 200 KW. generator. These losses constitute the electrical losses in the circuits of the machine. The rapid reversals of magnetism to which the iron in the armature and in the pole-pieces is subjected, during the operation of the machine, set up E. M. F.'S in these masses which in their turn produce local, deleterious currents in the iron, called eddy currents. Although the E. M. F. producing these currents may be only a small fraction of a volt, yet the resistance of the mass of metal in which they are set up, being also very small, .the actual currents produced may be seriously large; hence it is necessary to check the establishment of these wasteful currents by limiting the mass of metal in which they can exist as a single circuit. This is accomplished, in practice, by laminating the iron in a direction parallel to the direction of the magnetic flux. It is not neces- sary elaborately to insulate the separate laminae or sheets of iron so employed, since the film of oxide always pre- sent on their surfaces is sufficient to prevent the feeble E. M. F.'S from crossing them. Where, however, great mechanical pressure is brought to bear upon such surfaces during construction, they are generally insulated, either by dipping them in shellac varnish, or by interposing sheets of tissue paper. 151. Similar eddy currents are also set up in the substance of the copper wire on the armature ; and, when such conductors are of large cross-section, it is necessary to subdivide that cross-section by transposing 140 and stranding the conductors; i.e., by dividing them into a number of separate conductors. If, however, the wire be wound in grooves on the armature core, as in the case of toothed armatures, or armatures having iron pro- jections, lamination of the conductors is rendered un- necessary, since the copper is practically insulated from the flux which links with it, and which passes almost en- tirely through the iron teeth. All electrical losses of the character of eddy currents belong to the P R type, and, since f, increases with the E. M. F., which in its turn increases with the speed, such losses increase as the square of the speed of rotation. If, therefore, the losses due to eddy currents in a given machine are 300 watts, at its normal speed, they would amount to 1,200 watts, if the speed were doubled. 152. The magnetic losses, which occur in the iron of the armature core, are due to what is called mag- netic hysteresis (his-ter-ee'-sis). The word hysteresis, de- rived from the Greek, means a lagging behind, thus characterizing the lagging of the magnetization in the iron, or other magnetic metal, behind the magnetizing flux. That is to say, when a magnetizing flux is brought to bear upon a piece of iron, the molecules of the iron become aligned, as already explained. On the with- drawal of the magnetizing flux, the bar does not in- stantly lose its magnetism ; i.e., its alignment, but tends to retain the same for a short time, and does not reach a condition of demagnetization until the flux has not only disappeared but has actually been reversed. In other words, the magnetic flux in the iron lags behind the magnetizing flux. 141 153. When a magnetic flux is produced in air around a conductor, energy is absorbed into the ether and air from the circuit ; but, on the cessation of the current, all this energy is returned to the circuit electrically. If, however, iron be magnetized by the current, energy will be absorbed from the circuit, both by the ether and by the iron, with this difference, that while the energy in the ether will be restored to the circuit as electrical en- ergy, on the withdrawal of the magnetizing flux, that in the iron will be only partially restored to the circuit, as electrical energy, the balance being expended in the iron as heat. Since the same amount of heat is produced at each cycle, at every reversal, if the iron be carried through 50 cycles (50 double reversals of magnetism per second), there will be 50 times as much energy expended in a cubic centimetre, or cubic inch, as if only a single cycle were made per second. As the limiting flux den- sity, through which the iron is carried in each cycle, is increased, the hysteretic loss increases in greater ratio, and a doubled range of flux density is accompanied by approximately a trebled loss of energy in hysteresis. Thus, if an iron armature be revolved in a bipolar mag- netic field 20 times a second, every cubic inch of iron will be magnetized from, say, a flux density of 5,000 gausses in one direction, to 5,000 gausses in the opposite direction, in 20 complete cycles or double reversals per second, and every cubic inch of such iron will have expended in it 0.0543 joules per second. If now the field magnets be excited by an increased current, so as to bring the flux density in the armature up to 10,000 gausses, the range of reversal will be doubled, and the hysteretic loss in every cubic inch will be practically 142 trebled ; i.e., increased to 0.165 joule per second, or an activity of loss of 0.165 watt. (GAUSSES) DENSITY FIG. 65. Hysteretic Diagrams of Charcoal Iron Rings and of Hard Cast Steel. Charcoal Iron -.-Full line to indicated scale. From observations of Kennelly. JC 6, (B 10,600. Hard Cast Steel : Broken line, to 10 times indicated scale. From observations of Steinmetz. 3C 82, (B 11,500. 143 154. Fig. 65 represents what is called a liysteretic diagram or cycle, and shows how the flux density in iron varies with the cyclic variation of the magnetiz- ing flux. Thus the prime intensity or magnetizing flux which will bring this sample of iron to an intensity of (B = 10,600 gausses is 6.1 gausses; carrying back the mag- netizing force, i. e., reversing it from 3C back to zero, the intensity in the iron does not return along the same path A B c, it took during ascension, but descends along the more slowly returning curve ODE, and, when the magnetizing flux reaches zero; i.e., when the magnetiz- ing flux is completely withdrawn, there is still a residual magnetic flux of (B = 8,900 gausses in the iron. In fact, the magnetic flux has to be carried back to z or 2.2 gaus- ses, in order to destroy the magnetic flux in the iron, i. 00 foot-pounds per minute = 10.8 KW., or 14.47 H. P. If now the flux could be increased tenfold, that is, to 40 megawebers, the same total torque could be obtained with 10 amperes. The armature drop being 0.5 volt, the c. E. M. F. 124.5 volts, the speed would be reduced to 93.4 revs, per minute. The work done at the pulley, with the same allowance for machine frictions, would be 1.12 KW. and the energy absorbed from the mains by the armature circuit 1.25 KW. In practice, however, the range which is under con- trol is a limited one ; the maximum limit of <^, being set by the rapidly increasing reluctivity of iron at den- sities approaching saturation, while the minimum limit is established by armature reaction, which seriously dis- 181 torts the weakened magnet flux, and may even over- power it, causing violent sparking at the brushes, and irregular operation. In shunt machines, the ratio of speed usually obtainable by variation of M. M. F. is about 25 per cent., while in series machines, it may, under favorable circumstances, amount to doubling the speed. 191. The case of variable torque and constant speed is continually met with in practice ; for example, when a lathe, drill, planer or other large machine to'ol, has to be driven at a constant speed by an electric motor under very variable loads. A shunt motor, connected with constant-potential mains, accommodates itself very closely to this requirement; for, neglecting the second- ary effects of armature reaction, the only diminution of speed between light and full loads is due to the drop in the armature resistance, representing a fall of speed of, say, 2 per cent, in a 100 KW. motor, and 5 per cent, in a 1 KW. motor. A series motor, however, is very far from complying with these requirements, since an increase in load automatically increases the M. M. F. of the field magnets, thus increasing both the flux through the arma- ture and the c. E. M. F., allowing the speed to diminish, with a further retardation due to drop in the resistance of the machine. Series motors, therefore, are not applic- able to cases where a constant speed is automatically re- quired under conditions of variable load. A compound-wound generator is, however, capable of being directly employed as a compound motor without any change in its electrical connections. The series winding here exerts a counter M. M. F., (abbreviated, c. M. M. F.), and thus diminishes the flux through the armature at full load, thus requiring an increase in 182 speed, to compensate for the drop in the armature re- sistance. Such machines are rarely needed, since the regulation in speed obtained by shunt machines is usually sufficient for practical requirements and, moreover, they are simpler to construct, 192. Atypical case of variable torque and variable speed is encountered in the electric street-car motor. For the speed has to be varied within wide limits under very varying conditions of torque, accord- ing to the number of passengers carried and the gradi- ent of the track. Here the same methods are adopted, as have already been alluded to in dealing with constant torque at variable speed ; that is to say, either resistance is inserted in circuit with the armature, or the M. M. F. of the h'eld magnets is varied, or, in some cases, both means of regulation are employed. While these afford sufficient regulation for street car motors, they fail to secure a perfect automatic control of speed under varied conditions of torque ; and, in this respect, the electric motor appears to least advantage. 193. In consequence of the reversibility of a gene- rator and motor, the same dynamo electric ma- chine can, as already observed, be employed in either capacity ; but its output as a generator, will, with con- stant excitation and speed, be always greater than its out- put as a motor. For, suppose a 50 KW. generator supplying a full-load current of 100 amperes at 500 volts terminal pressure. With a given excitation and speed, the frictional losses, magnetic, electric, and mechanical, will, perhaps, amount to 5 KW. These are supplied by the engine when the machine is acting as a generator, with an intake of 183 55 KW. at the shaft ; but, as a motor, at the same speed and excitation, the armature can only maintain a current strength of 100 amperes without overheating, while the frictions must now be supplied electrically so that the output of the machine will only be about 45 KW. INTAKE (WATTS) FIG. 83. Curves showing Expenditure of Power in a 750- Watt Shunt- Wound Motor operated trom constant potential mains. 194. Fig. 83 represents curves taken from the test of a particular 0.75 KW. shunt motor, wound for 500 volts. It will be seen that at full load, that is, at a de- livery of 0.75 KW. at the pulley, the intake is 1130 watts, representing a commercial efficiency of 66.4 per cent. 184 Of this 1130 watts, 90 are expended as i z /, in the shunt field, 70 as i* /, in the armature, 220 in mechanical, eddy, and hysteretic frictions, leaving the balance of 750 as output. SYLLABUS. The condition of constant torque and variable speed may be obtained by inserting resistance in the circuit of a shunt motor, or by commuting the fields of a series motor. The condition of variable torque and constant speed is very fairly met by shunt motors supplied from con- stant-potential mains. It can be still more closely met by compound-wound motors, but is not met by series motors. The condition of variable torque and variable speed cannot at present be automatically obtained in a single motor operated from constant potential mains. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, by THE ELECTRICAL ENGINEER.] WEEKLY. No. 24. NOVEMBER 24, 1894. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INXEf^JYIEDIATTE CRADE. ELKCTRIC MOTOR. (CONTINUOUS CURRENT TYPE.) 195. Motor armatures, like generator armatures, are either of the smooth-cored or toothed-cored jfcype. In a smooth-cored armature, the electrodjnamic force is largely exerted upon the substance of the wires, so that they are liable to be dislodged by momentarily powerful currents. In the toothed-cored armature, the wires, em- bedded in iron grooves, merely exert their M. M. F., and direct the flux through the surrounding iron, so that the electrodynamic force is entirely exerted, under the dis- tribution of , between the polar surfaces and the sur- 8 71 faces of the iron armature projections. (Sec. 125.) 196. Moreover, the eddy currents that are set up in the substance of the wires, when situated on the surface of a smooth-cored armature, in passing through the field flux, are avoided in toothed-cored armatures, where the flux is bodily guided, from side to side of the buried wires, through the mass of the iron. For these reasons, Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York, N. Y. [Entered asusecond-class matter at the New York }N. Y., Post Office, June 14, 1894.] 186 motors with toothed-cored armatures are rapidly displac- ing smooth-cored armatures. Especial care, has, however, to be taken in the design of toothed-cored armatures, in order to prevent excessive sparking, which is liable to be set up at the brushes, by reason of the increased induct- ance of coils nearly surrounded by iron. (Sec. 137, 160 and 162.) 