UC-NRLF 
 
 ELECTRO-TECHNICAL 
 SERIES 
 

 
Digitized by the Internet Archive 
 
 in 2007 with funding from 
 
 Microsoft Corporation 
 
 http://www.archive.org/details/electricityineleOOhousrich 
 
BY THE SAME AUTHORS 
 
 Elementary Electro -Technical Series 
 
 COMPRISING 
 
 Alternating Electric Currents. 
 Electric Heating. 
 
 Electromagnetism. 
 
 Electricity in Electro-Therapeutics. 
 Electric Arc Lighting. 
 Electric Incandescent Lighting. 
 Electric Motors. 
 
 Electric Street Railways. 
 Electric Telephony. 
 
 Electric Telegraphy. 
 
 Cloth, Price per Volume, $1.00. 
 
 Electro-Dynamic Machinery. 
 Cloth, $2.50. 
 
 THE W. J. JOHNSTON COMPANY 
 
 253 Broadway, New York 
 
ELEMENTARY ELECTRO-TECHNICAL SERIES 
 
 ELECTRICITY 
 
 IN 
 
 ELECTRO-THERAPEUTICS 
 
 BY 
 
 EDWIN J. HOUSTON, Ph. D. 
 
 AND 
 
 A. E. KENNELLY, Sc. D. 
 
 new Tea ^^Lll!_i.^.^ i 
 
 THE W. J. JOHNSTON COMPANY 
 
 253 Broadway 
 
 1896 
 
Copyright, 1896, by 
 THE W. J. JOHNSTON COMPANY. 
 
CONTENTS. 
 
 R/n<n\ 
 
 
 CHAPTER 
 
 I. Introductory, 
 
 II. Electromotive Force, 
 
 III. Electric Resistance, 
 
 IV. Electric Current, 
 
 V. Varieties of Electromotive Force, 107 
 
 VI. Electric Work and Activity, 
 VII. Frictional and Influence Ma 
 
 chines, .... 
 VIII. Magnetism, . . 
 IX. Induction of E. M. F. by Magnetic 
 Flux, .... 
 
 X. The Medical Induction Coil, 
 
 iii 
 
 1 
 13 
 63 
 
 80 
 
 124 
 
 138 
 184 
 
 221 
 
 248 
 
IV CONTENTS. 
 
 CHAPTER PAGE 
 
 XL Dynamos, Motors and Trans- 
 formers, 299 
 
 XII. High Frequency Discharges, . 329 
 
 XIII. Electrolysis and Cataphoresis, 356 
 
 XIV. Dangers in the Therapeutic Use 
 
 of Electricity, . . .365 
 
 Index, 373 
 
PREFACE 
 
 This little book, entitled Electricity 
 in Electro-Therapeutics, is intended to 
 meet a growing demand which exists not 
 only on the part of general medical practi- 
 tioners, but also on that of the general 
 public, for reliable information respecting 
 such matters in the physics of electricity 
 applied to Electro-Therapeutics, as can be 
 readily understood by those not specially 
 trained in electro-technics. 
 
 Electricity has recently made such rapid 
 strides in application to both surgical and 
 medical practice, that recent information, 
 concerning electrical developments in ap- 
 
VI PREFACE. 
 
 paratus and in theory, is much in request 
 by those interested in the healing art. 
 
 The method of treatment adopted 
 throughout the book in the description of 
 electro-technics has been the circuital 
 method ; that is to say, all the phenomena 
 of electricity and magnetism have been 
 considered as pertaining either to the 
 electro-static, the electric, or the magnetic 
 circuit, and the laws of these three circuits 
 have been developed upon analogous lines. 
 The authors believe that this treatment is 
 the key-note to a clear comprehension of 
 the numerous and often complex electro- 
 magnetic phenomena met with in the 
 application of electricity to electro-thera- 
 peutics. 
 
 In thus aiding the general public to read- 
 ily comprehend the principles underlying 
 the physics of electro-therapeutics, the 
 authors trust that they are aiding the gen- 
 
PREFACE. Vll 
 
 eral cause of humanity in enabling electricity 
 to be employed more intelligently in the 
 healing art, as well as permitting truth to 
 be discerned from fraud. 
 
 The authors therefore present this book 
 to the public in the hope that it may 
 prove serviceable. 
 
ELECTRICITY IN ELECTRO- 
 THERAPEUTICS. 
 
 CHAPTER I. 
 
 INTRODUCTORY. 
 
 It has not infrequently happened in the 
 history of scientific discovery, that from 
 various causes, the discoverer has obtained 
 so incomplete a view of the new fact, as to 
 entirely lose sight of its true significance, 
 and to regard it merely from the stand- 
 point of some of its unimportant char- 
 acteristics. This was the case, in the cele- 
 brated discovery made by Luigi Galvani, 
 in 1786, concerning the existence of what 
 he at first believed to be the vital fluid, or 
 
2 ELECTRICITY IN 
 
 essence of animal vitality, but which was 
 afterwards proved by Volta to be essen- 
 tially a new method of producing elec- 
 tricity by chemical action. 
 
 Galvani discovered that if the hind legs 
 of a recently killed frog be deprived of 
 
 Fig. 1.— Galvanoscopic Frog. 
 
 their integument, and the lumbar nerves, 
 suitably exposed in their position on either 
 side of the vertebral column, be connected 
 with the crural muscles by a metallic strip, 
 as shown in Fig. 1, that these muscles 
 will be brought into a spasmodic activity 
 
ELECTRO-THERAPEUTICS. 6 
 
 closely resembling their action during life. 
 It has been alleged that this classic experi- 
 ment of Galvani was the result of chance ; 
 that he had prepared some frogs' legs for 
 supper, and, happening to hang them 
 against an iron balcony, he noticed that 
 the muscles twitched as soon as they 
 touched the iron; that is, went convul- 
 sively through their motions as in life, and 
 that these motions were repeated when- 
 ever the contact was renewed. Moreover, 
 the power of producing these convulsive 
 movements was retained by the limbs for 
 an hour or more after removal from the 
 body. This account would seem improb- 
 able, since Galvani was well aware of 
 the fact that an electric discharge, sent 
 through the legs of a recently killed frog, 
 would produce convulsive movements in 
 them; and, indeed, he was in the habit of 
 employing such legs as a form of sensitive 
 
4 ELECTRICITY Iltf 
 
 galvanoscope, though, of course, Gal van i 
 did not know it under this name, but he 
 did know that it formed a much more 
 sensitive apparatus for detecting an electric 
 current than the pith-ball electroscope em- 
 ployed in those early days, as almost the 
 only available means for detecting an 
 electric charge. 
 
 Unfortunately, Galvani failed to rec- 
 ognize the extreme importance of his dis- 
 covery. Working, as he had been for a 
 long time, with the hope of discovering the 
 seat of animal vitality, he was only too 
 willing to find in this observation the 
 principle of that vital fluid for which he 
 so long and ardently had sought. He was, 
 therefore, handicapped in the search, and 
 unfitted, to a certain extent, to weigh 
 calmly the evidence presented. Though 
 thoroughly familiar with the convulsive 
 
ELECTRO-THERAPEUTICS. O 
 
 twitchings produced by the passage of the 
 electric discharge through the frog's legs, 
 it never seemed to occur to hi in that what 
 he had in reality discovered, was an en- 
 tirely new method of producing electric 
 discharges. He only saw in this observation 
 what he so ardently desired to see ; namely, 
 convulsive muscular movements, due to a 
 vital fluid, which, he believed, came from 
 the nerve of the animal, and was conveyed 
 through the metallic conductor to the 
 muscles, where it produced the char- 
 acteristic twitchings. 
 
 The announcement by Galvani of his 
 discovery, produced the most intense 
 excitement throughout the scientific world, 
 and his views as to the cause of the 
 phenomenon were at first generally 
 accepted. Among, perhaps, the most 
 ardent of his early followers was 
 
b ELECTRICITY IN 
 
 Alessandro Volta, who at once repeated 
 Galvani's experiments, and began a series 
 of extended researches on the phenomena. 
 Volta soon reached the conclusion that the 
 twitchings of the legs of Galvani's frogs 
 were to be ascribed, not to a vital fluid, 
 but to the presence of an electric dis- 
 charge, and that, consequently, sight had 
 been lost of the most important part of 
 Galvani's experiment ; namely, that it fur- 
 nished a new method of producing elec- 
 tricity. 
 
 Volta showed, among other things, that 
 the convulsive movements were more pro- 
 nounced when the nerves were connected 
 with the muscles by two dissimilar metals, 
 instead of by a single metal, and ascribed 
 the cause of the electricity produced as 
 the contact of dissimilar substances. In 
 the light of more modern scientific dis- 
 
ELECTRO-THERAPEUTICS. 7 
 
 covery, it would appear that both dis- 
 coverers saw but a partial truth, although 
 Volta had undoubtedly a more complete 
 grasp of the phenomena than Galvani. 
 Galvani observed the convulsive move- 
 ments, but improperly attributed them to 
 the presence of a vital fluid. Volta cor- 
 rectly ascribed the cause of the movements 
 to the passage of electricity, but incor- 
 rectly ascribed the cause of the continuous 
 supply of electric current to the contact 
 of dissimilar substances. Modern research 
 has shown that contact alone is unable to 
 account for the continuous production of 
 an electric current, and that such a dis- 
 charge occurs only under circumstances 
 when chemical actions take place. 
 
 In endeavoring, at the present time, to 
 gauge the value to the world of the dis- 
 coveries of these two pioneer investigators, 
 
8 ELECTRICITY IN 
 
 a tendency may exist to give, too unre- 
 servedly, the award to Volta, on the plea 
 that the result of his investigations pro- 
 duced, some ten years later, the great dis- 
 covery of the Voltaic pile, a discovery 
 which has done so much for the world's 
 progress, but it must not be forgotten 
 that it was the original observations of 
 Galvani, aided, it is true, largely by his 
 afterwork in the same field, that first 
 called attention to the wonderful effects 
 which electricity produces on the animal 
 organism, and if to-day electro-therapeu- 
 tics, or the application of electricity for 
 the restoration of the healthy condition 
 of the body, is an actual power, the 
 beneficial effects of which are apparently 
 being more and more clearly established 
 every year, it is undoubtedly to Galvani that 
 the guerdon of the discovery must be 
 awarded. 
 
ELECTRO-THERAPEUTICS. 9 
 
 Without attempting to trace the history 
 of the extended experiments that were 
 made on Galvani's original observation, 
 experiments that have continued up to the 
 present day, and without stopping to con- 
 sider the extended and bitter controversy 
 that was waged between the disciples of 
 Galvani on one side, and those of Volta on 
 the other, as to the cause of the phenomena, 
 or the equally extended controversy which 
 existed as to the origin of the electric 
 current produced in the voltaic pile, or cell, 
 it will suffice, for our present purpose, to 
 consider animals as electric sources, and 
 the effects produced by the passage of 
 electricity through animals. These can be 
 briefly summarized as follows ; viz., 
 
 (1) That the body of an animal is, in 
 itself, the seat of electric currents. 
 
 (2) That these currents exist not only 
 during the abnormal or diseased condition 
 
10 ELECTRICITY IN 
 
 of different parts of the body, but also in 
 the normal condition of the body. 
 
 (3)^ That electric discharges, when sent 
 through the body of an animal, are cap- 
 able of producing marked effects therein, 
 the character of which depends upon 
 the nature of the discharge. 
 
 (4) That the passage of an electric dis- 
 charge through a nerve, muscle, or indeed 
 through any organ of the body of an ani- 
 mal, produces an alteration in its functional 
 activity. 
 
 (5) That a sufficiently powerful dis- 
 charge through the body of an animal may 
 produce death. 
 
 The effects of electricity on the human 
 body have been very generally recognized 
 since the time of Galvani, and it is gener- 
 ally believed that electricity possesses 
 powerful remedial properties ; but, like 
 
all remedra^ agencies, unless lnteilige 
 applied, it^^f^groduce more ha^rti^^m 
 good. That ma^J^^Ms r T^en^«h5ne by 
 its improper use there can be no doubt. 
 Indeed, one of the results of such misuse 
 has been even at the present time to 
 greatly retard its general introduction. 
 Much of this difficulty has arisen from 
 want of a general knowledge of the funda- 
 mental laws underlying the production 
 and action of electricity and magnetism. 
 As a result of a lack of such knowledge, 
 extravagant and ridiculous claims are fre- 
 quently made as to the wonderful curative 
 powers of certain apparatus, alleged to 
 produce electric currents ; apparatus that 
 even a tyro in electricity would at once be 
 able to show are incapable of producing 
 any current whatever. 
 
 A knowledge of the laws of electricity 
 
12 ELECTRICITY. 
 
 and magnetism will, therefore, not only 
 enable the general public to detect elec- 
 tro-therapeutic frauds, but will also tend 
 to increase confidence in legitimate elec- 
 tro-therapeutic applications, a confidence 
 justly merited by experience. 
 
 Since both electricity and magnetism 
 are exact sciences, their application to the 
 art of electro-therapeutics cannot fail to be 
 of benefit in scientific treatment. 
 
CHAPTER II. 
 
 ELECTROMOTIVE FOECE. 
 
 Although we are ignorant of the exact 
 nature of electricity, yet it is by no means 
 true that we are ignorant of the laws 
 under which electricity operates. In 
 other words, our ignorance relates to the 
 exact nature of the electric force rather 
 than to its laws. In the physical world we 
 can properly claim knowledge concerning a 
 force, when we are able to predict its 
 action under given conditions. Gauged in 
 this way, our knowledge of electricity is 
 both extensive and accurate, since it is 
 possible to fairly predict just what will 
 happen under a great variety of electric 
 conditions. 
 
 13 
 
14 ELECTRICITY IN 
 
 It will be generally acknowledged, how- 
 ever, that we certainly know this much 
 about electricity ; viz., that it cannot be 
 regarded as a form of matter ; at least 
 not matter in the ordinary sense of the 
 word. All matter with which we are 
 acquainted, exercises gravitational influ- 
 ence, that is to say, tends to attract other 
 matter towards it. No such tendency has 
 yet been shown to exist in the case of 
 electricity. But ordinary matter is not 
 the only material with which we are sur- 
 rounded. The entire universe is believed 
 to be pervaded by a highly tenuous 
 medium called the ether, which transmits 
 light, heat and gravitation. 
 
 As frequent reference will be made in 
 this book to the existence and properties 
 of the ether, it may be well to explain the 
 nature of the evidence which has con- 
 
ELECTRO-THERAPEUTICS. 15 
 
 vinced scientific men of its existence. We 
 know that the effects produced by a 
 sounding body, such, for example, as a 
 vibrating bell, are transmitted across the 
 space between the bell and the observer's 
 ear, by means of waves, or to-and-fro 
 
 Fig. 2.— Transmission of Sound through a Vacuum. 
 
 motions produced in the medium existing 
 between the bell and the ear. This 
 medium is usually the air. If a bell, 
 placed inside a glass vessel, as shown in 
 Fig. 2, be set vibrating, it can be heard by 
 an observer at some distance, since the air 
 the vessel contains transmits the vibrations 
 to the sides of the globe, which, in their 
 
16 ELECTRICITY IN 
 
 turn, transmit the vibrations to the external 
 air, and so to the observer's ear. But if the 
 vessel be exhausted ; i. e., deprived of its 
 air, the sound will no longer be trans- 
 mitted, since the medium is then removed 
 which carries its vibrations. If, however, 
 a similar experiment be tried with a hot 
 body, it will be found that the existence 
 of such a "medium as air is not essential to 
 permit the body to transmit its heat across 
 intervening space. For example, if, as in 
 Fig. 3, two reflectors A and B, be placed 
 inside the glass receiver of an air-pump, 
 and a delicate thermometer T y be suitably 
 supported at the focus of one of these 
 reflectors, and a platinum wire be placed 
 at the focus of the other reflector, then, 
 if an electric current be sent through the 
 platinum wire, of such strength as to ren- 
 der it incandescent, the heat radiated from 
 the wire will be reflected successively from 
 
ELECTRO-THERAPEUTICS. 
 
 17 
 
 A and J?, and be focused on the ther- 
 mometer, which will immediately indicate 
 an increased temperature, and this effect 
 will occur, whether the receiver contains 
 
 ■**#* 
 
 %3B^ 
 
 p* 
 
 Fig. 3. Transmission of Heat through a Vacuum. 
 
 air or is exhausted. Evidently, the pres- 
 ence of a gross medium like air is not 
 essential to the transmission of radiant 
 heat, and in this respect differs from the 
 transmission of sound just referred to. 
 Moreover, the light emitted by the glow- 
 
18 Electricity in 
 
 ing platinum wire is also transmitted 
 through the empty space in the globe and 
 renders the wire visible. 
 
 A little reflection will show that the 
 preceding experiment is not, in reality, 
 needed to prove the possibility of radiant 
 light and heat being readily transmitted 
 across space devoid of ordinary matter ; for, 
 radiant light and heat reach the earth from 
 the sun, and from the fixed stars, across 
 space existing between the earth and the 
 heavenly bodies, which space we believe to 
 be devoid of ordinary matter. In the early 
 history of physical science this fact led to 
 the belief that light and heat were effects 
 produced by specific fluids or effluvia ; 
 that a hot body sent off a specific efflu- 
 vium which constituted heat ; and, in a 
 similar manner, a luminous body sent off a 
 specific effluvium constituting light. The 
 
ELECTRO-THERAPEUTICS. 19 
 
 particular effluvium in the case of heat 
 received the name of caloric, a term 
 which, unfortunately, even to-day, is still 
 loosely employed in the science of heat. 
 Without entering into minute details, it 
 suffices to say that the theory which 
 ascribes light or heat to the existence of 
 special fluids is now considered absolutely 
 untenable. Light and heat, like sound, 
 are believed to be produced by vibrations, 
 or to-and-fro motions. A medium, there- 
 fore, is necessary to carry the vibrations 
 of light and heat, and, consequently, the 
 highest vacuum which can be obtained is, 
 for this reason, believed to be filled with 
 ether, that is to say, the ether is not 
 pumped out of a reservoir by the action 
 of the air-pump. 
 
 The ether is sometimes called the lumi- 
 niferous ether because it transmits the 
 
20 ELECTRICITY IN 
 
 vibrations of light. It is also called the 
 universal ether because it is supposed to 
 exist everywhere. Even in the densest 
 of solid bodies it is assumed to exist, 
 between the ultimate particles, of which 
 such bodies are formed ; namely, between 
 the atoms and the molecules. 
 
 Electricity and magnetism, like heat and 
 light, are also capable of manifesting their 
 influence through the air-pump vacuum. 
 For example, the filament of an incandes- 
 cent lamp, which, as is well known, is 
 placed in a very high vacuum, can, never- 
 theless, be deflected by the electric 
 attraction produced by a charged body 
 A, Fig. 4, at some distance from the 
 lamp. Here the electric attraction tra- 
 verses the apparently empty space sur- 
 rounding the filament, and, consequently, 
 must have acted through the ether which 
 
Fig. 4. — Transmission of Electrostatic Force 
 Across a Vacuum. 
 
 is believed to fill the exhausted lamp 
 chamber. 
 
 In a similar manner, magnetic attraction 
 is capable of acting through empty space ; 
 
22 
 
 ELECTRICITY IN 
 
 for, if, as in Fig. 5, an incandescent electric 
 lamp be brought near a powerful magnet, 
 when a continuous current is passing 
 
 Warn ^HI^^^^^^^^Hh 
 
 Fig. 5. — Transmission of Magnetic Force Across 
 a Vacuum. 
 
 through the filament, the filament has 
 thereby acquired magnetic properties, and 
 a deflection of the filament will take place, 
 
ELECTRO-THERAPEUTICS. 23 
 
 although no medium, save the ether, can 
 apparently convey this influence within 
 the chamber. Here, as before, this par- 
 ticular experiment is not necessary to 
 show that magnetic influence can traverse 
 apparently empty space ; for, during the 
 prevalence of an unusual number of spots 
 on the surface of the sun, there are pro- 
 duced marked disturbances on delicately 
 suspended compass needles on the earth. 
 This influence is apparently transmitted 
 through the ether which we believe Alls 
 interstellar space. 
 