197. Since the motor armature revolves under the influence of a distribution of flux between the poles and armature, whereby the attractive force is in- creased on one side and diminished on the other, (Sec. 125) the direction of M. M. F. in a motor armature must be such as will increase, by the flux it produces, the in- tensity at the polar edge which the armature approaches, i.e., the leading polar edge, and decrease the intensity at the polar edge which it leaves, i.e., the following or trailing polar edge. We have seen, however, that in a generator, the armature has to be moved by mechanical force, against an electrodynamic force ; and, consequently, the leading polar edge in a dynamo is weakened, while the trailing polar edge has its intensity strengthened by the armature reaction and M. M. F. The M. M. F. in a motor armature, is, therefore, opposed to the direction of M. M. F. in a generator armature, when the direction of rotation and the direction of field M. M. F. are the same- Tins is the key to all the relations existing between the direction of rotation of a machine when acting as a generator or as a motor. Thus, when the direction of current through the ma- chine, or the direction of E. M. F. at the terminals of the machine, remains the same, a shunt-wound motor will have the same direction of rotation as when employed 187 as a generator, while a series-wound machine will, on the contrary, have the opposite direction of rotation, as a MOTOR9 DIRECTION Of j DIRECTION OF TERMINAL CURRENT PRESERVED! TERMINAL E. M. r. PRESERVED MOTOR I I MOTOR 8EPARATELY|EXC. I SEPARATEl Y|EXC. GENERATORS Uc.Rngivttr- FIG. 84. Showing Relative Direction of Rotation in Generators and Motors. motor, that it has when driven as a generator. It also follows that in order to reverse the direction of rota- 188 tion of a motor it is only necessary to reverse the M. M. F. either of the field magnets, or of the armature, while, if both be reversed, the direction of rotation will remain unchanged. For this reason the mere reversal of the terminals of any motor will not alter its direction of rotation, unless the field magnets are separately excited. These conditions are exemplified in Fig. 84, where the uppermost row of motors are represented as sepa- rately excited, the middle row as shunt-wound, and the lowest row, as series-wound. The large straight arrows indicate the directions of the M. M. F.'S in fields and arma- tures, while the curved arrows indicate the direction of rotation. The direction of E. M. F. in the armature is also in the direction in which the letters are marked. 198. Motors, like generators, are capable of being operated in series. In practice, however, they require either to be mechanically coupled together, so that they are forced to maintain the same speed, or, their load must be so adjusted that their speed is automatically controlled. If this condition is not complied with, the motors are likely to race, and thus give rise to trouble- some irregularities of speed. 199. The advantages which an electric motor possess over other motors may be enumerated as follows. (1.) Facility of reversal of direction of rotation. (2.} Small size and weight per kilowatt (weight aver- age 100 to 180 Ibs. per kilowatt of output). (3.) Self governing power, or the capability of auto- matic control. (4.) A high efficiency. (5.) Rotary as opposed to reciprocating motion, with facility of operation and freedom from repairs. 180 (6.) Portability in small sizes, when connected with machine tools, so that a tool can be brought to the work, rather than the work to the tool. (7.) Cleanliness ; i.e., protection from dust, liquids, etc. (8.) Convenience and efficiency of distributing power to distances by means of insulated conductors. (9.) Facility with which the power can be metered to consumers and observed at any moment. 200. In reversing a motor, it is merely necessary, as we have seen, to reverse the M. M. F. either of the field magnets, or of the armature, and, in practice, it is usually the armature which is so reversed. It is neces- sary, however, to avoid making the reversal suddenly, unless resistance be temporarily inserted in the armature circuit, for the reason that the momentum of the arma- ture, carries it in its previous direction, and the E. M. F. of the armature under such conditions is no longer a c. E. M. F. to the circuit, but is a direct E. M. F. (contracted D. E. M. F.) tending to increase the current strength that will flow through the armature, when connected with the mains. Thus, suppose a 10 KW. 120- volt shunt-motor, with 100 amperes full-load intake, making 1,000 revolu- tions per minute, is connected to a system of mains, maintaining a constant pressure of 120 volts. On cutting off the current from the armature, whose resistance may be 0.05 ohm, the motor may take, say, sixty seconds to come to rest, depending upon the amount of load to which it is connected. If, however, while still running at 500 revolutions per minute, the armature be con- nected reversed to the mains, the E. M. F. of the armature will be 60 volts in the same direction as the E. M. F. now impressed from the mains, so that the cur- 190 rent strength which would pass through the armature according to Ohm's law would be ' ^ = 3,600 0.05 amperes, or 36 times greater than the normal, full-load intake. Of course the inductance of the armature would tend to set up a temporary c. E. M. F., independently of the rotation of the armature, tending to check this rush of current, but it is easy to see that before the momentum of the armature can be overcome, and its c. E. M. F. established by acceleration in the opposite direction, a dangerously strong current may pass through it. This shows that either resistance, or inductance, or both, should be inserted in the circuit of the armature of a motor when it has to be reversed. The same necessity for avoiding excessive rush of cur- rent exists, although to a smaller degree, in starting shunt motors from rest. A starting rheostat, therefore, has generally to be introduced, especially with large motors, in order slowly to accelerate the armature and develop its c. E. M. F. For this reason series motors can be more safely started from rest suddenly, owing to the resistance and inductance of the field magnet coils, which auto- matically check the first rush of current through the machine before the c. E. M. F. has had time to develop. 201. In electric locomotors, it is essential that the weight should be reduced as far as possible. The torque to be exerted by such a motor varies with the weight to be moved, the friction of the track, and the gradient. Fig. 85 represents a motor shaft M, geared directly to the car wheel w. A motor used in connection with such gearing is commonly called a single reduction motor. 191 If the pull, which would have to be exerted upon the car in a direction parallel with the track, be P, Ibs., due to gradient and friction combined, then the torque at the axle of the car TF, will be P f Ibs.-feet, where /*, is the radius of the car wheel (usually 1.25 feet in a street car). If j, be the gearing ratio of the motor and wheel, so that the motor makes J revolutions to each revolution of the car wheel, the torque at the motor shaft will be P f f~ Ibs.-ft. It is necessary, therefore, that the torque T = Elec. Engineer FIG. 85. Single Reduction Gear between Street Car Motor and Car Wheel. ( -^. cm. dynes, which the motor can exert with the maximum permissible current *', amperes, shall be P f equal to jL Ibs.-f t. for the maximum gradient and fric- J tion which the car has to overcome. Since the weight of the motor adds to JP, it is necessary to obtain the maximum amount of torque from the motor, with the smallest weight consistent with perfect mechanical se- curity, freedom from sparking, and other difficulties. 192 This is accomplished in practice, for street cars and railway motors, by employing toothed-cored armatures, carbon brushes, and cast steel multipolar Held magnet frames, so that the maximum flux is obtained with the minimum material. SYLLABUS. Smooth-cored armatures are mechanically weaker than toothed-cored armatures. The relative directions of M. M. F. in armature and field, for the same direction of rotation, are reversed in motors and generators. The leading pole edge has its flux density strengthened in a motor, and the trailing polar edge has its flux density strengthened in a generator by armature M. M. F. It is essential to introduce resistance or inductance into the armature circuit of a motor which is being re- versed or started from rest. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1804, by THK ELKCTRICAI. ENGINEER. WEEKLY. No 25 T)TrrTf\raTTK 1 18 98 D^n 99 1KQ4- Price ' 10 Cents< 4J, 1 W. Subscription, $3.00. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. IWTEWJYIEDIATE ARC 231. The carbon voltaic arc is produced by sending a sufficiently strong current through two carbon electrodes, which are at first in loose contact and are afterwards gradually separated to a short distance. When the current is sufficiently powerful, the space between the ends of the electrodes is filled with incandescent car- bon vapor, in the form of a luminous bow, which, from its shape, is called the voltaic arc. The carbon vapor is disengaged mainly from the end of the positive carbon, which soon thereby becomes hollowed out in the form of a minute crater. 232. To produce and maintain the voltaic arc, a cer- tain electric activity is necessary, depending upon the distance between the electrodes, their magnitude, position and incandescent surfaces. The average arc, as employed in the U. S. for commercial lighting, requires a current of about 10 amperes, and a total pressure at lamp terminals of about 45 volts, representing an activity of 450 watts. Of the total c. E. M. F. of 45 volts, from Published by THE ELECTRICAL ENGINEER, 303 Broadway, New York, N. Y, [Entered as second-class matter at the New York, N. Y., Post Office, June 14, 1894.] 218 38 to 40 volts are developed at the surface of the posi- tive electrode, about five volts are developed in the arc itself, and the remainder are developed in the resistance of the lamp mechanism. The activity at the surface of the positive carbon is, therefore, from 380 to 400 watts, and this is the principal source of radiant energy. 233. In an electrolytic cell, a c. E. M. F. is set up at the surface of the electrode, and the value of this c. E. M. F. is practically independent of the c. E. M. F. due to the resistance of the intervening liquid, so that work is expended in liberating the products of electrolysis. The development of c. E. M. F. in the arc lamp, is ana- logous to the development of c. E. M. F. by electrolysis ; and, in point of fact, the voltaic arc with its carbon elec- trodes, forms a species of electrolytic cell, the carbon vapor being analogous to the electrolyte. The energy is here expended in volatilizing carbon at an extremely high temperature, estimated at 3,500 C.; in fact the temperature, which can be attained by means of the electric arc, is probably greater than can be obtained in any other way. The c. E. M. F. of an arc lamp is practi- cally the same for the same distance between the carbon points for all dimensions of carbon electrodes, or areas of incandescence. But the larger the carbon, and the greater the surface of incandescence, the greater the cur- rent strength that must be supplied to it. For very large arcs, such as in powerful search-lights, a current strength of as much as 200 amperes, is sometimes em- ployed, requiring, therefore, an activity of about 10 K. w. 234. During the maintenance of the arc, the positive carbon, that is, the carbon from which the current enters the arc, attains at its crater, a much 219 higher temperature than that of the incandescent carbon. Indeed, the temperature of the negative carbon is suffici- ently lower to permit the condensation of carbon vapor on its surface, so that after the arc has been maintained for some time, a nipple will be formed on the end of the negative carbon opposite the crater in the positive. The material so deposited is pure graphite. During the maintenance of the arc, the carbon vapor being exposed to the air, is consumed by oxidation. The rate of con- sumption of the positive carbon, however, is greater than that of the negative, owing to the fact that it is vola- tilized. Roughly speaking, the rate of consumption of the positive carbon is twice that of the negative. 235. In the early history of arc lighting, the carbon electrodes employed were sawn out of blocks of the hard deposits of carbon found inside the gas" re- torts, employed in the manufacture of illuminating gas by the destructive distillation of coal. The enormous demand for carbon electrodes, however, soon rendered this source insufficient, and now, all carbon electrodes are manufactured. The process is essentially as follows : Pulverized carbonaceous substances, such as powdered coke or charcoal, are mixed into a stiff paste with some carbonaceous liquid, such as coal-tar, and are then moulded or forced through a die under great hydraulic pressure, dried, and submitted to a carbonizing process by baking in a furnace. During this process the cohesion of the carbon powders is increased by the carbon deposited from the decomposition of the carbonizable liquid under the influence of the heat. Since, in nearly all the arc lights in practical use, the carbons are placed vertically one above the other, it is necessary to make the carbon rods 220 or pencils very nearly straight, so that their axes may coincide during feeding. Where an exceedingly hard variety of carbon is required, the expedient is some- times adopted of soaking the carbons, after carbonization, in some carbonaceous liquid, and again subjecting them to a further process of carbonization, but this is only adopted in the manufacture of carbons for special pur- poses. 236. The steadiness of the arc light, though dependent on a variety of circumstances, is largely influenced by the position occupied by the arc. In order to prevent a travelling of the arc around different portions of the edge of the carbon, the expedient is sometimes adopted of making the central portions of the electrodes softer, that is, more readily volatilized, than, the remaining ma- terial, by the introduction of a different kind of carbon. Such carbons are called cored carbons. Owing to their expense, they are not extensively used in commercial lighting. The carbon in general use, is the ordinary coreless car- bon which has been electrolytically coated with a deposit of metallic copper. Although uncoated carbons are fre- quently employed, yet, unless special care is taken in their manufacture, they are apt to burn irregularly on the sides, and becoming pointed, are apt to interfere with the proper operation of the lamp. 237. The candle-power of an arc-lamp is very differ- ent in different directions, and, since in practice, the arc rarely remains for any length of time in a fixed posi- tion between the carbons, the candle-power as indicated by a photometer, is constantly varying. Fig. 92 represents 221 diagram matically | the physiologically effective luminous intensity of an ordinary arc lamp, at different angular positions ahout the carbons as an axis. It will be observed that at an angle of about 50 below the horizontal plane, when the carbons are vertical, the intensity is a maxi- mum, and that it rapidly diminishes both above and be- low this position. The mean spherical candle-power is the average candle-power taken all over the surface of a sphere having the arc at its centre, and is usually about one third of the maximum candle-power, and capable of Elec. Engineer FIG. 92. Diagram Indicating Luminous Intensity of an Arc Lamp in Different Directions. being expressed with a fair approach to accuracy by the numerical formula Mean spherical candle-power = Mean horizontal candle-power maximum candle power ~2~~ ~T~ The existing practice of rating the luminous power of an arc lamp by its maximum luminous intensity, is very defective, and could be preferably replaced by a state- ment of the mean spherical candle-power, or the total flux of light, (physiologically effective radiation). 222 It may be supposed that the horizontal candle-power of a lamp, that is, its candle-power in the horizontal plane, would be the same in all directions. This, how- ever, is not the case, owing to the fact that the carbons burn irregularly, and that the crater, which forms the main source of light of the voltaic arc, is seldom either located exactly centrally at the end of the positive carbon, or is surrounded by walls of equal height. It becomes neces- sary, therefore, to take the mean horizontal candle-power of a lamp, which is usually only a small fraction of its maximum candle-power. 238. Since the carbons are consumed during use, and the steadiness of the light produced requires, among other things, that the length of the arc be main- tained a constant distance apart, it is necessary that some regulating device be employed, whereby the carbons can be maintained at this distance during use. This is ac- complished by means of various feeding mechanisms connected with the lamp. A great variety of feeding mechanisms have been devised depending upon the posi- tion of the carbon and upon whether both carbons are fed toward each other, or whether, as is generally the case, the negative carbon is fixed and the positive or upper carbon alone is fed. The carbons have been placed in various positions ; parallel or oblique, that is, included at all angles from zero to 180. At one time in the history of the art, the carbons were employed parallel, as in the Jablochkoff candles, the carbons being maintained at a constant distance apart, not by the use of the feeding mechanism but by means of non-conducting substances such as kaolin placed between and consumed with them. 223 Nearly all commercial systems of arc-lighting at the present day, employ the carbons vertically over one another, with the positive carbon uppermost, except where the walls of buildings are to be illumined from lamps placed beneath, when the negative carbon may be placed beneath. This, of course, is on account of the fact that the crater in the positive carbon is the main source of light, the greater intensity of light being pro- jected directly from the crater, which, when the positive is the upper carbon, will be downwards as will be seen by an inspection of the carve in Fig. 92. 239. For most commercial purposes a slight change in the height of the arc within the lamp globe is immaterial. Consequently the use of mechanism, feed- ing one of the carbons only is not objectionable, provided the length of the negative carbon is so adjusted that the position of the arc shall never fall outside the surrounding glass globe. For light-house purposes, and for search- lights generally, where the arc is used in connection with reflectors and the position of the arc is therefore impor- tant, the mechanism of the lamp is adapted to feed both carbons. In such cases the negative carbon is fed about one half as rapidly as the positive carbon. 240. The result of experience has been to limit the length of the carbon used for the positive elec- trode to about 12 inches. For long runs during winter, varying from, say, 13 to 15 hours, a single pair of car- bon pencils, would be insufficient and would, therefore, necessitate recarboning during the night. In order to avoid this, the device of employing two separate pairs of carbons has been adopted, the circuit connections be- 224 ing such that when, by consumption, the length of one pair of carbons has been reduced a certain predetermined amount, the current is automatically trans- ferred to the second pair. A doiible-carbon, or all-night arc lamp of this character is shown in Fig. 93. SYLLABUS. In the carbon voltaic arc, an activity of about 450 watts is usually expended, with about 10 amperes at 45 volts pressure. The c. E. M. F. in the circuit is principally developed at the surface of the positive carbon or crater where work is being done in volati- lizing carbon against its cohesive attraction. During the maintenance of the arc, car- bon is volatilized mainly from the end of the positive carbon, some of the volatilized Double carbon carbon being deposited as a nipple of graphite, Arc Lamp. on ^g Qppogijjg surface of the negative carbon. The mean spherical candle-power of an arc light is usually half the mean horizontal candle-power, plus one quarter of the maximum candle-power. Laboratory of Houston & Kennelly, Philadelphia. Copyright, 1894, by THE ELECTRICAL ENGINEER.] WEEKLY. Price ' 10 Cents - Subscription, $3.00. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE CRADE ARC LIOHTINQ. 24:1. In arc lamps when the current is off, the feeding mechanism permits the upper carbon to fall, until it rests on the top of the lower carbon, so that the carbons are in contact when the current is started. On the passage of the current through the lamp, an electromagnet in the main circuit, by the attraction of its armature, separates the positive carbon the proper distance from the negative, thus establishing an arc be- tween them, and holds the upper carbon in this position by the operation of a device called a clutch. When the consumption of the carbons increases the distance between them, the pressure rises at the terminals of the lamp, due to the extra drop in the increased resistance and length of the arc, and as soon as the pressure at the lamp ter- minals has risen sufficiently high, i. e., when the carbons have burnt a certain distance apart, a special magnet, wound with fine wire, so that its resistance is, say 500 Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York, N. Y. [Entered as second-class matter at the New York, N. Y., Post Office, June 14, 1894.] 22G ohms, placed in shunt to tlie lamps, attracts its armature, releases the clutch and permits the upper carbon to fall. In a well regulated lamp, the upper carbon, while in use, never actually touches the lower carbon, since the decrease in potential, caused by the decrease in the re- sistance of the arc, reduces the attraction of the shunt magnet, thus allowing the clamp to clutch the upper carbon swiftly. 242. In all series-connected arc lamps, some device must be employed to prevent the failure of any lamp from opening the entire circuit. This is generally Etec. Engineer FIG. 94. Diagram of a Form of Arc Mechanism. accomplished by means of a special cut-out magnet? placed in the shunt circuit, and so arranged, that, whenever the pressure at the lamp terminals rises beyond a certain working maximum, this magnet shall operate and cut- out the lamp, by releasing a spring, short-circuiting the lamp terminals. The connections of such an automatic cut-out are di- agram matically represented in Fig. 9, as applied to a form of feeding mechanism for a series-connected lamp. Here the lifting magnet M, wound with coarse wire and having a resistance of about 0.05 ohm, is connected di- 22' rectly in circuit with the arc. On the attraction of the armature, which is pivoted at B, the clutch grips the lamp rod, and thus raises the upper, or positive carbon, and establishes an arc. When, by the consumption of the carbons, the arc becomes too long, the pressure between the carbon electrodes increases and more current flows through the magnet N, of about 400 ohms resistance, wound with fine wire and placed in a shunt circuit around the electrodes. As soon as this current reaches a certain criti- cal strength, the attraction on the armature momentarily releases the clutch and permits the upper carbon to fall, until by decrease in the length of the arc, the current through the shunt magnet decreases, when the upper mag- net again overpowers it arid reclutches the upper carbon. The automatic cut-out mechanism is shown at s. It consists of an electro-magnet, wound witii fine wire^ and placed in shunt around the carbon electrodes. If the carbon for any reason fails to feed, the increased pres- sure on the terminals of the shunt circuit causes the mag- net s, to be so highly energized as to attract its armature and thereby permit a spring automatically to close the short-circuit at A, and cut the lamp out. Besides the mechanism described, a great variety of other forms have been designed for operating arc lamps. The two windings N and M, may be placed on the same core ; or they may be placed on separate magnets attract- ing separate armatures, but in all cases a series-winding is employed for the lifting magnet, and a shunt winding for the feeding magnet. 243. The number of arc lights connected in series in a single circuit may sometimes be as great as 200, representing an aggregate pressure at the generator 228 terminals of roughly 10,000 volts (10 kilovolts). More usually, however, 125 lights is the limit, and in ordinary practice, from 50 to 65. For street lighting purposes, from 9 to 10 amperes is the strength of current main- tained. Taking the average number of arc lights on a single circuit at, say 60, representing an aggregate pres- sure of 3000 volts, such a system readily adapts itself to lighting an extended area, since the size of wire employed, usually No. 6. A.W.G., has a resistance of only about 2.1 ohms per mile. Thus a circuit of, say, live miles in length would only have a resistance of 10.5 ohms in con- ductors, producing a drop of 105 volts, with 10 amperes of current, which would represent an activity of 1050 watts, and would be capable of supplying about 2 arc lamps. The price asked for arc lighting service per year will, of course, depend upon a variety of circumstances, such as the size and nature of the plant, the cost of coal or water power, and the area of lighting, etc.; but taking the average case, the price would probably be from $70 to $110 per 450-watt, 50-yolt arc lamp per annum. 244. Arc lamps are frequently operated on incandes- cent circuits, usually two in series on 110 volt circuits, or four in series across the outside conductors of three wire circuits (220 volts). In all constant-potential lamps, it is necessary to insert a resistance in the circuit of the lamp so as to ensure their proper operation. This is not necessary in series-connected lamps, since the lamps tend to automatically check one anothers variations. For this reason the pressure at the terminals of a constant- potential arc lamp will, by reason of the drop in the re- sistance, be two or three volts greater, than in the case of 229 series lamps. It would appear, therefore, that the effi- ciency of a series arc system would necessarily be greater than that of the same number of lamps operated on constant-potential circuits. This, however, is not always the case, owing partly to the fact that a series generator has a somewhat lower efficiency than a constant-potential generator. Moreover, when a constant-potential incan- descent circuit already exists, and but comparatively few arc lamps are required, it may be more economical to connect these directly to the incandescent circuit than to Q) I 1 D.P Snap Switch i i ELEC.ENGH. NV. ^ FIG. 95. Incandescent Circuit, with Standard Lamps. install a separate generator and circuit for their special ac- commodation. Fig. 95 shows the connections of a pair of arc lamps operated in series from a pair of incandescent mains at about 110 volts pressure. 245. In a station for the operation of an extensive system of arc lamps, where a number of dynamos are employed in supplying the different circuits, and where the load on such circuits may vary, a switchboard becomes necessary, whereby this load can be readily shifted from one dynamo to another. Such a switch-board re- 230 quires a high insulation on account of the pressures em- ployed and means must be adopted to prevent an arc being accidentally drawn from one bar to another. Fig. 96 represents a form of such switchboard, in which the connections are established by flexible cords connected with suitably insulated handles and protected by rubber tubes. FIG. 96. Form of Series Arc Switchboard. 