 It is interesting to note in this connec- 
 tion, that the early views concerning the 
 nature of electricity regarded it as a fluid 
 or fluids, just as heat and light were 
 originally regarded. It is now the belief, 
 however, that electricity and magnetism 
 are phenomena connected with some active 
 
24 ELECTRICITY IN 
 
 condition of the ether. For example, light 
 is almost universally regarded as being 
 transmitted by a particular transverse vi- 
 bration of the ether, and some particular 
 forms of disturbances in the ether are also 
 believed to be the causes of electric and 
 magnetic phenomena. 
 
 Before, however, discussing at greater 
 length the nature of electricity, let us con- 
 sider a well-known electric source, as, 
 for example, the dynamo-electric machine, 
 such as is used for generating electric cur- 
 rents for arc or incandescent lights. Here, 
 popularly, the machine is spoken of as pro- 
 ducing electricity. Strictly speaking, how- 
 ever, it is not electricity which the machine 
 primarily produces. The machine produces 
 a variety of force capable of starting, or 
 producing, an electric current under suit- 
 able conditions. This force produced by 
 
ELECTRO-THERAPEUTICS. 25 
 
 the dynamo is termed electromotive force, 
 or the force which tends to set electricity 
 in motion, so as to cause an electric now. 
 It is essential to bear in mind that in no 
 case can electricity be produced by any 
 machine without the prior production of 
 electromotive force, just as no motion can 
 exist in any material object without the 
 antecedent application of a material force ; 
 i. e. y of a body-moving force. Whenever, 
 therefore, electricity is produced, no matter 
 what the nature of the machine, or source 
 producing it may be, the machine or 
 source must necessarily first produce an 
 electromotive force, and this electromotive 
 force in its turn will or will not produce 
 electricity, according to the conditions 
 under which it acts. Electromotive force 
 is usually abbreviated E. M. F. 
 
 Various devices are employed in prac- 
 
ELECTRICITY IN 
 
 tice for the production of E. ML F. Such 
 devices may be classified as follows : 
 
 (1) Those produced by chemical action ; 
 such as a voltaic cell or primary cell, and 
 a charged storage cell or secondary cell. 
 
 (2) Those produced by the action of 
 radiant energy ; i. e., radiant light or heat ; 
 such, for example, as a thermo-electric cell. 
 
 (3) Those produced by the action of 
 mechanical energy ; such, for example, as 
 a dynamo-electric machine, a frictional 
 machine, an electrostatic induction machine, 
 or a liquid flowing through a capillary 
 tube. 
 
 (4) Those produced by vital energy, 
 such as an animal or a plant regarded as an 
 electric source. 
 
 As already stated, in any of the preced- 
 ing sources it is E. M. F. which is pri- 
 marily produced. 
 
ELECTRO-THERAPEUTICS. 27 
 
 Take, for example, a form of voltaic cell, 
 shown in Fig. 6, known as the Leclanche 
 
 Fig. 6.— Leclanche Voltaic Cell. 
 
 cell. Here it is the chemical energy of 
 combination between the zinc plate A, and 
 the solution of ammonium chloride, which 
 enables the electric current to be sustained, 
 but the cell always produces an E. M. F., 
 
28 ELECTRICITY IN 
 
 although it will not supply an electric cur- 
 rent until its terminals A and B, are con- 
 nected together by means of a conductor 
 or external circuit. 
 
 In order to measure the E. M. F. of 
 any electric source, a unit of E. M. F. 
 has been internationally adopted. This 
 unit is called the volt, after Alessandro 
 Volta, the inventor of the voltaic cell. 
 The E. M. F. of the Leclanche cell shown 
 in Fig. 6, is, approximately, 1\ volts, 
 and that of the ordinary blue-stone cell, 
 as shown in Fig. 7, is about one volt. 
 
 In both the Leclanche and the blue- 
 stone voltaic cells, it will be observed that 
 there are two metallic substances immersed 
 in a liquid. For example, in the Le- 
 clanche cell, shown in Fig. 6, the two sub- 
 stances are carbon and zinc, and the solu- 
 
ELECTRO-THERAPEUTICS. 29 
 
 tion in which they are plunged is an aque- 
 ous solution of sal-ammoniac. In the blue- 
 stone, or gravity cell, shown in Fig. 7, the 
 
 Fig. 7.— Bluestone or Gravity Voltaic Cell. 
 
 two metals are zinc and copper, marked 
 Zn and On, but here there are two sepa- 
 rate exciting liquids ; namely, a dense solu- 
 tion of copper sulphate, which occupies the 
 lower part of the cell, and a lighter solu- 
 tion of zinc sulphate, which surrounds the 
 
30 ELECTRICITY IN 
 
 zinc plate and floats upon the copper sul- 
 phate solution. All voltaic cells may be 
 divided into two general classes ; namely, 
 
 (1) The single-fluid cells, or those which 
 have a single exciting fluid ; and, 
 
 (2) The double-fluid cells, or those 
 which, like the bluestone cell, have two 
 exciting fluids. 
 
 Every voltaic cell, whether of the 
 double- or single-fluid type, consists of two 
 essential parts ; namely, 
 
 (1) Of a voltaic pair or voltaic couple, 
 consisting of two dissimilar electrically con- 
 ducting substances. 
 
 (2) Of an exciting liquid called the elec- 
 trolyte, capable of conducting electricity, 
 and of being decomposed by it. The 
 double-fluid cells have two liquids or 
 electrolytes. The two substances forming 
 a voltaic pair or couple, are called the 
 
ELECTRO-THERAPEUTICS. 31 
 
 elements of the cell. Voltaic elements are 
 generally made in the form of plates or 
 rods, and are known respectively as the posi- 
 tive and the negative plates or elements. 
 
 During the action of a voltaic cell, that 
 is, while it is furnishing electric current 
 to the circuit connected with it, a chemical 
 action takes place between one or both of 
 the electrolytes and one of the plates. 
 The result of this action is that one of the 
 plates, the positive, is gradually dissolved, 
 or enters into chemical combination with 
 part of the electrolyte, the other plate 
 remaining unacted on. In nearly all forms 
 of voltaic cells, there results from this 
 decomposition a tendency to liberate hydro- 
 gen at the surface of the negative plate, or 
 the plate which is unacted on. If hydro- 
 gen be permitted to be liberated on the 
 surface of the negative plate, a marked 
 
32 ELECTRICITY IN 
 
 decrease occurs in the ability of the cell 
 to furnish current, for reasons which will 
 be pointed out hereafter. 
 
 In single-fluid cells no provision is made 
 to prevent the evolution of hydrogen at 
 the negative plate ; or, as it is generally 
 called, the polarization of the negative 
 plate. In the double-fluid cell the second 
 fluid consists of a substance which sur- 
 rounds the negative plate, and is provided 
 for the express purpose of entering into 
 combination with the hydrogen and so 
 preventing its being liberated. There are 
 some forms of voltaic cells which are ap- 
 parently single-fluid cells, since they possess 
 but a single fluid, or electrolyte, but which 
 properly come under the type of double- 
 fluid cells, since they are uon-polarizable, 
 being provided with a solid substance in 
 contact with the negative plate, that is 
 
ELECTRO-THERAPEUTICS. 33 
 
 capable of combining with hydrogen and 
 thereby preventing its liberation. For 
 example, in the Leclanche cell, shown in 
 Fig. 6, the elements of the voltaic couple are 
 zinc and carbon, immersed in a solution of 
 sal-ammoniac in water, but the carbon is 
 surrounded by granulated, solid peroxide 
 of manganese, which possesses the power 
 of readily entering into combination with 
 hydrogen. 
 
 A great variety of conducting sub- 
 stances are employed in pairs for the 
 couples of voltaic cells. The most impor- 
 tant of these, however, are zinc, carbon, 
 copper, lead, silver, and platinum. Of 
 these substances, zinc in nearly all cases 
 forms the positive element ; that is to 
 say, it forms a voltaic couple either 
 with carbon, copper, lead, silver, or 
 platinum. 
 
34 ELECTRICITY IN 
 
 When a voltaic cell Las its circuit closed, 
 for example, when the zinc-carbon couple 
 shown in Fig. 6, is connected to an exter- 
 nal circuit, the E. M. F. it produces causes 
 a current to flow through such circuit. 
 For purposes of convenience it has been 
 agreed, conventionally, to regard the electric 
 current as leaving a voltaic cell at a par- 
 ticular point, and, after passing through 
 the circuit, to re-enter it at another point. 
 The point at which the current is conven- 
 tionally assumed to leave the cell is called 
 its positive pole, and the point at which 
 it is assumed to re-enter it, after having 
 passed through the circuit, the negative 
 pole. The positive pole of the cell is the 
 pole connected with the plate which is not 
 acted on, that is with the negative plate, 
 while the negative pole is the pole con- 
 nected with the plate which is chemically 
 acted on, or the positive plate. For 
 
ELECTRO-THERAPEUTICS. 35 
 
 example, in the battery shown in Fig. 7, 
 the positive pole is connected with the 
 copper plate, and is marked with a plus, 
 and an arrow, indicating the fact that the 
 current leaves the cell at this pole ; while 
 the negative pole is the terminal of the 
 zinc plate, and is indicated by a minus 
 sign, and an arrow flowing towards the 
 cell, indicating the fact that the current 
 enters the cell at this pole. In the battery 
 shown in Fig. 6, the positive pole is the 
 terminal B, of the carbon element, while 
 the negative pole is the terminal A, of the 
 zinc element. 
 
 Voltaic cells form an important electric 
 source much employed in electro-therapeu- 
 tics. We will, therefore, briefly describe 
 some of their more important practical 
 forms. 
 