24-6. Arc lamps are sometimes operated on alterna- nating current circuits. In such cases, since the direction of the current rapidly changes, a definite posi- tive crater and its opposing negative nipple are never formed. Consequently, the temperature of the two carbons is approximately the same, as also the amount of light they emit. For the same reason the distribution of 231 light is more regular, than in the case of the continuous current arc, and possesses two points of maximum inten- sity, one directed upwards and one downwards, as is shown in Fig. 97. The mean horizontal intensity also bears a greater proportion to the mean spherical intensity, or to the maximum intensity than in the case of the con- tinuous current arc, but this proportion is more variable in alternating than in continuous current arcs. A B KORIZQ FIG. 97. Distribution of Light from an Alternating Current Arc as measured in a particular case. Arc lamps on alternating current circuits require from 28 to 35 volts at their terminals, according to the charac- ter, size and separation of the carbons. All alternating arcs are apt to produce a humming sound, the pitch of which depends upon the frequency of alternation. This is due to the periodic expansion and contraction of the air in the successive waves of heat produced. Alterna- ting-current arc lamps are usually supplied by transfor- mers, whose primaries are connected in series in the main circuit and whose secondaries are locally connected o*/ mn [TJNI7BESIT7; 232 to each arc lamp. The current employed in the primary circuit, instead of being from 9 to 10 amperes, the usual strength for continuous series-connected circuits, is com- monly about 30 amperes, the secondary current strength in the lamp circuits being, however, about 9 amperes. The consumption of the carbons is nearly uniform in an alternating current arc lamp. SYLLABUS. Most arc lamp mechanisms in use, consist of an elec- tromagnet placed in the main circuit for causing a sepa- ration of the carbons, and another electromagnet, placed in a shunt circuit, for causing their approach. In all se- ries-connected arc lamps, an automatic cut-out device, operated by a shunt magnet, is provided for closing a short circuit past the lamp on the failure of its carbons to feed. Arc lamps are sometimes connected in a single series circuit up to the number of 200. More commonly 125 is the limiting number, while from 50 to 65 is the number commonly employed. Under certain circumstances it is more economical to connect arc-lamps to constant-potential mains. Such lamps require a special resistance introduced into their circuit in order to control them. Alternating current arcs provide a more general dis- tribution of light, than constant current arcs, owing to the fact that both carbons are at approximately the same temperature. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, by THE ELECTRICAL ENGINEER.] WEEKLY. "N" n 30 TAMTTAWV * 1 QQ^ Price, - 10 Cents. JANLAKI o, 1 Subscription, $3.00. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE GRADE. Alternating Currents. 247. A continuous E. M. F. or current not only con- tinuously preserves the same direction, but, un- less otherwise specified, maintains the same strengthr~An alternating E. M. F. or current is one which changes its direction, being alternately positive and negative. A con- tinuous current may become fluctuating or pulsatory : i.e., it may, while preserving the same direction of current flow, vary either periodically or irregularly in its strength, but an E. M. F. or current does not become alternating unless it actually changes its direction. The difference between an alternating and a fluctuat- ing or pulsatory current will be seen from an inspection of Fig. 98, where a fluctuating E, M. F. or current, although represented as periodically varying in intensity, is not alternating since it is constantly directed or flows in the same direction through the conductor, being at all times represented by a line above the zero line o A ; while an alternating E. M. F. or current is represented by Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York, N. Y. [Entered as second-class matter at the New York, N. Y., Post Office, June 14, 1894.] 234 a line which is alternately on the positive and negative sides of the zero line, o A. The term alternating E. M. F. or current, a? employed Elec.bnginter FIG. 98. Fluctuating and Alternating E. M. F.'S or Currents. in practice, conveys the conception not only of periodic alternation of direction, but also of periodic recurrence of magnitude. In other words, if an alternating current or E. M. F. be graphically represented by a curve, whatever may be the shape of this curve as representing direction and magnitude, this shape must be repeated in successive waves. There may be an infinite variety of alternating E. M. F.'S and currents, not simply in regard to their magnitude, but also in regard to their manner of variation, as shown in Fig. 99. c / j . IP 1 8 = c I ' k FIG. 99 Periodic Alternating E. M. F. or Current. Rectangular Type. Elec. Engineer FIG. 100. Periodic Alternating E. M. F. or Current. Zig-zag Type. The E. M, F. may suddenly reverse its direction, as, for example, by the action of a commutator, so that the E. M. F. may suddenly change from a positive maximum 235 to a negative maximum, and vice versa, of which the graphical representation is the flat-topped type of wave ; or, the E. M. F. may gradually increase and decrease at a uniform rate from the positive to the negative maxima, and vice versa as shown in Fig. 100, whose graphical representation is a wave of the zig-zag type. E. M. F.'S or currents of this type seldom exist in practice, but approximations to them exist, of the types shown in Fig. 101, which represents a type of alternating wave of the flat-topped variety, and in Fig. 102, which represents a type of the peaked variety of wave, such as some alter- nators produce. FIG. 101. FIG. 102. ec. Engineer FIG. 103. Periodic Alternating E.M.F. Periodic Alternating E.M.F. Periodic Alternating E.M.F or Current. or Current. or Current. Flat Topped Curve. Peaked Curve. Sinusoidal Curve. The E. M. F. may assume a wave form intermediate be- tween the flat-top and the peaked varieties, and called the sinusoidal form, shown in Fig. 103, because its graphical representation is a sinusoid or curve of sines. The sinusoidal form of wave may be understood from a consideration of the following preliminary principles ; namely, if the disc Q R s, Fig. 104, supported on a hori- zontal axis A B, at o, be uniformly rotated about this axis, the vertically falling shadow ^>, of the point p, situated on the radius o P, intercepted by a horizontal sheet of paper E F G H, will execute a to-and-fro motion along the line, p o q, whose length will be twice the radius o P, and the 236 shadow will occupy different positions on this path accord- ing to the different positions of the disc. The motion of the shadow thus produced is called a simple-harmonic or simple-periodic motion and the E. M. F. or current, whose magnitude varies in accordance with such motion is called a simple-harmonic or simple-periodic E. M. F. or current. If now, the sheet of paper be moved steadily forwards in the horizontal plane, parallel to the axis A B, the moving shadow will trace on its surface a wave curve of the type shown in Fig. 103, and called a sinu- soid, because the distance of any point on the curve Elec. Engineer Fro. 104. Diagram of Simple Harmonic Motion. from the zero line a &, measures the sine of the angle at that moment included between the radius vector o P, and the vertical plane. The shape of the sinusoid will depend upon the length of the radius vector and on the speed with which the disc rotates, as shown in Fig. 105. For example, if the radius vector have the value o j, as shown at A, the sinu- soid traced for a particular speed of disc and paper is shown by the curve A B c D E F. If now, the velocities re- maining the same, the radius vector be halved, as at B, 23' the resulting sinusoid will be flattened. On the con- trary, if, as at D, the radius vector remain as before, but the velocity of rotation be doubled, the sinusoid will be sharpened. When a conducting loop or coil is steadily rotated about any diameter in a uniform magnetic flux, a sinu- soidal or simple-periodic E. M. F. will be generated in it. JHec+Engineer FIG. 105. Graphical Representations of Simple Harmonic E. M. F.'. or Currents. Commercial alternators, i. e., alternating current gen- erators, do not produce true sinusoidal E. M. F.'S, but in many instances they produce so close an approximation thereto that, for the purposes of computation, their val- ues may be regarded as sinusoidal. Even when the wave of E. M. F. generated by an alternator is distinctly flat-topped or peaked, its E. M. F. is usually regarded as 238 sinusoidal to a first approximation, and corrections are subsequently introduced for the effects of deviation. An alternation, or semi-period, consists of a single wave in either the positive or negative direction. A com- plete double alternation, that is, a double reversal, consti- tutes a cycle. Thus the wave o a ft 9 volts. If the inductance-reactance of 125.7 ohms could be separated from its accompanying resistance in the coil, the drop on the resistance itself would be 18.27 X 50 = 913.5 volts ; but, since the in- ductance and resistance of a coil of wire cannot be sepa- rated, all that can be observed is the drop at the termi- nals of the two coils ; namely, at the terminals of the 135.3 ohms impedance, as represented in Fig. Ill, and in this case the pressure at the terminals of the resist- ance would be 18.27 X 135.3 = 2472 volts. It follows, therefore, that a sinusoidal effective E. M. F. of 1100 volts, which never exceeds 1555 volts at the peak of the waves, can produce a pressure that could be measured witli a suitable voltmeter, of 2472 volts across the terminals of the resistance coil, and a further pressure in series with this of 2909 volts across the terminals of the condenser, making a total pressure arithmetically of 5381 volts; but geometrically the sum of these two pressures can only be 1100 volts, because the impressed E. M. F., as shown in Fig. 112, is out of phase with the c. E. M. F'S. SYLLABUS. When two sinusoidal E. M. F.'S are connected in series their resultant will be their geometrical sum. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, by THE ELECTRICAL EMGIIOIWI.] WEEKLY. "NTo 39 Price ' ' 10 Cents. Subscription, $3.00. Electrical Engineering Leaflets, BV Prof. E. J. Houston, Ph. D. A. E. Kennelly, F. R. A. S. INTERMEDIATE Alternating Currents. 257. In a continuous-current circuit, the reciprocal of a resistance is called a conductance (Sec. 33). In an alternating-current circuit, the reciprocal of an im- pedance is called an admittance. If an impedance A B, such as represented in Fig. 113, of 1.5 ohms, be transformed into an admittance, its length will be the reciprocal of the length A B, or y 1 ^ 0.667 mho, and its inclination to the horizontal will be reversed, as shown at a b. In a continuous-current circuit, as has already been stated (Sec. 33), the joint conductance of a number of con- b* ductances in parallel is the sum of the Fin. 113. separate conductances. In an alter- lllustra ting Plane Vector . , . . Reciprocals. nating-cuiTent circuit, the joint ad- mittance of a number of admittances in parallel is the geometrical sum of the separate admittances. Published by THE ELECTRICAL ENGINEER, 303 Broadway, New York, N. Y. [Entered as second-class matter at the New York, N. Y M Post Office, Jane Elec. Engineer 250 258. If an alternator, Fig. 114, producing a sinu- soidal E. M. F. of 1100 volts effective, at a fre- quency of 125 ~, be connected to two impedances in parallel consisting of (1) a condenser of 10 microfarads capacity and (2) a coil of 30 ohms resistance with an in- ductance of 0.2 henry, the angular velocity of the K. M. F. will be 6.283 X 125 = 785.4 radians per second, and the product of this and the capacity of 10~ r ' farad = 7.854 X 10~ 3 . The impedance of the condenser is. BE FIG. 114. Illustrating Joint Impedance of Impedances in Parallel. therefore, - oh rns, in a down ward direction, as ' 7.854 X 10- 3 represented bv the line D E. The impedance of the resist- ance coil will be the geometrical sum of the resistance A B, of 30 ohms, and a reactance B c, of 785.4 X 0.2 = 157.1 ohms, so that the impedance is 159.9 ohms, along the line A c. The reciprocal of the condenser-impedance D E, will have a length or 0.007854 mho, and will be di- 127.o 251 reeted vertically upwards instead of downwards, as shown by the line D E, set off on a suitable scale. The acjmit- tance of the coil will have a length . l = 0.006252 i oy.y mho, and will be set off downwards, at an angle with the horizontal, equal to the angle CAB, as shown by the line a , where the maximum amplitude or crest of the wave, in- stead of occuring midway between the zero passages, as in a sinusoidal wave, is displaced along the surface, owing to the influence of hysteresis in the iron, because a large change of current is necessary to produce a small change in flux at the turning point in the magnetic cycle (Sec. 1 54, Fig. 05). It is difficult to assign an angle of lag to such a wave ; and, consequently, the ratio of the projection to the actual current strength cannot be determined, while the apparent and actual activities in a circuit can always be found by means of a suitable voltmeter, am- meter and wattmeter. In transformers, however, the cur- rent tends to become more nearly sinusoidal as their load, that is, their output, is increased, so that the waves of cur- rent supplied by a sinusoidal alternator to a circuit sup- plying transformers at different loads are seldom so dis- torted as those shown in Fig. 1 1 f>. 202. The power factor of an alternating current transformer with ferric circuit varies from 0.7 at no load, to, perhaps, 0.99 in large transformers at full load, but in aero-ferric transformers, whose magnetic cir- cuits are formed only partly of iron, the power factor at no load may be as low as 0.4. The power factor of an alternating-current, synchronous motor, may vary from 0.9 to 1.0, and in an alternating current induction mo- 255 tor, from 0.5, on light load, to 0.85 or 0.9 at full load. The average alternating-current circuit has a power fac- tor of about 0.95, so that the apparent activity is only about 5 per cent, in excess of the actual activity. 263. The ratio of the impedance of a circuit or con- ductor to its resistance is called its impedance factor. The impedance of a line or conductor is almost always greater than its resistance, owing to the induc- tance of the conducting loop, and the impedance factor shows how many times greater than the resistance this impedance is. The impedance factor depends upon the frequency of alternation and increases with the size of conductor. Thus at 120 ~, the impedance factor of two No. 4 A. w. G., copper wires, suspended in air parallel to each other, at an interaxial distance of five feet, is 1.6, so that the apparent resistance of such a pair of conduc- tors would be 60 per cent, in excess of their ohmic re- sistance. The ratio of the reactance of a conductor or circuit, to its ohmic resistance is called its reactance fac- tor, and measures the tangent of the angle of lasr or lead in the case of sinusoidal currents. 264. It has already been stated (Sec. 45) that the ap- parent resistance of a rod or cylinder is greater for alternating than for continuous currents. The reason for this is to be found in the fact that if we consider, for example, a long straight conductor, carrying 10 amperes, the magnetic flux will encircle the axis of this wire in an alternately right-handed and left-handed direction at each alternation of the current. Of this flux, perhaps 80 per ;ent. will lie outside the wire, and the remainder, or 20 per cent., will be contained in the substance of the wire, there being no magnetic flux at the axis or centre. The 256 pulsating magnetic flux induces a c. E. M. F. directed along the wire and opposing the establishment of the current in it. While, however, the central portions of the wire have the full c. K. M. F. produced by all of this flux, the external or superficial portions have a c. E. M. F. only 80 per cent, as great, since it is produced by the external flux only. The result will be that the impedance of any filament of wire near the centre will he greater than that of a corresponding filament near the outside, and the current will, therefore, be distributed more densely in the outer layers. 265. The imperfect penetration of an alternating cur- rent into the interior portions of a conducting wire is called the skin effect of alternating currents. At high frequencies, and in large sizes of wire, the skin effect may be very considerable, but for commercial frequencies and with the sizes of wire employed in overhead con- struction, the impedance due to skin effect is very small. Thus in the case of a No. 000 copper wire, carrying cur- rents whose frequency is 14-0 ^, the impedance, owing to the influence of imperfect current penetration is only 1.6 per cent, greater than the ohmic resistance. In iron wires, however, this influence is much more marked, owing to the greater proportion of magnetic flux existing within the substance of the alternately magnetized wire. The impedance of a Xo. 7 A. w. G. iron wire at 140 - may be double that of its ohrnic resistance, owing to the effect of imperfect current penetration. In telephony the advantage of copper wires over iron wires is principally ascribed to their reduced skin effect. Liaboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, by THE ELECTRICAL ENGINRKK.] WEEKLY. Nn T3 TA v 1SQ* Price, - 10 Cents. JANLARI Jt>, 1 J5. Subscnption, $3.00. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. I1MXERJYIEDIATTE CRADE. 266. The number of poles on a continuous-current generator is largely a matter of economy and con- venience in construction. In an alternator, the number .of poles is prescribed, as soon as the frequency and the number of revolutions of the armature per second has been deter- mined upon, since these two considerations determine the frequency of alternation. A bipolar alternator, generates one cycle for each complete revolution of its armature ; a four-pole machine, generates two cycles for each complete revolution of its armature; and a machine poles will, therefore, generate cycles for each revolu- 2 tion of its armature. Consequently, the frequency of an alternator is -^S cycles per second, where n, is the number of revolutions of its armature per second ; thus an alternator of 16 poles, making 16 revolutions per sec- ond, would have a frequency of 128 ~. Some inductor Published by THE ELECTRICAL ENGINEER, 203 Broadway, New York, N. Y. [Entered as econd-class matter at the New York, N. Y., Post Office, June 14, 1894. ] 258 alternators, however, which revolve masses of soft iron instead of wire, produce twice as great a frequency, or a frequency of n p, cycles per second. 267. The character pf the E. M. F. wave generated by an alternator, depends upon the dimensions of the pole pieces and winding spaces, so that by varying the distance between the poles, or their shape, or the dis- tance between the coils on the armature, as well as the shape of the space these occupy, the type of E. M. r. wave may be varied. 268. In most alternators, the armature is revolved in a fixed field frame. From this the E. M. F. gen- erated is connected with the circuit they supply through brushes resting on collector rings, in lieu of the commu- tators employed in continuous-current generators. In other alternators, however, the armature is maintained at rest, and the field frame revolved about it. In such machines the current is supplied through brushes and collector rings to the magnets, while the armature is con- nected directly to the line. In still other forms of alter- nators, the field and armature are both fixed, and masses of iron are used to vary, by their revolution, the magnetic circuits between the two. Such alternators are called inductor alternators. 269. The coils in alternating armatures are either of the Gramme ring, the drum, the disc, or the pole arma- ture type. The most usual, owing to its convenience of construction, is the pole type, but other forms are in common use, especially in Europe. The armature coils are sometimes connected in series and sometimes in par- allel-series, as shown in Figs. 117 and 118. When wound 259 in parallel-series, twice the number of armature turns is required for the same E. M. r. FAec. Engineer FIG. 118. SUe. engineer FIG. 117. FIG. 118. FIG. 119. 270. When a wave of E. M. F. or current is not a simple sinusoid, for example, irithe case of such a wave ae is represented in Fig. 119, it is sometimes convenient to SUCTANT FIG. 120. FIG. 121. 260 regard the wave as capable of being analyzed or decora- posed into components, all of which are sinusoids ; that is, into a fundamental sinusoid and its harmonics. It can be shown that any periodic wave possessing a definite frequency, no matter how complex its form, can be re- solved into a fundamental sinusoidal wave of the same frequency, and a numbef of ripple chains, or harmonics, each harmonic having a frequency some integral multi- ple of the fundamental frequency. Thus Fig. 120, rep- resents at s, a sinusoidal wave of E. M. r. having a fre- quency of 100 ~, and an amplitude of 800 volts. Its first harmonic, that is, a sinusoidal wave of double the frequency, and which in this case has an amplitude of 400 volts, and starts in phase with the fundamental, is represented at P. Its second harmonic, having three times the frequency of the fundamental and in this case with the amplitude of 500 volts, also starting in phase with the fundamental is seen at Q. If two alterna- tors, respectively producing the waves s, and P, were rigidly coupled on the same shaft, the E. M. F. they would jointly produce in the circuit, would be represented in Fig. 121 where the wave o G H j K, is the sum of the com- ponent waves o a b c d, and o A B c v, Fig. 120. The re- sultant wave is observed to be asymmetrical ; that is to say, if the positive wave o G H j K, Fig. 121, be revolved about the line c K, so as to be completely reversed in di- rection, it will not coincide with the following negative wave K L M N P. This lack of symmetry is owing to the addition of the odd harmonic ; for, the first, third, fifth, etc., harmonics have the property that when added to the fundamental wave, either singly or in combination, they produce asymmetry about the zero line, and since 261 all properly constructed alternators produce symmetrical waves of E. M. F. and current, in which each wave differs from its successor or antecedent in direction only, such harmonics do not exist in the forms of wave com- mercially employed. Similarly if in Fig. 120, the three waves P, Q, and s be combined, their resultant will be the wave shown in Fig. 119, whose amplitude is about 1330 volts. This wave is also asymmetrical, owing to the presence of the first harmonic. 271. Fig. 121 shows the effect of combining a funda- mental wave F, of a particular amplitude and phase, with its second harmonic. F -|- A, the resultant of F and A, is of the flat-topped type, while F -(- B, similarly com- pounded of F, and B, is of the peaked type. A and B, have the same amplitude, but differ in phase by half a period, or 180. Both the resultant waves are symmetri- cal, and it can be demonstrated that the addition to a fundamental wave of any number of even harmonics, of any amplitude or phase, will always produce a symmetri- cal wave, no matter how complex its form. The flat- topped or peaked type of alternating E. M. F. or current may, therefore, be equivalent to the result produced by the presence of a prominent second harmonic. 272. When a complex-harmonic E. M. F. is impressed upon a circuit, the current may be considered as the sum of all the component currents which each com- ponent of E.M. F., considered as a separate alternator, could independently produce in the circuit. The upper har- monics have so high a frequency that the reactance offered to them by inductance in the circuit produces a high impedance to their current, and, consequently, in a circuit containing considerable inductance, the upper harmonics in the current are greatly weakened, so that the wave of current tends to approach the fundamental sinusoid. In fact, it is seldom necessary to introduce more than a second and fourth harmonic into the har- monic analysis of any practical alternating-current wave. On the other hand, the effect of hysteresis in iron cores linked with a conducting circuit, is, as we have already seen, likely to produce considerable distorsion of cur- rent wave type. 273. When an alternating E. M. F. or current is spoken of as possessing harmonics, it is not, therefore, to be inferred that those harmonics are actually present, but that the type of curve is such as could be produced by the admixture of certain harmonics, with a funda- mental having the frequency of the wave, and that the effects of such an E. M. F. or current, would be dupli- cated in the E. M. F. or current under consideration. In fact, with the alternating E. M. F.'S and currents in com- mercial use, the consideration of harmonics may for many purposes be neglected. 274. Two methods of winding alternators are in com- mon use, viz., the series and the parallel-series, as already shown in Figs. 117 and 118. In the former, the total E. M. F. of all the coils is utilized, but the full pressure of the alternator is developed between the two neighboring extremities A, and B. In the latter, twice as many turns have to be wound on the machine to produce the same E. M. F. as in the former case, but the points of maximum pressure are now as far removed on the arma- ture as possible. 263 Alternators may be self-excited by commuting a cur- rent from a small special winding, and directing this rectified current through the field magnets. In almost all cases, however, alternators are separately excited by means of a small, continuous- current generator, operated on the same shaft or by a belt running from an arm- ature shaft. Alternators are frequently compound-wound. This winding may be arranged in one or two ways ; viz., either the main current supplied from the armature is led through a shunted commutator, 011 the armature shaft, which is connected with a special winding on the field "-I!*M_J!_ ^ 3 SECONDS => Mec. Engineer FIG. 122. Elec. Engine FIG 123. magnets, so that a portion of the outgoing current is commuted and sent through the field winding, as shown in Fig. 1'22; or a transformer is placed in the path of the outgoing current, and the secondary coil is connected through the commutator with the field winding, so that as the outgoing current increases in strength the field winding receives additional excitation. 