 Fig. 8, shows a form of voltaic cell, 
 
36 
 
 ELECTRICITY IX 
 
 called the silver-chloride cell. This cell 
 consists of a zinc-silver couple, immersed 
 
 Fig. 8. — Form of Silver-Chloride Cell. 
 
 in a dilute aqueous solution of sal-am- 
 moniac. The silver plate has the form of 
 a wire, and is surrounded by a fused mass 
 of silver chloride. The arrangement of 
 
ELECTRO-T 
 
 Fig. 9.— Silver-Chloride Voltaic Cell. 
 
38 ELECTRICITY IN 
 
 the plates is shown in the figure. It will 
 be seen that a thread B, is wrapped around 
 the silver and silver chloride, so as to pre- 
 vent the possibility of contact between the 
 silver element and the zinc plate. The 
 two plates are also kept apart by a small 
 block of wood W. The couple so formed 
 is placed in a small glass or rubber jar J, 
 containing the exciting solution of sal- 
 ammoniac. Another form of silver- 
 chloride cell is shown in Fig. 9. The 
 advantage of the silver-chloride cell con- 
 sists in its portability. As many as fifty 
 of these cells can be set in a frame, as 
 shown in Fig. 10, and enclosed in a small 
 wooden box weighing only ten pounds. 
 The silver-chloride cell is very nearly uni- 
 form in its electromotive force, which has a 
 value of about 1.03 volts. The cell pos- 
 sesses, however, the disadvantage of not 
 being able to supply powerful currents 
 
ELECTRO-THERAPEUTICS. 39 
 
 continuously, owing to the fact that in 
 order to obtain portability, its elements are 
 made so small. Were the cell constructed 
 of such a size as would permit it to supply 
 
 Fig. 10.— Battery of Silver-Chloride Cells. 
 
 powerful currents, the cost of the silver 
 and silver-chloride would render its use 
 impracticable. The cell is particularly 
 well adapted to supply feeble currents, 
 requiring a considerable E. M. F., espe- 
 cially when portability is desired. 
 
40 
 
 ELECTRICITY IN 
 
 Fig. 11, shows a single-fluid cell, consist- 
 ing of a zinc-carbon couple in an exciting 
 
 Fig. 11. — Zinc- Carbon Cell. 
 
 solution of sal-ammoniac in water. Here, 
 as before, the terminal of the zinc plate 
 forms the negative pole and that of the 
 
ELECTRO-THERAPEUTICS. 41 
 
 carbon plate, formed of a number of 
 rods of carbon, the positive pole. This 
 cell furnishes a strong current for a short 
 time. Its advantage consists in the fact 
 that it will supply a moderately strong 
 current for a brief interval, and that it 
 suffers very little chemical loss on open 
 circuit. Its E. M. F. is about 1J£ volts. 
 The disadvantage of the cell is that it 
 polarizes considerably, so that it cannot 
 continue to furnish current of any con- 
 siderable strength for a long time. 
 
 Fig. 12, shows another form of a couple 
 of zinc-carbon immersed in a solution 
 called electropoion fluid, consisting of 
 chromic acid and water, or of bichromate 
 of potash, sulphuric acid and water. This 
 cell is called the Grenet or bichromate cell. 
 The advantage of the Grenet cell is that it 
 is capable of supplying a fairly strong cur- 
 
42 ELECTRICITY IN 
 
 rent for a short time, though longer than 
 in the case of the simple carbon-zinc cell. 
 Its disadvantage is that the chemical action 
 
 Fig. 12.— Grenet Plunge Cell. 
 
 continues even when the cell is on open 
 circuit, so that the zinc becomes dissolved. 
 Consequently, in practice provision has to 
 be made in ■ this cell, for raising the zinc 
 
ELECTRO-THERAPEUTICS. 43 
 
 plate from the solution when the battery is 
 not in use. 
 
 Fig. 13, shows two different forms of 
 
 Fig. 13.— Edison-Lalande Cells. 
 
 Edison-Lalande cell. Here the couple is 
 formed of plates of zinc and copper im- 
 mersed in a solution of caustic soda, or 
 potash, in water. Although this cell em- 
 
44 ELECTRICITY IN 
 
 ploys but a single liquid, yet, like the 
 Leclanche cell, a special provision is made 
 to prevent polarization. This is accom- 
 plished by placing a plate of compressed 
 copper oxide in contact with the copper 
 plate, so that the hydrogen, which tends 
 to be liberated at the surface of the 
 copper plate, is prevented from doing so 
 by entering into combination with the 
 oxygen of the oxide of copper. The Edi- 
 son-Lalande cell possesses the advantage of 
 furnishing powerful currents for a con- 
 siderable length of time without sensible 
 polarization, and also of suffering negligi- 
 ble local action or chemical loss on open 
 circuit. Its disadvantage lies in its low E. 
 M. F., which is only about two thirds of a 
 volt, when at work. 
 
 Another form of Edison-Lalande cell is 
 shown in Fig. 14. Here a large copper 
 
ELECTRO-THERAPEUTICS. 45 
 
 Fig. 14. Edison-Lalande Cautery Cell. 
 
 plate is placed between two zinc plates. 
 The object of this form of cell is to 
 
46 
 
 ELECTRICITY IN 
 
 provide powerful currents suitable for 
 heating electric cauteries; i. e., metallic 
 
 Fig. 15.— Pahtz Gravity Cell. 
 
 wires or strips raised to a white heat by 
 the passage of an electric current, and 
 employed for removing diseased growths. 
 
ELECTRO-THERAPEUTICS. 47 
 
 Fig. 15, shows a form of zinc-carbon 
 cell, called the Partz gravity cell. This is 
 a double-fluid, gravity cell, the fluids be- 
 ing a solution of common salt, or sulphate 
 of magnesia, and a dense solution of a salt 
 called sulpho-chromic salt, practically a 
 salt which, dissolved in water, forms the 
 electropoion solution before referred to. 
 In order to charge the cell, the jar is first 
 partly filled either with a solution of com- 
 mon salt or magnesium sulphate, and the 
 sulpho-chromic salt added through the fun- 
 nel shaped tube on the left-hand side of 
 the cell, thus forming a dense solution sur- 
 rounding the lower or carbon plate. The 
 advantage of this cell is its high E. M. 
 F., nearly two volts, and the strength of 
 current it can supply. Its disadvantage 
 is local action on open circuit. 
 
 Fig. 16, shows a form of cell called a 
 
48 ELECTRICITY IN 
 
 dry cell. This name is badly chosen, 
 since, although the cell does not actually 
 contain free liquid, yet its action is depend- 
 
 Fig. 16.— Form of Dry Cell. 
 
 ent upon the presence of a liquid electro- 
 lyte, in this case the liquid being absorbed 
 either by a gelatinous substance, or by 
 some pulverulent material. The advan- 
 tage of a cell which contains no free 
 
ELECTRO-THERAPEUTICS. 49 
 
 liquid, is that the cell can be readily car- 
 ried about without spilling any of the ex- 
 citing liquid. The disadvantage of the cell 
 is that the current it will supply is compar- 
 atively feeble, owing to its high resistance. 
 
 Dry cells give E. M. Fs. varying from 
 two-third volt, to about two volts. 
 
 Where the circuit to be supplied is 
 such that the E. M. F. furnished by a 
 single cell is not capable of overcoming 
 what is called the resistance of the circuit, 
 it is necessary to connect a number of 
 separate cells so that they may all supply 
 their currents into the same circuit. A 
 number of separate cells capable of acting 
 as a single cell is called a battery, a term fre- 
 quently incorrectly applied to a single cell. 
 
 A number of voltaic cells may be con- 
 nected to form a battery, in a variety of 
 
50 ELECTRICITY IK 
 
 ways, but at present we will discuss only 
 one of such connections ; namely, connec- 
 tion in series. The method adopted in 
 this connection is shown in Fig. 17, where 
 three Daniell gravity cells are shown, con- 
 
 Fig. 17.— Series Connection of Three Daniell 
 Gravity Cells. 
 
 nected in series. Here the negative pole 
 of the cell A, is connected with the posi- 
 tive pole of the cell B ; the negative pole 
 of B y is connected with the positive pole 
 of O, while the free positive and the free 
 negative poles of A and C\ respectively, 
 form the terminals of the battery. In the 
 
ELECTRO 
 
 case of the series 
 cells, the E. M. F. 
 the sum of the E. M. 
 
 Fig. 18.— Edison-Lalande Battery. 
 
 cells composing it. Consequently, the 
 battery shown in Fig. 17, will have an E. M. 
 F. of approximately three times 1.05 volts. 
 
 Fig. 18, shows a battery of two series- 
 connected, Edison-Lalande cells. Fig. 19, 
 
52 
 
 ELECTRICITY IN 
 
 shows a battery of the general type shown 
 in Fig. 12. Here each cell is formed of a 
 number of plates of zinc and carbon. 
 When the battery is desired for use, the 
 
 rrrrrrrrrfrf 
 
 HUIU 
 
 Fig. 19.— Plunge Battery. 
 
 handle is turned so that the plates are let 
 down into the jars filled with the exciting 
 liquid. When out of use, the plates are 
 raised from the jars, in order to preserve 
 them from corrosion, or local action. 
 
ELECTRO-THERAPEUTICS. 53 
 
 Should the plates not be removed from 
 the exciting liquid, the zincs will rapidly 
 be corroded. 
 
 Fig. 20, shows a battery of 50 series- 
 connected, silver-chloride cells, so arranged 
 
 Fig. 20.— Silver-Chloride Battery of Fifty Cells. 
 
54 ELECTRICITY IN 
 
 that any number from one up to 50 can be 
 connected to the main terminals. 
 