275. In continuous current generators the drop in the armature is almost entirely owing to the resist- ance of the armature ; some little being, however, due to a c. E. M. F. of self-induction in the coils undergoing com- 264: mutation, and some to armature reaction and c. M. M. F. In an alternator, however, the drop is not only due to the resistance, but also to the reactance of the armature, so that the drop is increased from IP, to /-/, volts ; J, be- ing the impedance of the armature. In addition to this, there is usually some drop due to armature reaction and c. M. M. F., but as there is no commutator, there is no loss due to commutative action. SYLLABTTS. The frequency which an alternator has to supply de- termines the number of its poles when its speed of rotation is given. The character of the E. M. F. wave of an alternator de- pends upon the relative size and spacing of the poles and armature winding. Most alternators revolve their armatures ; some revolve their fields, and a few revolve a mass of iron, forming a portion of their magnetic circuit. Alternating waves of E. M. F. and current, when not strictly sinusoidal, may be resolved into a fundamental and a member of harmonics. The combination of a fundamental with any of its odd harmonics produces an asymmetrical wave with respect to the zero line, but its combination with any of its even harmonics produces a symmetrical wave. Laboratory of Houston & Kennelly, Philadelphia. [Copyright, 1894, bv THK ELECTRICAL ENGINEER.] WEEKLY. No 34 FTTRT?TTAT?V O 1QQK Price, - 10 Cents. 4 L Subscription, S3.00. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE ALTERNATORS. 276. Alternators employed for incandescent lighting usually have a frequency of from 125 ~ to 133 ~, and should have a frequency above 35 ~ in order to ensure steadiness of the light. Below 30 ~ incandes- cent lamps appreciably flicker, showing pulsations in the light emitted corresponding to the pulsations of the cur- rent, especially with high pressure, high efficiency fila- ments, which have necessarily a very small cross section and a high temperature. A type of alternator suitable for incandescent lighting at a frequency of 133 ~~, is shown in Fig. 124. This machine has a commutator provided at M, for rectifying the induced current through the compound-wound field magnets, so as to maintain a constant E. M. F. at collector rings R, R', under all conditions of load. It has 28 poles and makes 571 revolutions per minute with a ca- pacity of 450 K w. The E. M. F.'S supplied by such alternators are 1000, Published by THE ELECTRICAL ENGINEER, 903 Broadway, New York, N. Y. [Entered as second-class matter at the New York, N. Y., Post Office, June 14, 1894.] 266 2000 or 3000 volts effective, representing a maximum E. M. F. in each cycle of about 1414, 2828, or 4242 volts, on the assumption that the waves of E.M. F.are sinusoidal. The effective pressures at machine terminals may l>e five to fifteen per cent, in excess of the E. M. F.'S to be sup- plied in the mains in order to allow for drop in con- ductors. FIG. 124. Alternator for Incandescent Lighting. 277. In all incandescent alternators, the inductance and, therefore, the reactance of the armature is kept as low as conveniently possible, so as not to obtain an unduly large impedance in the armature of the ma- chine, and so as to prevent an excessive drop in the armature under load. In alternating arc generators, however, the armatures are required to vary automati- 267 callj the E. M. F. at the collector rings in conformity with the number of lamps that are operated in series in the circuit through the medium of their respective trans- formers. This is accomplished by giving to the armature a large inductance and consequent reactance, and also by arranging for a powerful reactive effect between the c. M. M. F. in the armature and the M. M. F. of the Held. By this means the drop of pressure in the armature, and the reactive M. M. F., keep the pressure at collector rings down to that required for supplying under all conditions of load, a practically uniform current through the line. 21 &. Alternators supplying incandescent or arc lamps, furnish a single alternating current through one pair of mains from the collector rings. Such a current is capable of driving a similar alternator as a motor, but only when the motor is in step with the alternator. Such motors can either not be started at all, or can only be started from rest under light load, but once in step with the generator will run under full load from its current. These motors are called synchronous motors. In order to employ an alternating current motor, capable of being started with full torque from rest, which is the requirement of most machinery, multiphase currents have at present to be employed ; that is, two or more currents, differing in phase by different amounts, require to be simultaneously sent through the motor in different circuits. At present there are only three varieties of multiphase currents in commercial use; namely, diphase, triphase and monoeyclic. 279. Two alternating E. M. F.'S are called diphase E. M. F.'S, when they have the same frequency, mag- nitude and wave character, but differ in phase by a quarter 268 Cjcle,or90,beingvtherefore,tw quart rutur^. Such K.M.F.'S are as shown in Fig. 1:25, where o A, indicates an E. M. F. of lino volts rotated at a definite angular velocity about the point o, but always in quadrature with an equal E.M.F. o B, which rotates around o, with it, so that when o A, has its full length the projection of o B, on a horizontal line vanishes. When o A, reaches the position shown in Fig. l!2' ; namely, after | of a period has elapsed, the pro- jection of o A', on the horizontal line will be o a', or 778 volts, and the projection of o B, still at right angles to o A, will be o by or 778 volts negative. It is evident, there- B 6- -778 FIG. 125. FIG. 126. Diagtam of Diphase E. M. F.'S. Diphase E. M F. Diagram. fore, that when one E. M. F. has its maximum, the other E. M. F. has its zero. The current, which these two E. M. F.\S will send through independent circuits, will also be in quadra- ture, if the impedances of those circuits are equal ; for, the lag of each current behind its own E. M. F. will be the same in each circuit. In some cases, four wires and two separate circuits are employed for the dis- tribution of diphase currents as shown in Fig. 127, while in other cases three wires are employed, one wire forming a common return, as in Fig. 128. Each circuit, considered separately, is an ordinary uniplia.se circuit in which incandescent lamps, arc lamps or synchronous motors can he operated, but the combination of the two currents enables non-synchronous or inductive motors to n Etec. Engineer FIG. 127. Biphase Connections, Separate C'iicuits. l>e operated. In Fig. 1.28, the E. M. F. between neighbor- ing wires is seen to he 2000 volts effective, while be- tween outside wires, the E. M. F. is 2828 volts effective, and this will be true whether the E. M. F.'S are sinusoidal or not; for, as shown in Fig. 125, the E. M. F., A E, is 1.414 times greater than either o A, or on, by geometry. Diphase E. :sr. F/S are generated by two sets of coils so wound on the armature, with respect to the field poles, that the E. M. F. generated in one is 90, or cycle, ahead of the E. M. F. generated in the other. Eltc.Enginer FIG. 128. Diphase Connections, Interconnected Circuits. 280. Three alternating E. M. F.'S are called triphase E. M. F.'S, when they have the same frequency, magnitude and wave character, but differ in phase cycle or 120. Such a system of E. M. F.'S is represented in 270 . Fig. 129 where o A, o B, and o c, are three triphase K. M. F.'S, each of 1000 volts effective, revolving together about the point o, with a definite angular velocity. 281. Triphase E. M. F.'S are generated by three sets of coils so wound on the armature with respect to the field poles, that the E. M. F.'S in them are 120 apart. There are two methods of connecting the windings ex- ternally ; namely, the star-rnMliod, indicated in Fig. 130, where the three windings are brought to a common con- nection o, and the triangular method represented in Fig. 131, where the three windings are connected in one loop B B _ R. FIG. 129. Triphase K. M. F. Diagram. . Elec. JKnffineer FIG. 130. FIG. 131. Star Triphase Winding. Triangle Triphase Winding. or series D E F. Whichever method is adopted the E. M. F. is always measured between any two of the three terminals A B c, or D E F. In the star winding, the E. M. F. between any two terminals as A and c, Fig. 129, is 1732 volts effective or 1.732 times the E. M. F. in the winding o A, o B, or o c, as is evident from the geometry of the figure, so that if the E. M. F. between three termi- nals is 1732 volts, that between any terminal and the common connection is 1000 volts. On the contrary, when connected in the triangular system, the E. M. F. be- tween terminals is the E. M. F. of the winding. The out- 271 put, however, of a machine will, under both conditions, he the same, and in fact will be the same whether the machine he divided in three parts connected in triphase, or into a single winding and worked Uniphase. 282. A recent combination of the uniphase and mul- tiphase systems is called the monocyclic system. This system is intended to he a Uniphase system in so far as regards electric, lighting over an extended area by two wires, but when multiphase motors are to be driven, a third and smaller wire called \\\v power wire is employed carrying a special pressure to such multiphase motors. A Elee. Engineer FIG. 132. Mouocyclic E. M. F. Diagram. The arrangement of E. M. F/S in a monocyclic genera- tor is represented in Fig. 132, where o A, is the principal E. M. F. of the generator and is here represented as of 2000 volts effective E. M. F., revolving at definite angular velocity about the extremity o. This i:. M. F., connected to two collector rings, furnishes a uniphase current for incandescent and arc lighting, and also for synchronous motors. A separate winding of smaller cross-sectional area and fewer turns, produces the E. M. F., B c, of 577 volts effective, which is connected between a third collector ring and the middle of the principal winding o A. This K. M. F. is arranged to be generated in quadrature with o A, as shown in the figure. Between terminals o, ancj 272 A, there will thus be an E. M. F. of 2000 volts, between o, and c, 1154 volts, leading o A, by 30 and between c, and A, 1154 volts, lagging 30 behind o A, consequently o c and c A, are separated in phase by 60. The power wire from c, being carried to the premises where an in- duction motor is to be operated, two transformers are installed each for half the power required. One trans- former is connected to the wires o and o, as shown in Fig. 133, and the other to the terminals A and c. The E. M. F. induced in the secondary winding of these trans- nsww 1 PRIMARY lO 1154 PRI FIG. 133. Monocyclic Triphasc Transformer Connections. FIG. 134. Combination of Secondary Monocycl.c E. M. F. into Triphase System. formers may each be, say, 100 volts effective, but a rela- tive angular position of o c and c A, in Fig. 132, or o' c' and c' A', in Fig. 134. By reversing the connection of the second E. M. F., c' A', we obtain an E. M. F. c' A", Fig. 134, so that the three terminals of the two trans- formers o', c' and A'', have between them three triphase E. M. F.'S, each equal to 100 volts, as shown in Fig. 134, and to these terminals the triphase motor wires are at- tached. Laboratory of Houston & Kennelly, Philadelphia. (.Copyright, 1894, by THE ELECTRICAL ENGINEER. } WEEKLY. No. 85. FKHBUAKY 9, 1895. ggV^ Cent*. Electrical Engineering Leaflets, Prof. E. J. Houston, Ph. D. AND A. E. Kennelly, F. R. A. S. INTERMEDIATE GRADE. Alternating Cnrrent Transformers. 283. An alternating-current transformer consists es- sentially of an induction coil in which an alter- nating E. M. F. is induced in a secondary circuit b}^ the variations of an alternating current in the primary circuit. Suppose that a laminated ring of iron wire cc c, Fig. 135, be wrapped with a primary coil P, and an alternat- ing E. M. F. of 1000 volts be impressed on its terminals. If the coil has 500 turns and a resistance of 7?, ohms, then a certain effective current strength /, w r ill pass through the coil. Since the current is alternating, the M. M. F. it produces sets up an alternaing flux through the coils and establishes in it a c. E. M. F. The geometrical sum of this c. E. M. F. and the drop, will be 1000 volts ; thus, if the resistance 7?, be two ohms, and the current strength /, one ampere, the impedance of the coil will be 1.000 ohms, and the geometrical sum of the c. E. M. F. and the drop of 2 volts, will be equal to the E. M. F. of Published by THE ELECTRICAL ENGINEER, 903 Broadway, New York, N. Y. [Entered as second-class matter at the New York, N. Y., Post Office, June 14, 1804.] 274 1000 volts at the terminals. The c. E. M. r. must there- fore be very nearly 1000 volts. 284. If now a secondary coil s. be wound on the ring as shown, the flux from the primary coil may, neglecting leakage, be considered as passing entirely through the secondary coil. If the number of turns in the secondary coil be 50, the E. M. F. induced in it will be very nearly -/^ths of that at the primary terminals, 100 volts. If the secondary circuit be opened, the presence of the secondary coil has no effect upon the primary circuit, but if the secondary coil be closed through a resistance, a current will flow through the secondary circuit, and will produce a M. M. F. in the magnetic circuit, counter to the M. M. F. of the primary current. The primary M. M. F. is, therefore, weakened, and the c. E. M. F. in the primary coil weakened, and the impedance in the primary coil being reduced, an increased current strength flows through it from the primary mains. This increase in current is sufticient, under the new conditions, to re-establish the flux and c. E. M. F. required in the primary circuit. As the load in the secondary circuit is increased, the impedance of the primary circuit diminishes, and, not only does the primary current increase, but it comes more nearly into phase with the primary impressed E. M. F., that is, both the current strength and the power factor increase. In other words, an alternating-current transformer is self- regulating, under all variations of load, up to the limit of the apparatus. 285. In the alternating-current transformer shown in Fig. 135, the primary and secondary coils are wound on the outside of the iron wire ring forming the 275 magnetic circuit. This arrangement is objectionable in practice on account of leakage, as illustrated in Fig. 136. Preferable forms are shown in Fig. 137, where .the pri- mary and secondary coils are brought nearer together and where they are more closely surrounded by iron, thus reducing the leakage. The flux paths are roughly indicated by the arrows. Other forms, in which the iron lies outside the coils, are shown in Figs. 138 and 139, where the primary and secondary terminals are represented by the letters p p, and s s, respectively. Here the primary and secondary coils are surrounded by U-shaped stampings of sheet metal, alternately placed FIG. 135. FIG. 136. above and below so as to produce a thick and short mag- netic circuit. The leakage in such a transformer is com- paratively small, but the transformer has to be entirely dismantled, in order to replace an injured coil. Still an- other form is represented in Fig. 140, where two coils c c arid c Coulomb, Definition of .... 42 Coulomb Meter 43 Counter Elect ro motive Force of Voltaic Arc .... 218 Couple, Voltaic 66 Crater, Positive, of Voltaic Arc, Formation of, 219 286 INDEX. Crater, Positive, of Voltaic Arc, Temperature of 219 Critical Output of Dynamo- Electric Machine 145 Current, Alternating, De- finition of 14, 233 Current , Alternating or Periodic. Peaked Type . 235 Current, Continuous, De- finition of 14, 44 Current, Electric 41 to 48 Current, Electric, Electro- lytic Effect of 42, 43 Current, Electric, Heating Effect of 3 Current, Electric, Moment- ary 3 Current, Electric, Rate of Flow 43 Current, Fluctuating or Pulsatory 233 Current, Periodic or Alter- nating, Flat-Topped Type of 235 Current, Periodic or Alter- nating, Sinusoidal -Type of 235 Current, Pulsatory, Defini- tion of 44 Current, Simple-Periodic . . 236 Current, Thermal Activity of in Conductor 193 Currents, Alternating, De- finition of 45 Currents, Diphase 267 Currents, Electric, Varieties of 44 Currents, Monocyclic 267 Currents, Multiphase 267 Currents, Table of Varying Commercial Strengths of 45 Currents, Triphase 267 Curves of Reluctivity 99 Cut-out, Automatic, for Arc Lamps 226, 227 Cycle, Definition of 238 Cycle, Hysteretic 142, 143 Daniell Voltaic Cell 76, 77 Depolarizer, Solid, for Vol- taic Cell 69 Depolarizer for Voltaic Cell 68 Diameter of Commutation . 150 Difference of Potential 16 Diphase Currents 267 Diphase E. M. F. 's 267 Direction of Magnetic Flux, Convention as to 88, 89 Discharge, Electric, Mo- mentary 2 Dissymmetrical E. M. F. , De- finition of 14 Direction of Magnetization, Dependence of, on Direc- tion of Current 114 Divided Circuit, Application of Ohm's Law to 52 Double-Carbon Arc Lamps 224 Drop in Conductor 50 Drum- Wound Armature. . . 130 Dynamic Force, Definition of 169, 170 Dynamo and Motor, Co- existence of Electrodyna- mic and Electromotive Force in 169 Dynamo and Motor, Re- versibility of 169 Dynamo-Electric Induction 121 INDEX. 287 Dynamo-Electric Induction in Conductor, three cases of 122, 123 Dynamo-Electric Machine, Building up of 158, 1 59 Dynamo- Electric Machine, Classification of Losses in 138 Dynamo-Electric Machine, Commercial Efficiency of 137, 138 Dynamo-Electric Machine, Critical Output of 145 Dynamo-Electric Machine, Electrical Capability of.. 135 Dynamo-Electric Machine, Essential Parts of . 129 Dynamo-Electric Machine, Lead of Brushes in ...... 1 50 Dynamo-Fvlectric Machine, Limitation of Output by Excessive Drop of Arma- ture 145, 146 Dynamo-Electric Machine, Limitation of Output by Excessive Heating. . . 146, 147 Dynamo-Electric Machine, Limitation of Output by Excessive Sparking at the Brushes 148, 149 Dynamo-Electric Machine, Relation Existing be- tween Output and Inter- nal Resistance 134, 135 Dynamo, Electrical Losses of 138 Dynamo, Hysteretic Losses in 140 Dynamo, Magnetic Losses of 138 Dynamo, Mechanical Losses of 138 Dynamo, Output of 134 Dynamo, The 129 to 152 Dynamos, Series Connec- tion of 59, 60 E. M. F., Alternating, Defini- tion of 14, 233 E. M. F., Complex-Harmo- nic 261 E. M. F., Continuous, Defi- nition of 14 E. M F., Counter, of Motor. 170 E. M. F., Definition of Direc- tion of 9 E. M. F., Dissymetrical, De- finition of 14 E. M. F., Fluctuating, De- finition of 14 E. M. F. in Armature Con- ductors, Calculation of Value of 131 E M. F. Induced in Rotating Conducting Loop, Direc- tion of 132 E. M. F. of Armature Con- ductor, Rule for Calculat- ing 134 E. M. F. or Current, Periodic or Alternating, Rectan- gular Type of 234 E. M. F. or Current, Periodic or Alternating, Zigzag Type of 234 E. M. F. or Current, Periodic Peaked Type of 235 E. M. F., Periodic, or Alter- nating Current, Flat- Topped-Type of 235 288 INDEX. E. M. F., Periodic, or Alter- nating Current, Sinus- oidal Type of 235 E. M. F., Production of, by Mutual Induction 128 E. M. F., Pulsating, Defini- tion of . . 14 E. M. F., Simple-Harmonic. 236 E. M. F., Simple-Periodic... 236 | E. M. F., Steady, Definition of 14 E. M. F., Symmetrical, De- finition of 14 E. M. F., Triphase, Diagram of 270 E. M. F., Unit of ii E. M. F.'S, Conjoined 9, 10 E. M. F.'S, Diphase 267 E. M. F.'S, Opposed Action of 9, 10 E. M. F.'S or Currents, Fluc- tuating or Alternating, Graphic Representation of 234 E. M. F.'S or Currents, Sinu- soidal, Harmonics of 260 E. M. F.'S, Triphase 269, 270 Earth's Crust, Average Re- sistivity of 27 Earth's Flux, Production of E. M. F. in Coil by Rotation in 127 Eddy-Ctirrent Losses 139 Eddy-Current Losses in Copper Wire 139, 140 Edison-Lalande Voltaic Cell 79 Effect, Skin, Definition of.. 198 Effective or Joint Resistance 23 Efficiencies, Varying, Com- mercial, for Incandescent Lamps 207, 208 Efficiency and Size of Trans former, Effect of Fre- quency of Alternation on 279 Efficiency, Commercial, of Dynamo 137, 138 Efficiency, Electrical, of Dynamo 137 Efficiency of Alternating- Current Transformers. . . 278 Efficiency of Distribution of Electrical Heating. ... 195 Efficiency of Incandescent Lamp 206 Efficiency of Machine 7, 8 Electric Charge i to 8 Electric Current 41 to 48 Electric Currents, Magnetic Effects of 4 Electric Currents, Varieties of 44 Electric Motor, Advantages Possessed by 188, 189 Electric Motor, Conditions for Constant Torque and Variable Speed 178, 179 Electric Motor, Conditions for, Variable Torque and Constant Speed 181 Electric Motor, Conditions for, Variable Torque and Variable Speed 182 Electric Sources, Series- Connected 59, 60 Electrical Capability of Dynamo-Electric M a - chine 135, Electrical Effects 136 l to 8 INDEX. 289 Electrical Effects, Classifi- cation of 4 Electric Heater, Practical Efficiency of 196 Electric Losses of Dy- namo 138 Electricity, Unit Quantity of 42 Electricity, Unit Rate of Flow of 43 Electrification , Effect of Surface-Dissimilarity on 2 Electrode, Negative, of Voltaic Cell 69 Electrode, Positive, of Vol- taic Cell 69 Electrodes for Arc Lamps, Length of 223 Electrodynamic Attractions and Repulsions 167, 168 Electrodynamic Force 162 Electrodynamic Force, Cir- cumstances Affecting Value of 162, 163 Electrodynamic Force, Source of Work Done by 165 Electrodynamics 161 to 168 Electrodynamics, Defini- tion of 161 Electrolysis, Definition of.. 4 Electrolyte, Definition of . . 65 Electrolytic Effect of Elec- trical Current 42, 43 Electromagnet, Aero-Ferric Circuit of 108 Electromagnet, Definition of 113 Electromagnets 1 13 to 120 Electromotive Force ... 9 to 16 Electromotive Force, Dy- namo-Electric Induction of, in Conducting Loop 124, 125 Electromotive Force, Gen eration of, in Conducting Loops 124 Electromotive Force, Origin of 16 Electroplated Arc-Light Carbons 220 Element, Voltaic, Defini- tion of 66 Energy Constancy of 6 Energy, Definition of 5 Energy, Expenditure of . . 5 Energy, Wasted 7 Ether, Universal 2 Exhaustion of Lamp Cham- ber of Incandescent Lamp 203 Exploring Magnetic Needle 92 Feeding Mechanism for Arc Lamps 222 Ferric-Magnetic Circuit 100 Field Magnets, Function of 129 Filament of Incandescent Lamp, Cause of Decrease in Diameter of 209 Filament of Incandescent Lamp, Cross Sectional Area of 205 Filament of Incandescent Lamp, Decrease of Di- ameter and Efficiency of. 210 Filament of Incandescent Lamp, Flashing Process for 203 2 9 INDEX. Filament of Incandescent Lamp, Manufacture of . . 201, 202 Filament of Incandescent Lamp, Materials Employ- ed in 202 Filaments for Incandescent Lamp, Mounting of 203 Flashing Process for Fila- ment of Incandescent Lamp 203 Flat-Topped Type of Peri- odic E.M.F. or Alternating Current 235 Fleming's Hand Rule 123 Fleming's Hand Rule for Motors 163 Fluctuating Alternating E. M. F.'S, Graphic Repre- sentation of 234 Fluctuating E. M. F., Defini- tion of... 14 Fluctuating or Pulsatory Current 233 Flux-Paths, Magnetic, De- finition of 89 Flux, Prime 113 Focusing Arc Lamps 223 Following or Trailing Edge of Motor Armature 186 Force, Dynamic, Definition of 169, 179 Force, Electrodynamic 162 Force, Electrodynamic, Cir- cumstances Affecting Value of 162, 163 Franklin's Kite 3 Frequency, Definition of.. 238 Frequency, Effect of, on Skin Effect 256 Frequency of Alternation, Effect of, on Size and Effi- ciency of Transformer. . . 279 Frequency of Incandescent Lighting Alternators 265 Galvanometers 46 Galvanometer, Thomson Mirror. ... 47 Generator, Compound- Wound, Diagram of ... 155 Generator, Series-Wound, Diagram of 154 Generator, Shunt- Wound, Diagram of 155 Generators and Motors, Relative Direction of Rotation in 187, 188 Generators, Classification of Continuous-Current ... 156 Generators, Dynamo Elec- tric, Classification of 156 Gilbert, Definition of . . .92, 95 Gradient, Hydraulic 15 Gradient, Potential 15 Gramme-Calorie, Definition of 193 Gramme-Ring Armature, Commutator 134 Gramme Ring Armature Dynamo- Electric Induc- tion of E. M. F. in 133 Graphite, Formation of, in Voltaic Arc ... 219 Gravity Daniell Voltaic Cell 7 7 Grenet Cell, E. M. F. of 73 Grenet Voltaic Cell 73, 74 Ground Plates, Telegraph- ic, Definition of 27 INDEX. 291 Ground-Return Circuit, De- finition of 28 Grove Voltaic Cell 75 Hand Rule, Fleming's 123 Hard Iron, Action of Mag- netic Flux on 113 Heat, Commercial Applica- tions of Electrically Gene- rated 194 Heater, Electric, General Construction of 195 Heater, Electric, Practical Efficiency of 196 Heating Effect of Alternat- ing Current 239 Heating Effect of Electric Current 3 Heating, Electric. ... 193 to 200 Heating, Electric, Efficien- cy of Distribution of 195 Hefner- Alteneck Lamp . . . 206 Horizontal Candle-Power for Arc Lamps 222 Horse -Power, Definition of 12 Human Body, Electric Re- sistance of 39 Hydraulic Gradient 15 Hysteresis, Magnetic 140 Hysteretic Cycle. ...... 142, 143 Hystere tic Diagrams. .142, 143 Hysteretic Loss in Dynamo 140 Illumination, Definition of 209 Illumination , Proposed Unit of 209 Impedance, Definition of.. 244 Impedance Factor of Alter- nating-Current Circuit.. 255 Impedance of Alternating- Current Circuit, Quanti- ties Entering into Value of 244 Incandescent Arc Light Circuits 228, 229 Incandescent Electric Lamp, Mounting of Fila- ment in 203 Incandescent Lamp, Ac- tivity Absorbed by 205 Incandescent Lamp Bases, Varieties of 204, 205 Incandescent Lamp, De- crease of Diameter of Filament Attending Use 210 Incandescent Lamp, Defini- tion of 201 Incandescent Lamp, Effect of Variation in Diameter of Filament on Efficiency of v .. 210 Incandescent Lamp, Exces- sive Non-luminous radia- tion of 197 Incandescent Lamp Fila- ment, Materials Employ, ed in Manufacture of 202 Incandescent Lamp, Incor- rect Statement of Effici- ency of 206 Incandescent Lamp, Lead- ing- in Wires of 202 Incandescent Lamp, Manu- facture of Filament for . . 201, 202 Incandescent Lamp, Steps in Manufacture of 201 Incandescent Lamp, Single- Pole Switch 213, 214 2 9 2 tNDEX. Incandescent Lamp Switch- es 213 Incandescent Lamps, Candle-Powers of 207, 208 Incandescent Lighting 201 to 216 Incandescent Lighting, Al- ternating-Current Cir- cuits for 212 I ncandescent Lighting , Series Circuits for 212 Incandescent Lighting , Systems of Distribution for 212 Induced Electromotive Force 121 to 128 Inductance of Alternating- Current Circuit 244 Inductance-Reactance .245, 246 Induction, Dynamo-Elec- tric 121 Induction, Magneto-Elec- tric 121 Induction, Mutual 122 Induction, Self- 121 Induction, Self-, Production of E. M. F. by 127, 128 Inductor Alternators.. 257, 258 International Ampere, De- finition of 43 International Unit of Activ- ity or Power, Definition of 12 International Unit of Work, Definition of u, 12 International Ohm 17, 18 Insulation, Apparent, of Telegraphic Line 29 Insulation, Average Appar- ent per mile of . . 29 Insulators, Definition of . . . 19 Insulators, Effect of Tem- perature on Resistivity of 22 Insulators, Varieties of 28 Intake of Machine, Defini- tion of 7 Iron, Action of Magnetic Flux on 113 Iron, Eddy-Current Losses in 139 Iron, Reluctivity of 104 Jablochkoff Candles 222 Joint Admittance.. 249, 250, 251 Joint Resistance 19 Joule, Definition qf u, 12 Joule per Second, Defini- tion of. . . 12 Kilovolt, Definition of u Kilowatt, Definition of .... 