 A voltaic cell will continue to furnish 
 current to the circuit connected with it, as 
 long as it contains any positive metal to be 
 dissolved, and any electrolyte to dissolve 
 it. As soon as either the electrolyte or the 
 positive plate is consumed, the cell ceases 
 to give current until it is furnished with 
 another plate or with more electrolyte, or 
 both. In the form of voltaic cell called 
 the storage cell, or secondary cell, to dis- 
 tinguish it from the ordinary voltaic or 
 primary cell, the selection of the elements 
 of the couple and the electrolyte are such, 
 that after the cell is completely exhausted 
 or run down, it is capable of being restored 
 or charged, and of again being brought 
 into a condition ready for action by the 
 passage of an electric current through it in 
 
ELECTRO-THERAPEUTICS. 55 
 
 the opposite direction to that of the cur- 
 rent which it furnishes when charged. 
 The passage of this charging current has 
 the effect of producing a series of decom- 
 positions, which practically restore the 
 condition both of the elements of the 
 couple and of the exciting liquid or elec- 
 trolyte. 
 
 Secondary or storage cells are made in 
 a variety of forms, but those in common 
 use, consist practically, when charged, of a 
 voltaic couple of porous lead and lead 
 peroxide, immersed in an electrolyte of 
 dilute sulphuric acid. The lead peroxide 
 forms the positive plate, and the metallic 
 finely divided, porous lead, the negative 
 plate. 
 
 In most forms of storage battery, the 
 substances forming the positive and nega- 
 
56 
 
 ELECTRICITY IN 
 
 tive elements are packed in perforations in 
 a supporting plate or grid, made of an 
 alloy of lead containing a small percentage 
 
 
 Fig. 21. — Plate of Chloride Storage Battery. 
 
 of antimony. The object of the antimony 
 is to prevent the action of the sulphuric 
 acid on the lead in the grid during charg- 
 ing. Fig. 21, shows a plate of a well- 
 known form of storage cell, called the 
 
ELECTRO- 
 
 chloride storage ce 
 as shown, of a lead-a 
 
 Fig. 22. — Interior View of Chloride Storage 
 Battery. 
 
 ing pastels or discs of metallic lead or lead 
 peroxide, according to the nature of the 
 plate. A number of such plates are 
 
58 
 
 ELECTRICITY IN 
 
 generally grouped together, to form a 
 single storage cell, the connections being 
 such that all the positive plates are con- 
 
 Fig. 23.— Portable Chloride Storage Battery. 
 
 nected together to form a single positive 
 plate, and all the negative plates are 
 similarly connected to form- a single nega- 
 
ELECTRO-THERAPEUTICS. 59 
 
 tive plate, the whole being immersed in a 
 jar containing a solution of sulphuric acid 
 in water. In Fig. 22, two of such storage 
 cells are connected together to form a 
 storage battery. A portable form of 
 chloride storage battery suitable for medi- 
 cal purposes is shown in Fig. 23. 
 
 Another form of storage cell, called the 
 Julien Cell, is shown in Fig. 24. The 
 advantage of storage cells is that they 
 are capable of supplying a very powerful 
 current, and that the E. M. F. of each cell 
 is two volts. Their disadvantage is that 
 they require to be periodically charged. 
 
 Storage cells are generally charged by 
 connecting them with the terminals of a 
 dynamo-electric machine, the positive 
 terminal being connected to the positive 
 terminal of the dynamo. The dynamo 
 
60 ELECTRICITY IN 
 
 may be provided especially for the pur- 
 pose, or the charging current may be 
 
 Fig. 24— Julien Storage Cell. 
 
 obtained from the mains supplying elec- 
 tric incandescent lamps. Sometimes, how- 
 
ELECTRO-THERAPEUTICS. 61 
 
 Fig. 25. — Cabinet Containing Eleven Primary Vol- 
 taic Cells for Charge of Two Storage Cells. 
 
62 ELECTRICITY. 
 
 ever, a primary battery is employed for 
 this purpose. For example, as shown in 
 Fig. 25, 11 gravity cells are connected in 
 a battery so as to charge two storage cells. 
 The current supplied by the primary bat- 
 tery is a feeble but steady current, and the 
 storage cells when charged by the action of 
 this current, may be many times stronger 
 for a correspondingly shorter length of 
 time. 
 
CHAPTER III. 
 
 ELECTEIC KESISTANCE. 
 
 We have pointed out, in the preceding 
 chapter, that it is not electricity which an 
 electric source primarily produces, but an 
 electromotive force, or a force capable of 
 producing a flow or current of electricity, 
 when the terminals or poles of the source 
 are suitably connected by a conducting 
 path or circuit. We have also seen that 
 electromotive force is measured in units 
 called volts, and that an ordinary blue- 
 stone gravity cell produces, under ordinary 
 conditions, an E. M. F. of a little more 
 than one volt. When the terminals of a 
 single voltaic cell of this type are con- 
 
 63 
 
64 ELECTRICITY IN 
 
 nected with a circuit, the electric current 
 which will flow through the circuit, that 
 is, the amount of electricity which will 
 pass through it per second, will depend 
 upon another quantity called the resist- 
 ance of the circuit. Electric resistance is 
 that which opposes the flow of electricity 
 through a circuit. 
 
 Resistance is measured in units of resist- 
 ance called ohms, after Dr. Ohm, who first 
 pointed out the law which governs the 
 flow of electricity through a circuit. The 
 resistance of a circuit depends upon its 
 length, its area of cross-section, and the 
 material of which it is composed. A 
 short, thick circuit, of some good conduct- 
 ing material like copper, will have a small 
 resistance; a long, thin circuit of some 
 poor conducting material, like wet string, 
 will have a high resistance. The greater 
 
ELECTRO-THERAPEUTICS. 65 
 
 the length of a conductor, the greater 
 will be its resistance. Thus ; if we double 
 the length of any uniform metallic wire, 
 we double its resistance ; or, if we halve 
 the length of the wire, we halve its resist- 
 ance. The greater the area of cross-sec- 
 tion of a conductor the less its resistance, 
 so that if the area of cross-section of a 
 wire be doubled, retaining the same 
 length, its resistance will be halved. 
 
 The ohm is the resistance of a definite 
 length of a definite material, having a defi- 
 nite area of cross-section. The ohm is de- 
 fined as the resistance of a column of pure 
 mercury having a length of 106.3 cms., and 
 a cross-sectional area of one square milli- 
 metre, at the temperature of melting ice, 
 or 0° C. The actual definition requires, 
 not that the cross-sectional area should be 
 one square millimetre, but that the weight 
 
66 ELECTRICITY IN 
 
 of the column of mercury whose length 
 is 106.3 centimetres, should be 14.4521 
 grammes, but this is equivalent, in a uni- 
 form column, to a cross-section of one 
 square millimetre. In defining the length 
 and the area of cross-section of the column 
 of mercury which represents the ohm, it is 
 necessary to specify the temperature, since 
 the resistance of metallic bodies increases 
 with an increase in temperature. The 
 ohm may also be roughly stated as being 
 the resistance offered by two miles of ordi- 
 nary copper trolley wire, or by one foot of 
 copper wire No. 40 A. W. G. (American 
 Wire Gauge) having a diameter of 
 0.003145 in., at 45° F. 
 
 In order to compare the relative resist- 
 ances of wires of different materials, hav- 
 ing the same length and area of cross-sec- 
 tion, as well as for the purpose of being 
 
ELECTRO-THERAPEUTICS. 67 
 
 able to calculate the resistance of a given 
 length and area of cross-section of any 
 material, it is usual to consider the specific 
 resistance or resistivity which bodies offer 
 to the passage of electric currents. The 
 resistivity of a substance is numerically 
 equal to the resistance offered by a wire of 
 such substance having unit length and 
 unit cross-section. Any units of length 
 and cross-section might be adopted for 
 this purpose, but the units actually 
 adopted are the centimetre, for the unit 
 of length, and the square centimetre for 
 the unit of cross-sectional area. The 
 resistivity of a substance, therefore, is 
 numerically equal to the resistance offered 
 by a wire of the substance, one centi- 
 metre long and one square centimetre in 
 area of cross-section. In the case of 
 metals, the resistivity is always a very 
 small fraction of an ohm, and is, in fact, 
 
68 
 
 ELECTRICITY IN 
 
 usually expressed in microhms; i. e., in 
 millionths of an ohm. In the case of 
 many liquids, the resistivity is conven- 
 iently expressed in ohms, but in the case 
 of materials which possess very poor con- 
 ductive power, generally called insulators, 
 the resistivity becomes enormously great 
 and is more conveniently expressed in 
 megohms, or millions of ohms, in begohms, 
 or -billions of ohms, i. e., thousands of mil- 
 lions, or in tregohms, or trillions of ohms, 
 i. e. y millions of millions. 
 
 The following is a table of resistivities 
 of various substances : 
 
 Silver, annealed, 
 Copper, " 
 Iron, " 
 
 Mercury, 
 Platinum, 
 Pure water, 
 Tap water, . 
 Hard rubber, 
 Porcelain, 
 
 1.53 microhms, at 0° C. 
 1.594 " " 
 
 9.687 " " 
 
 94.84 " " 
 
 9.03 " " 
 
 about 3.75 megohms, at 10° C. 
 " 200,000 ohms, at 10° C. 
 . . 28,000 tregohms. 
 . . 540,000 " 
 
In order to show t«.4^e of this table, 
 suppose it to be requirafc<t5 calculate the 
 resistance of a wire of platmufeaaje foot 
 
 long (30.48 cms.), and j^ of an inch in 
 diameter ; i. e. 9 having a cross-sectional 
 
 area of ~j® ^^ incn=a0005067 
 square centimetre. 
 
 Looking at the table of resistivities we 
 
 find for platinum the value 9.03 microhms, 
 
 and the resistance of the wire will, there- 
 
 . 9.03X30.48 . , 
 
 fore, be a 000^0fi7 ~543,200 microhms = 
 
 0.5432 ohm. Here we multiply the resis- 
 tivity by the length because the longer 
 the wire, the greater will be its resistance. 
 If the wire be 30. 48 centimetres long, and 
 one square centimetre in area of cross- 
 section, it would have a resistance of 
 
70 ELECTRICITY IN 
 
 9.03X30.48 microhms. We divide by 
 the area, because the greater the area, the 
 less the resistance. The resistance of a 
 platinum wire, one centimetre long, and 
 0.0005067 square centimetre in cross-sec- 
 tional area, would have a resistance of 
 
 9 03 
 
 QQOQ - Ofi7 = 17,820microhms=0.0l782ohm. 
 
 The resistance of any metal can, the- 
 oretically, at least, be computed in the 
 same manner, but slight impurities in the 
 material are liable to affect markedly its 
 resistivity, and consequently its resist- 
 ance, so that, except in the case of very 
 nearly pure metals, the resistances of com- 
 putation are not very reliable. The effect 
 of impurities is always to increase the 
 resistivity, and, therefore, the resistance. 
 
 It will be observed that most of the re- 
 sistivities are given at the temperature 
 
ELECTRO-THERAPEUTICS. 71 
 
 0° C. Ordinarily, the effect of temperature 
 is to increase the resistivity of all metallic 
 substances. This effect is nearly the same 
 for all pure metals, and is, roughly, 4-10ths 
 of one per cent, per degree centigrade 
 increase of temperature above zero centi- 
 grade. In the case of liquids, and of non- 
 metallic substances generally, the resistiv- 
 ity diminishes as the temperature rises. 
 Carbon behaves in this respect like an 
 insulator, rather than a conductor. Tem- 
 perature has a marked influence in dimin- 
 ishing the resistivity upon the best 
 insulators. 
 
 It may be seen from the table, that the 
 resistivity of pure water is very high ; 
 namely 3.75 megohms, and it is believed 
 by some, that if water could be obtained 
 in absolute purity it would not conduct, or 
 that its resistivity would be indefinitely 
 
72 ELECTRICITY IN 
 
 great. Very slight degrees of impurity- 
 suffice, however, to greatly reduce the re- 
 sistivity, and the addition of a soluble salt 
 reduces it to a few ohms. Thus the re- 
 sistivity of a strong solution of zinc sul- 
 
 ^<^v\aaaaaaaaa^— 
 
 Fig. 26.— Connection of Resistances in Series. 
 
 phate in water is about 30 ohms at ordi- 
 nary temperatures. 
 
 If a wire AB, of 10 ohms resistance, 
 be connected with a second wire CD, of 
 20 ohms resistance, as shown in Fig. 
 26, in such a manner that the current first 
 passes through one and then through the 
 other, they are said to be connected in 
 series, and the total resistance of the series 
 AD, will be 20+10=30 ohms. 
 
ELECTROTHERAPEUTICS. 73 
 
 If two conductors, AB and CD, Fig. 27, 
 
 •each of 10 ohms resistance, be connected in 
 
 parallel, as shown, so that the current 
 
 divides between them, as indicated 
 
 
 // 
 
 'ION 
 
 w 
 
 OF 
 
 
 
 
 (XAA/ x^ 
 
 
 Fig. 27.- 
 
 —Connect 
 
 Resistances in 
 
 Parallel. 
 
 by the arrows, the joint resistance of 
 
 10 
 the pair will be -^-=5 ohms. Similarly, if 
 
 three such wires be connected in parallel, 
 their joint resistance would be ~w~ = 3.333 
 
 ohms, and so on, for any number of par- 
 allel wires. 
 
 The resistance of any instrument wound 
 with wire, such, for example, as a tele- 
 phone, depends upon the length and cross- 
 
74 ELECTRICITY IN 
 
 section of the insulated wire employed, as 
 well as on the character of the wire itself. 
 For a given size of coil ; i. e., a given vol- 
 ume of winding space, the resistance in- 
 creases rapidly as the diameter of the wire 
 employed to fill that space is reduced. 
 Neglecting the variation of the insulating 
 thickness of the wire, and its effects, the 
 resistance will increase inversely as the 
 fourth power of the diameter ; that is to 
 say, if we halve the diameter, we shall in- 
 crease the resistance of the winding ap- 
 proximately 16 times (2 4 =2x2x2X2) = 16. 
 
 If one Daniell gravity cell, represented 
 in Fig. 28, be connected through 200 feet 
 of No. 25 A. W. G. copper wire, to a tele- 
 phone, the resistance of the telephone is 
 say 50 ohms, the resistance of the wire 
 6.5 ohms, and the resistance of the cell, 
 say 5 ohms. The total resistance of the 
 
ELECTRO-THERAPEUTICS. 75 
 
 circuit will, therefore, be 50+6.5+5=61.5 
 ohms. 
 
 The electric resistance of any organic 
 substance, such as moist flesh, is very diffi- 
 cult to determine. As soon as flesh is 
 dried, it becomes a non-conductor, and it is, 
 
 Fig. 28.— Resistances in a Circuit Consisting op 
 a Voltaic Cell, Telephone, and Connecting 
 Wibes. 
 
 therefore, evident that the conducting 
 properties of the mass are due almost en- 
 tirely to the conducting power of the 
 liquids contained in it. These liquids 
 may be regarded as forming physically a 
 •continuous mass, although, in reality, the 
 mass is divided by numerous porous parti- 
 tion walls, or septa. The effect of these 
 
76 ELECTRICITY IN 
 
 division walls is to increase greatly the 
 resistance offered by the liquids. 
 
 For the same reason, the resistance of 
 the human body, or of any portion of the 
 body, is a very complex quantity, and 
 varies from time to time. It also varies 
 with the nature of the area of the contact 
 surfaces between which the measurement 
 is taken. Thus if a bare copper wire be 
 grasped in each hand, the resistance of 
 the body, as measured between the two 
 copper wires, may be 100,000 ohms, or 
 more, depending largely upon the dryness 
 of the skin. If now, instead of holding 
 the two wires, the wires be connected to 
 metallic electrodes, such as those shown 
 in Fig. 29, the resistance apparently 
 offered by the body, as measured between • 
 them, will, perhaps, be only 30,000 ohms, 
 for the reason that the area of cross-sec- 
 
ELECTRO-THERAPEUTICS. 77 
 
 tion of skin, through which the current 
 enters and leaves the body, has been 
 greatly increased; i. e., the area of cross- 
 section of the conductor in the skin itself 
 has been greatly increased. If now, the 
 
 Fig. 29. — Metallic Handle Electrodes. 
 
 same electrodes be held in the hands, while 
 wrapped in absorbent cotton, thoroughly 
 wetted by, say salt water, the resistance of 
 the body, as measured between them, will 
 be, perhaps, 5,000 ohms, owing to the fact 
 that the skin has become thoroughly mois- 
 tened by contact with the saline solution, 
 
78 ELECTRICITY IN 
 
 and has thereby become a better con- 
 ductor, or has had its resistance lowered. 
 Again, if the hands be dipped to the 
 wrists in jars containing salt and water, or 
 dilute caustic soda, so that the entire sur- 
 face of the hands is brought into contact 
 with the liquid, the resistance of the body, 
 as measured between the two jars, will, 
 probably, be only 1,000 ohms. If, finally, 
 the hands be dipped still deeper in tall 
 jars containing the same solution, the re- 
 sistance will still tend to slightly diminish, 
 owing to the greater area of skin offering 
 passage to the current, and the reduced 
 effective length of the circuit through the 
 body. 
 
 The principal resistance which the body 
 offers to the passage of an electric current 
 is that of the skin, owing, not only to the 
 nature of its substance, but also to the fact 
 
ELECTRO-THEKAPEUTICS. 79 
 
 that under ordinary circumstances it is 
 dry. The strength of current, therefore, 
 that will pass through the human body, in 
 the event of an accidental contact with an 
 electric circuit, such for example as a 
 trolley wire, will depend markedly on the 
 nature of the contact, as well as on the 
 electromotive force in the parts of the cir- 
 cuit in contact ; in other words, the cur- 
 rent received will depend not only on what 
 is touched, but also on how it is touched. 
 
CHAPTER IV. 
 
 ELECTRIC CURRENT. 
 
 When we speak of an electric current 
 flowiug through a circuit we mean the rate 
 at which electricity passes through the cir- 
 cuit, and this rate can be expressed by the 
 amount of electricity which passes through 
 the circuit in a given time, say in one 
 second, just as the flow of water through a 
 pipe can be expressed by the quantity of 
 water which passes in a given time. The 
 unit quantity of electricity is called the 
 coulomb. A little consideration will show 
 that in the case of liquid flowing through 
 a pipe, since the entire mass of the liquid 
 in the pipe is in motion, the quantity 
 which passes any cross-section of the pipe, 
 
 80 
 
ELECTRO-THlj^PElfTKJS. r -. r ^ 81 C Q 
 
 in a given time, mu&t\B& the same as that £ j 
 
 which passes through aHjf^tlier cross-sec- c •• 
 tion in the same time, sure^otneovise, 
 there would tend to be a surplus in some 
 parts of the pipe, and a deficit in others. 
 The same is true concerning an electric 
 flow or current. The amount of electricity 
 passing any point of the circuit being 
 always rigorously equal to the quantity 
 which passes any other point in the same 
 time. We distinguish between the 
 quantity of electricity and the rate at 
 which it flows, just as we distinguish 
 between a gallon of water, and the rate of 
 flow of a gallon-per-second. The electric 
 unit rate of flow, or unit of current strength, 
 is called the ampere, and is equal to one 
 coulomb-per-second. 
 
 The ampere is the unit rate of flow 
 invariably employed in all practical appli- 
 
82 ELECTRICITY IN 
 
 cations of electricity. Thus, an electric 
 incandescent lamp may require for its 
 operation a current strength of from 0.25 
 to 10 amperes, according to its dimensions, 
 and the amount of light it is designed to 
 produce. In some applications of elec- 
 tricity, as, for example, in electric welding, 
 or metal working, enormous currents have 
 to be employed, 50,000 amperes being 
 sometimes required. In the application of 
 electricity in electro-therapeutics, the cur- 
 rent strength is always a small fraction of 
 an ampere, and is generally measured in 
 milliamperes, or thousandths of an ampere, 
 for the reason that a current strength of 
 one ampere would be dangerously great. 
 In the application of the death penalty 
 by electricity, as practiced in the State of 
 New York, the alternating current strength 
 passed through the body of the criminal is 
 usually 7 or 8 amperes. 
 