12 Kite, Franklin's 3 Lamp Chamber of Incan- descent Lamp, Exhaus- tion of 203 Lamp, Incandescent, De- finition of 201 Lamp, Life of 211 Lamp Rod, Clutch for 225 Lead of Commutator Brushes, Effect of, on Sparking 150, 151 Leading Polar Edge of Motor Armature 186 Leading-in Wires of Incan- descent Lamp 202 Leading in Wires of Incan- descent Lamp, Method Employed for Connect- ing with Filament. 202 INDEX. 293 Leclanche Voltaic Cell 78 Leclanche Voltaic Cell, E. M. F. Of 78 Lesser Calorie, Definition of 193 Life of Incandescent Lamp 211 Lifting Magnets for Arc Lamps, Shunt and Series Winding for 227 Line, Average Apparent Insulation per mile of ... 29 Lines of Magnetic Force, Definition of. 89 Losses, Eddy-Current 139 Losses of Dynamo 138 M. M. F 92 M. M. F.'S, Joint, or Op- posed, Action of 95 M. M. F., Relation of, to Am- pere Turns 94 Machine, Definition of Out- put of 7 Machine, Dynamo-Electric, Essential Parts of 129 Machine, Efficiency of .. .7, 8 Machine, Intake of, Defini- tion of 7 Machines, Dynamo-Elec- tric, Armatures of 130 Magnet, Polar Surfaces of 115 Magnets, Attractive 115 Magnets, Portative 114 Magnetic and Electric Re- sistances, Differences be- tween 97, 98 Magnetic Circuits, Compu- tation of Reluctance in . . 100, 101, 102, 103 Magnetic Effects of Elec- tric Currents 4 Magnetic Figures, Methods of Obtaining, 91, 92 Magnetic Flux 105 to 112 Magnetic Flux, Action of, on Hard Iron 113 Magnetic Flux, Action of, on Soft Iron 113 Magnetic Flux, Calculation of 106, 107 Magnetic Flux, Convention as to Direction of 88, 89 Magnetic Flux-Paths, De- finition of 89 Magnetic Flux, Properties of 89 Magnetic Force, Definition of Lines of 89 Magnetic Hysteresis 140 Magnetic Losses of Dynamo 138 Magnetic Needle, Explor- ing 92 Magnetic Reluctance. 97 to 104 Magnetic Reluctance, De- finition of 97 Magnetism, Permanent ... 114 Magnetism, Temporary ... 114 Magnetization, Influence of Direction of Current on Direction of 1 14 Magneto-Dynamics, Defini- tion of 161 Magneto-Electric Induc- tion 121 Magnetomotive Force.. 8 7 to 96 Magnetomotive Force, De- finition of 92 Magnetomotive Force, Di- rection of 95 Magnetomotive Force, Per- manent 92 INDEX. Magnetomotive Force, Structural 114 Magnetomotive Force, Transient 92, 93 Magnetomotive Force, Unit of 92 Matthiessen's Standard of Conductivity 34, 35 Maximum Candle-Power of Arc Lamps 222 Mean Spherical Candle - Power of Arc Lamp 220 Mechanical Losses of Dy- namo 138 Megawatt, Definition of . . . 12 Megohm, Definition of 20 Megohm-Mile 29 Metallic Circuit 28 Method of Reversing Elec- tric Motors 189 Mho, Definition of 25 Microhm, Definition of. . . 20 Microvolt, Definition of ... 1 1 Millivolt, Definition of 1 1 Mine Hoist, Electric 178 Mirror Galvanometer, Thomson Tripod Form of 46, 47 Molten Platinum Lamp or Violle 206 Momentary Electric Cur- rent 3 Momentary Electric Dis- charge 3 Monocyclic Currents 267 Monocyclic System, De- scription of 271, 272 Motion, Simple-Harmonic. 236 Motion, Simple-Periodic. .. 236 Motor and Generator, Dis- tinctive Features of Dif- ferences between 170 Motor- Armature , Defini- tion of Leading Polar Edge of 186 Motor-Armature, Follow- ing or Trailing Polar Edge of 186 Motor- Armature, Smooth Core, Use of 185 Motor-Armature, Toothed Core 185 Motor, c. E. M. F., of .... 170 Motor, Electric, Conditions for Constant Torque and Variable Speed in. . . 178, 179 Motor, Electric, Method of Reversing 189 Motor, Electric, Variable Torque and Variable Speed, Conditions for... 182 Motor, Torque in Armature of, Definition of 166 Motors and Generators, Relative Direction of Rotation in. 187, 188 Motors, Fleming's Hand Rule for. . . 163 Motors, Single-Reduction. . 190 Motors, Starting Rheostat for 190 Multiphase Currents 267 Multiple Circuit, Definition of 60, 61 Multiple Circuit, Resistance of 61 Multiple- Series Circuit, De- finition of 62, 63 Municipal-Series Circuit. . . 58 INDEX, 295 Mutual Induction 122 Mutual Induction, Produc- tion of E. M. F., by , 128 Negative Lead, Definition of 62 Negative Plate of Voltaic Cell 70 Negative Pole of Electric Source, Definition of .... 13 Negative Pole of Voltaic Cell, Definition of 69 Negative Resistivity, Tem- perature Coefficient of . 22 Non-Conductors, Definition of 19 Non-Ferric Magnetic Cir- cuit 100 Non-luminous Radiation, Excessive Amount of, in Incandescent Lamp. ... 197 Occluded Gases, Process for Exhaustion of 204 Ohm, International 17, 18 Ohm, Multiples and Sub- Multiples of 19 Ohm's Law 49 to 56 Ohm's Law, Application of, to Circuit Containing c. E. M. F 53, 54 Ohm's Law, Application of, to Divided Circuit 52 Ohm's Law, Application of , to Shunt Circuit 52, 53 Ohm's Law, Formula of. . . 50 Ohm's Law, Limitation of 54 Output, Critical, of Dyna- mo-Electric Machine . . 145 Output, Electrical 196 Output of Alternating-Cur- rent Transformers, Limi- tations of . . 275 , 276 Output of Dynamo 134 Output of Dynamo, Limita- tion of ,by Excessive Drop in Armature 145, 146 Output of Dynamo, Limita- tion of, by Excessive Heating 146, 147 Output of Dynamo, Limita- tion of, by Dangerous Sparking at Brushes. 148, 149 Output of Machine, Defini- tion of 7 Overload of Safety Fuse . . 200 Parallel Connection of Elec- tric Sources ... 61, 62 Parallel Connection of Shunt-Wound Genera- tors 157, 158 Parallel-Series Alternator.. 262 Peaked Type of Periodic E. M. F. or Alternating Current 235 Period, Definition of 238 Periodic Alternating E. M. F. or Current, Flat-Topped Type of 235 Periodic Alternating E. M. F. or Current, Peak -Topped Type of 235 Periodic Alternating E.M. F. or Current, Sinusoidal Type of 235 Permanent Magnetism .... 114 Permanent Magnetomotive Force 92 Phase, Definition of 238 INDEX. Poggendorff's Voltaic Cell. 73, 74 Polar Surfaces of Magnet.. 115 Polarization of Voltaic Cell 63 Polarization of Voltaic Cell, Effect of Resistance of . . 71 Polarization of Voltaic Cell, How Avoided 68 Pole, Positive, of Voltaic Cell 69 Poles of Electric Source, Definition of 13 Porous Cell, of Voltaic Cell 71 Portative Magnets 115 Positive Carbon of Voltaic Arc, Rate of Consump- tion of 219 Positive Lead, Definition of 62 Positive Plate of Voltaic Cell 70 Positive Pole of Electric Source, Definition of .... 13 Potential Difference in Cir- cuit, Distribution of . .51, 52 Potential, Difference of. ... 16 Potential Gradient 15 Power, Factor of Alternat- ing-Current Circuit 253 Power, Factor of Alternat- ing-Current Transform- ers 278, 279 Prime Flux 113 Prime Magnetomotive Force 113 Production of E. M. F. in Coil by Rotation in Earth's Flux 127 Pulsating E. M. p., Defini- tion of 14 Pulsatory Current, Defini- tion of 44 Pulsatory or Fluctuating Current 233 Quegohm, Denfinition of . . 20 Radiation, Physiologically Effective, of Arc Lamp. . 221 Reactance, Definition of. . . 245 Reactance Factor of Alter- nating Current Circuit . . 255 Reaction of Armature 118 Regulation of Dynamo . . . 153 to 160 Reluctance, Specific Mag- netic ; 99 Reluctivity, Curves of 99 Reluctivity of Iron 104 Reluctivity, Variation of, with Flux Density 99 Resistance, c. G. s. Unit of 18 Resistance Coils 31, 32 Resistance, Effective or Joint 23 Resistance, Electric. .17 to 48 Resistance Frame, Connec- tions of 30 Resistance, Joint 19 Resistance of Commercial Electric Apparatu s, Tables of 37, 38, 39 Resistance of Conductor, Circumstances Affecting 17 Resistance of Contacts.. 35, 36 Resistance of Copper Wires, Table of 35 Resistance of Human Body 39 Resistance of Multiple Cir- cuit. . 61 INDEX. 297 Resistance of Series Cir- cuits 25 Resistance of Series Cir- cuit 59 Resistance, Specific, Defini- tion of 20 Resistance, Unit of 17, 18 Resistance, Variable. . . .29, 30 Resistivity, Average, of Earth's Crust 27 Resistivity, Definition of.. 20 Reluctivity, Definition of . . 98 Resistivity of Conductors, Effect of Slight Impur- ities on 20 Resistivity of Conductor, Effect of Temperature on 20 Resistivity of Insulators, Effect of Temperature on 22 Resistivity, Table 21 Resistivity, Temperature Coefficient of Conductor. 21 Reversibility of Dynamo and Motor 169 Return Circuit, Ground, Definition of 28 Rheostat, Carbon 36 Rheostat, Starting, for Elec- tric Motors 190 Ring- Wound Armature. ... 130 Rotation, Relative Direc- tion of, in Generators and Motors 187, 188 Rule for Determining Di- rection of E. M. F. Induced in Rotating Loop 132 Safe Carrying Capacity of Copper Wires 199 Safety Device of Series- Connected Circuit 59 Safety Fuse, Overload of. . 200 Search Lights, Use of Large Arcs in 218 Self-Excited Alternators. . . 263 Self-induction 121 Self -Induction, Production of E. M. F. by 127, 128 Self Regulating Character of Alternating-Current Transformer 274 Semi-Period, Definition of 238 Separately-Excited Alter- nators 263 Series Arc Circuits, Switch- board for 230 Series Circuit, Definition of 58, 59 Series Circuit, Municipal, for Incandescent Lamps 59 Series Circuit, Resistance of 59 Series Circuits for Incan- descent Lighting 212 Series Circuits, Resistance of 25 Series-Connected Circuit, Safety Device of 59 Series-Connected Electric Sources 59, 60 Series-Connection of Alter- nators 241 Series- Connection of Arc Lamps. 227, 228 Series- Wound Alternators . 262 Series -Wound Generator, Diagram of 1 54 Shunt Circuit, Application of Ohm's Law to 52, 53 TJKI7BRSITY 298 INDEX. Shunt Magnet, Use of, in Arc Lamp 226 Shunt Wound Generator, Diagram of 155 Shunt-Wound Generators, Parallel Connection of, 1 5 7, 158 Silver Chloride Voltaic Cell 79, 80 Silver Chloride Voltaic Cell, E. M. F. of So Simple-Harmonic E. M. F. . . 236 Simple-Harmonic Motion.. 236, 237 Simple-Periodic Current. . . 236 Simple-Periodic E. M. F 236 Simple-Periodic Motion,237, 236 Single-Pole Switch for In- candescent Light ,..213, 214 Sinusoidal Alternators,Con- nection of, in Quadrature 243, 244 I Sinusoidal Type of Peri- odic E. M. F. or Alternat- ing Current 235 Sizes of Incandescent Lamps 207, 208 Skin Effect, Definition of . . 198 Skin Effect, Effect of Fre- quency on 256 Smooth-Core Motor Arma- ture 185 Smooth-Core Motor Arma- ture, Effect of Eddy Cur- rents on 185, 186 Soft Iron, Action of Mag- netic Flux on 113 Source, Definition of Posi- tive Pole of 13 Source, Electric, Definition of 13 Source, Negative Pole of, Definition of 13 Sources Electric, Classifi- cation of 13 Sources, Electric, Defini- tion of Poles of, 13 Sources, Electric, Parallel connection of 61, 62 Sparking of Commutator, Effect Lead of Brushes on 150, 151 Specific Heat, Definition of 193 Specific Magnetic Reluc- tance 99 Specifications for Incan- descent Lighting 215 Standard Candle, Defini- tion of 206 Star-Triphase Winding 270 Steady E.M.F., Definition of 14 Standard Conductor, Defi- nition of 37 Structural Magnetomotive Force 114 Surfaces, Disimilarity of, Effect of, on Electrication 2 Switchboard for Series Arc Circuits 230 Switches, Incandescent Lighting 213 Symmetrical E. M. F., Defi- nition of 14 System, Astatic 48 System, Monocyclic, Des- cription of . .. 271, 272 System, Three- Wire 63 Table of Resistance of Cop- per Wires 35 Table of Resistivity 21 INDEX. 299 Table of Varying Strengths of Commercial Currents. 45 Tables of Resistance of Commercial Apparatus. . 37, 38, 39 Telegraphic Ground Plates, Definition of 27 Telephone Transmitter 36 Temperature Coefficient of Resistivity of Conductor. 2 1 Temperature Coefficient of Resistivity, Negative 22 Temperature Effect on Re- sistivity of Insulators. ... 22 Temperature Elevation in Dynamo Electric Machine 148 Temperature Elevation, Time Required for Maxi- mum, in Insulated or Covered Conductors 198 Temporary Magnetism. ... 114 Terminal, Negative, of Vol- taic Cell 69 Terminal, Positive, of Vol- taic Cell 69 Therm 193 Thomson Mirror Galvano- meter 47 Thomson Mirror Galvano- meter,TripodFormof,46, 47 Three-Wire System 62 Toothed-Core Motor Arma- ture 185 Toothed Dynamo Armature 152 Torque of Motor Armature, Definition of 166 Total Conductance, Value of 23 Transformer, Alternating- Current, Construction of 273 Transient Magnetomotive Force 92, 93 Transmitter, Telephone. .. 36 Tregohm, Definition of. ... 20 Triangle-Triphase Winding 270 Triphase Currents 267 Triphase E.M p., Diagram of 270 Triphase E. M. F.'S 269, 270 Triphase Generator, Star Winding of 270 Unit of Magnetomotive Fcrce 92 Unit of Resistance 17, 18 Unit of Work,International, Definition of . n, 12 UnitQuantity of Electricity 42 Unit Quantity of Heat, Names for 193 Unit Rate of Flow of Elec- tricity 43 Universal Ether 2 Variable Resistances . ..29, 30 Ventilation of Dynamo Ar- mature 148 Violle or Molten Platinum Lamp 206 Volt or Unit of E. M. F. . . . 1 1 Voltaic Arc, Counter-Elec- tromotive Force of .. .. 218 Voltaic Arc, Definition of. . 217 Voltaic Arc, Development of Counter-Electromotive Force in 218 Voltaic Arc, Formation of Graphite in 219 Voltaic Cell 65 to 88 Voltaic Cell, Bichromate, 73, 74 300 INDEX. Voltaic Cell, Effect of Po- larization on Resistance of 71 Voltaic Cell, Fields of Use- fulness for 87, 88 Voltaic Cell, Grenet .... 73, 74 Voltaic Cell, Phenomena Attending Action of 67 Voltaic Cell, Polarization of 68 Voltaic Cell, Positive Pole of 69 Voltaic Cell, Source cf En- ergy in 67 Voltaic Cells, Classification of 68 Voltaic Cells, Single-Fluid 69 Voltaic Couple 66 Voltaic Element 66 Voltmeter, Definition of , 5 1 , 52 Water - Gramme - Degree - Centigrade 193 Water-Motive Force, Orig- in of 16 Watt, Definition of 12 Waves, Sinusoidal, of E.M.F. or Current, Harmonics of 260 Weston Ammeters .... 46 Wheatstone's Balance... 31, 32, 33, 34 Wheatstone's Bridge. .. .31, 32, 33, 34 Winding, Star-Triphase... 270 Winding,Triangle-Triphase 270 Work, Defininition of 5 ERRATA. Page 45, ^ 53, New York City Lighting. For 50 read 60 kilo amperes. " 48, Syllabus. For 11.16 read 0.1116. " " 52, ^ 59. For 5, 10, 5 read 50, 100, 50. ~ " 79, ^ 88. For 0.007 read 0.07. - *' 120, Syllabus. For 5.771 X 10~ 7 read 5.771 (fc 8 X 10~ 7 . - DBI7BRSITT