ELECTRO-THERAPEUTICS. 83 
 
 When an electric current is sent through 
 a metallic solution, such, for example, as 
 an aqueous solution of copper sulphate, the 
 passage of the current is attended by a 
 decomposition of the salt, metallic copper 
 being deposited on the terminal or elec- 
 trode connected with the negative pole of 
 the battery, and an acid substance appear- 
 ing at the electrode connected with the 
 positive pole, and entering into combina- 
 tion with it. If such combination be 
 chemically impossible this substance will 
 be liberated at the electrode. Decompo- 
 sition by electricity is called electrolysis. 
 The amount of electrolytic decomposition, 
 that is, the amount of saline substance 
 decomposed, will depend upon the quantity 
 of electricity which passes, as well as upon 
 the nature of the substance itself. An 
 ampere may, therefore, be denned by the 
 amount of chemical decomposition which it 
 
84 ELECTRICITY IN 
 
 can effect in a given time. Thus, since an 
 ampere is a current of one coulomb-per- 
 second, and each coulomb of electricity 
 passing through a solution of silver de- 
 posits 1.118 milligrammes of silver, a cur- 
 rent strength of one ampere produces a 
 deposit of 1.118 milligrammes of silver per 
 second. 
 
 When the resistance of a circuit is 
 known in ohms, and the electromotive force 
 applied to the circuit is known in volts, 
 the strength of current, which passes 
 through the circuit in amperes, can be 
 readily calculated by a law known as 
 Ohm's law. Ohm's law is generally repre- 
 sented by the equation, 
 
 Volts 
 
 Am P eres = OW 
 
 Suppose, for example, a circuit having a 
 resistance of 10 ohms has an E. M. F. of 
 
ELECTRO-THERAPEUTICS. 85 
 
 5 volts acting on it; then the current 
 
 which flows through the circuit will be 
 
 5 1 
 
 — =- ampere. The practical electric 
 
 units of E. M. P., resistance and current 
 strength may, therefore, be denned, in 
 terms of Ohm's law, as follows ; 
 
 A volt is such a unit of E. M. F. as will 
 produce a current of one ampere in a cir- 
 cuit whose electric resistance is one ohm. 
 
 An ohm is such a unit of electric resist- 
 ance, as will limit the flow of electricity to 
 a current of one ampere, when under an 
 E. M. F. of one volt. 
 
 The ampere is the rate of flow of cur- 
 rent which will pass through a circuit 
 whose resistance is one ohm, under an 
 E. M. F. of one volt. 
 
 The above definitions, although con- 
 venient for defining electric units in 
 
86 ELECTRICITY IN 
 
 terms of one another, are not generally 
 employed. 
 
 The following examples of the applica- 
 tion of Ohm's law will show its impor- 
 tance. Required the E. M. F. necessary to 
 produce a current strength of 10 milliam- 
 peres through the human body, its resist- 
 ance being, under given conditions, 5,000 
 ohms. 
 
 10 mull amperes — i 000 = Too am P ere > 
 and the E. M. F. which divided by 5,000 
 
 ohms gives jttt: = 50 volts, the voltage re- 
 quired. 
 
 A battery of fifty silver-chloride cells, 
 each having an E. M. F. of 1.05 volts, and 
 an internal resistance of 10 ohms per cell ; 
 it is required to know whether a single cell 
 or the whole battery of fifty cells in series 
 
ELECTRO-THERAPEUTICS. 87 
 
 will give the greater current strength 
 through a short stout piece of copper 
 wire. 
 
 The resistance of the short piece of cop- 
 per wire being negligibly small compared 
 with the resistance of a single cell, we may 
 omit it altogether in the calculation. The 
 current strength from one cell will, there- 
 
 , 1.05 
 fore, be -j<7 = 0.105 ampere — 105 milliam- 
 
 peres. With fifty cells we have 50 times 
 
 as much E. M. F. and. also fifty times as 
 
 much resistance, and therefore, by Ohm's 
 
 .„ . 52.5 
 law, the current strength will be ^tjtj - = 
 
 0.105 ampere = 105 milliamperes, or the 
 same as before, so that it is evident that 
 through a negligibly small external resist- 
 ance, or, as it is called, on short circuit, 
 there is no advantage in adding similar 
 
88 ELECTRICITY IN 
 
 cells in series, since although each cell adds 
 its E. M. F. to the circuit, it also adds a 
 proportional amount of resistance. 
 
 If the silver-chloride battery of the pre- 
 ceding paragraph, is employed to send a 
 current through an external resistance of 
 1,000 ohms, what will be the current 
 strength with one cell and with fifty cells ? 
 
 With one cell, the total resistance will be 
 1,000+10 = 1,010 ohms, and the E. M. F. 
 1.05 volts, so that the current strength 
 
 will be jTyfA = 0.00104 = 1.04 milliampere. 
 
 With fifty cells, the total resistance will 
 
 be 1,000+500 = 1,500 ohms, and the E. M. 
 
 F. 1.05X50 = 52.5 volts. The current 
 
 52 5 
 strength will, therefore, be .. 5QQ = 0.035 = 
 
 35 milliamperes. 
 
ELECTRO-THERAPEUTICS. 89 
 
 As another example, let us suppose that 
 two Gravity-Daniell cells have each an 
 E. M. F. of 1.05 volts, and a resistance of 
 4 ohms. Find the current strength which 
 they can send through a resistance of one 
 ohm externally; (a) When connected 
 singly ; (b) In series ; (c) In parallel. 
 
 1.05 1.05 
 (a) jrrr = ^- = 0.21 = 0.21 ampere = 
 
 210 milliamperes. 
 
 1.05X2 2.1 
 (J) In series, ^^+1 = ~9~ = °- 233 = 
 
 233 milliamperes. 
 
 (c) In parallel. Here the positive pole 
 of one cell is connected to the positive pole 
 of the other, and the negative pole of one 
 cell, connected to the negative pole of the 
 other. The E. M. F. of the combination 
 will be that of a single cell, but the resist- 
 ance of the combination will be that of 
 two equal resistances in parallel, or one 
 
90 ELECTRICITY IN 
 
 half that of either; namely, 2 ohms, the 
 
 current strength will, therefore, be ^xr = 
 
 1.05 
 
 — Q- =0.35 ampere = 350 milhamperes. 
 
 An instrument for measuring the 
 strength of electric currents is called an 
 amperemeter, or ammeter. For electro- 
 therapeutic purposes, since the current 
 strength to be measured is usually ex- 
 pressed in milliamperes, the instrument is 
 frequently called a milliammeter. 
 
 Milliammeters are made in a variety of 
 forms. In nearly all cases, however, an 
 index or pointer is moved over a graduated 
 scale by the force exerted between a coil 
 of wire carrying the current to be meas- 
 ured, and a magnet in its vicinity. This 
 movement is due to the fact that a wire 
 
& DfESR 
 
 m 
 
 ELECTRO-THERAPBI^ICS/ Kl »^BllTV Lf 
 
 carrying an electric currentN&^mes tem- 
 porarily invested with magnetic 
 
 *fffe®|$ ! 
 
 Fig. 30.— Form of Milliammetek. 
 
 A simple form of milliammeter is shown 
 in Fig. 30. In this a pair of coils of wire, 
 situated beneath the horizontal face of the 
 instrument, become magnetized by the 
 passage of the current to be measured, and 
 
92 ELECTRICITY IN 
 
 deflect a magnetic needle, in the shape of 
 a split bell, from the position it assumes 
 under the influence of the earth's field at 
 the place where the measurement is made. 
 
 Fig. 31. — Vertical Form of Milliammeter. 
 
 The scale is marked directly in milli- 
 amperes. 
 
 Another form of instrument is shown in 
 Fio*. 31. Here a vertical needle is de- 
 
ELECTRO-THERAPEUTICS. 93 
 
 fleeted by the attraction of a coil of wire 
 upon a magnetized needle placed inside 
 the instrument. 
 
 Fig. 32. — Form of Milliammeter. 
 
 Still another form of instrument, differ- 
 ing only in constructive details from those 
 above described, is shown in Fig. 32. 
 
 While the preceding instruments are 
 simple in their construction, yet they are 
 all liable to have their indications attended 
 
94 
 
 ELECTRICITY IN 
 
 by changes of magnetic force occurring in 
 their neighborhood, and even when all 
 magnets are carefully removed from their 
 vicinity, their indications are often at- 
 
 Fig. 33.— Weston Milli ammeter. 
 
 tended by appreciable error, due to some 
 difference between the strength of the 
 earth's magnetic force at the place where 
 the instrument is employed, and the place 
 where it was originally calibrated. 
 
ELECTRO-THERAPEUTICS. 
 
 95 
 
 A form of instrument which is practi- 
 cally free from the earth's magnet inilu- 
 
 Fia. 34. — Working Parts of Weston Ammeter. 
 
 ence is shown in Fig 33. This freedom is 
 owing to the fact that the instrument has 
 
96 ELECTRICITY IN 
 
 no iron moving parts. The principal 
 working parts are shown in Fig. 34. A 
 horse-shoe permanent magnet MM, MM, 
 only partly visible in the figure, has soft 
 iron projections, or pole-pieces P, P, secured 
 to its poles. These projections are shaped 
 so as to enclose a cylindrical space. At 
 the centre of this space is supported a soft 
 iron, solid cylindrical core I I. Between 
 this core and the pole-j>ieces, there remains 
 a narrow annular gap, or space, which is 
 permeated by the magnetic flux from the 
 permanent magnet. In this space a deli- 
 cately supported coil C, of insulated 
 copper wire, is free to move. Coil springs 
 S, S, carry the current to be measured into 
 and out of the coil C. So long as there is 
 no current through the coil, it is unaffected 
 by the magnetic field in which it is placed, 
 and the pointer remains at the zero point. 
 When, however, a current passes through 
 
ELECTRO-THERAPEUTICS. 97 
 
 the coil, the electromagnetic action of this 
 current upon the magnetic field, causes 
 a mechanical force to be exerted upon 
 the coil, deflecting it against the spiral 
 springs S, S } in such a manner that the 
 pointer JS, moves over a scale beneath it 
 through a distance, which is almost di- 
 rectly porportional to the current strength. 
 The permanent magnet's field is so 
 very much more powerful than the earth's 
 magnetic field, that the influence of the 
 latter upon the coil is negligible by com- 
 parison. The accuracy ojf the instrument 
 depends upon the degree of permanence 
 with which the magnetism in the horse- 
 shoe magnet is retained. The instrument 
 has the disadvantage that it will not read 
 in either direction, so that if the current 
 passing through the instrument has the 
 wrong direction, the wires attached to the 
 instrument must be reversed. 
 
98 ELECTRICITY IN 
 
 In several of the instruments just 
 described, means are provided for varying 
 their sensibility and the range of their 
 indications. Thus, in Figs. 29 and 30, 
 a screw button is seen projecting from the 
 face of the apparatus, marked 10. If this 
 screw be pressed forward, until it abuts 
 strongly against its stop, a coil of wire will 
 be brought into connection with the termi- 
 nals, in such a manner that 9-10ths of the 
 current passing through the instrument 
 will pass through this wire, and only 
 1-1 0th will pass through the measuring 
 coils. The instrument under these con- 
 ditions is said to be shunted, or to have a 
 shunt applied to it whose power is 10. 
 The readings of the instrument in milliam- 
 peres must now be multiplied by 10, in 
 order to obtain the actual current 
 strengths. In Fig. 33, three terminals are 
 shown and the instrument is provided 
 
ELECTRO-THERAPEUTICS. 99 
 
 with two sets of graduations on its scale. 
 When the right hand and the corner left 
 hand terminals are used, the lower scale is 
 brought into use, by which the instrument 
 reads up to 10 milliamperes only, but by 
 using the right hand, and the inner left 
 hand terminals, the upper scale is utilized, 
 by which currents up to 500 milliamperes 
 can be measured. 
 
 Instruments of the preceding types are 
 sufficiently sensitive for all the ordinary 
 requirements of electro-therapeutic applica- 
 tions. When, however, physiological re- 
 searches have to be made, in which very 
 feeble electric currents are measured, it is 
 necessary to use a mirror galvanometer, as 
 a form of ammeter. Such an instrument 
 is made in a variety of forms, one of the 
 simplest of which is illustrated in Fig. 35. 
 A circular coil or spool of fine insulated 
 
100 
 
 ELECTRICITY IN 
 
 copper wire is mounted upon a tripod 
 frame, and a small magnetized needle 
 
 Fig. 35.— Mirror Galvanometer 
 
 is suspended by a fibre of silk at the 
 centre of the coil. A small glass mirror 
 is attached to the suspension in such 
 
ELECTRO-THERAPEUTICS. 
 
 101 
 
 a manner that any deflection of the 
 little magnetized needle will cause an 
 angular deflection of the mirror. Facing 
 the instrument is a lamp and scale, shown 
 in Fig. 36. The lamp, whose glass 
 
 Fig. 36.— Mirrok Galvanometer Scale. 
 
 chimney only is seen at G> throws a 
 beam of light through the window W, on 
 the mirror suspended in the galvanometer. 
 The latter reflects the beam on the scale S. 
 Since a deflection of the mirror through an 
 angle of 45°, would be sufficient to deflect 
 
102 ELECTRICITY IN 
 
 the beam through a right angle, or 90°, 
 and, therefore, to send the beam to the end 
 of an indefinitely long scale, it is evident, 
 that a veiy small angular deviation of the 
 rnaomet under the influence of the current 
 to be measured passing through the coil, 
 will suffice to produce a marked displace- 
 ment of the spot of light along the scale S. 
 A current in one direction will deflect the 
 beam to the right, and a current in the 
 opposite direction, to the left. Such an 
 apparatus has commonly a resistance of 
 500 ohms, and a current of one millionth 
 of an ampere, on one micro-ampere, will 
 deflect the beam through a distance of a 
 millimetre on a scale one metre distant. 
 
 In researches of very great delicacy, 
 where exceedingly feeble currents have to 
 be observed, special very sensitive mirror 
 galvanometers are employed. One of these 
 
ELECTRO-THERAPEUTICS. 
 
 103 
 
 is shown in Fig. 37. Here four sets of coils, 
 one above another, act on four little mag- 
 
 •""■TlUffll:" 
 Fig. 37.— Sensitive Mirror Galvanometer. 
 
 netic needles situated at their respective 
 centres. A single mirror, attached to the 
 upper part of the suspension, reflects its 
 
104 ELECTRICITY IN 
 
 beam of light through the window W. 
 The terminals of the coils are brought to 
 the connecting posts t, t. m, tn, are two con- 
 trolling magnets employed for bringing the 
 magnetic needles back to the same posi- 
 tion after the application of the current 
 to be measured. Such an instrument has 
 a resistance of about 15,000 ohms, and has 
 a sensibility such that one billionth of an 
 ampere, or one bicro-ampere, i. e. one 
 thousand millionth ampere, will produce a 
 deflection of 15 millimetres on a scale dis- 
 tant one metre. Except for delicate work, 
 and very feeble currents, the Thomson 
 galvanometer is undesirable, as the values 
 of its indications have usually to be con- 
 verted into amperes by careful measure- 
 ments and computations. 
 
 A form of galvauometer, very con- 
 venient when the greatest sensibility is not 
 
"<«?CF^?ry Cf " 
 
 required, is shown in Fig^&S-^and is called 
 the D'Arsonval galvanonn 
 
 ELECTRO-THERA»^J r 
 
 
 Fig. 38.— D'Arsonval Galvanometer. 
 
 coil of insulated wire C, is suspended 
 between the poles of a permanent magnet 
 M, and by means of the attached mirror, 
 
106 ELECTRICITY. 
 
 the deflection of this coil can be observed. 
 It is evident that while in the preceding 
 mirror galvanometers, the coil is fixed 
 and the magnet is movable, in this in- 
 strument the magnet is fixed and the coil 
 is movable. 
 
CHAPTER V. 
 
 VARIETIES OF ELECTROMOTIVE FORCE. 
 
 The voltaic or primary cell, and the 
 secondary cell already described, will pro- 
 duce an E. M. F. which, so long as the 
 chemicals remain unchanged, does not vary 
 in strength. Such an E. M. F. is, there- 
 fore, called a continuous E.M.F. A con- 
 tinuous E. M. F. is also produced by a 
 variety of other electric sources, such, for 
 example, as a continuous-current dynamo, 
 which, so long as its speed of rotation 
 remains the same, produces an E. M. F. 
 which is practically continuous. 
 
 Fig. 39, represents graphically a con- 
 tinuous E. M. F. The straight line AB, is 
 - 107 
 
108 
 
 ELECTRICITY IN" 
 
 drawn parallel to the base OS, at a dis- 
 tance representing 1.1 volts. Time is 
 measured along the base OS, and the fact 
 that the line AB, remains parallel to the 
 
 > 
 
 2.2C 
 
 1.1 A 
 
 
 
 -LIE 
 
 ■f S 
 
 Fig. 39.— Continuous E. M. F. 
 
 base, represents the constancy of the 
 E. M. F., which might be that of a single 
 Daniell cell. Two such cells, connected 
 in series, would produce a continuous 
 E. M. F. of 2.2 volts, represented by the 
 straight line CD, twice as far above the 
 line OS, as the line AB. 
 
ELECTRO-THERAPEUTICS. 109 
 
 An E. M. F. possesses direction, as well 
 as magnitude; that is to say, it may tend 
 to send a current through a circuit in one 
 direction or in the opposite direction. All 
 
 E. M. F.'s that tend to send the current in 
 one direction may be regarded as positive, 
 and all tending to send the current in the 
 opposite direction, as negative. Positive 
 
 F. M. Fh are represented graphically by 
 distances above the line OS, and negative 
 F. M. F.'s, by distances below. Thus, in 
 Fig. 39, the line FF, would indicate a 
 negative E. M. F. of 1.1 volts, or an 
 E. M. F. oppositely directed to that of 
 the line AB. 
 
 Fig. 40, shows the E. M. F. produced by 
 a continuous-current dynamo. Here the 
 line AB, is parallel to the base as before, 
 but instead of being straight, is a fine, 
 wavy line. These little waves represent 
 
110 
 
 ELECTRICITY IN 
 
 variations in the amount of E. M. F. pro- 
 duced every time that the bar in the com- 
 mutator passes underneath the collecting 
 brush. These wavelets exist in the 
 E. M. F. of every continuous-current 
 
 IIS 
 
 mi 
 111 
 
 CO 
 
 H10 
 £l09 
 
 m 
 
 A B 
 
 n 
 
 T 
 
 * 
 
 SECONDS 
 
 Fig. 40. — Type of E. M. F. Produced by a Contin- 
 uous-Current Dynamo. 
 
 dynamo. When they are very marked, 
 as represented in Fig. 41, the E. M. 
 F. is said to be pulsatory. Such E. 
 M. F.'s are produced by some con- 
 tinuous-current generators, usually for 
 
ELECTRO-THERAPEUTICS. 
 
 Ill 
 
 supplying arc lamps. It is evident, that 
 at different times the E. M. F. varies con- 
 siderably in its magnitude, but never 
 changes direction, the line AB, being 
 always on one side of the zero line 08; 
 that is to say, it always has the same direc- 
 
 ± A L 
 10 10 2 
 
 SECONDS 
 
 Fig. 41.— Pulsatory E. M. F. 
 
 tion in the circuit, just as though a battery 
 of voltaic cells were employed to send cur- 
 rent through a circuit, and that at inter- 
 vals, a certain number of these cells were 
 cut out and re-introduced. 
 
112 
 
 ELECTRICITY IN 
 
 When the waves start each time from 
 the zero line, the E. M. F. is said to be 
 intermittent. Fig. 42, shows that, at cer- 
 tain intervals, an E. M. F. exists in the cir- 
 cuit in one direction, and that at interven- 
 
 P TIME 
 
 Fig. 42.— Intermittent E. M. F. Undirectional. 
 
 ing intervals there is no E. M. F. The 
 intermittent E. M. F. can be obtained by 
 connecting a continuous E. M. F., say a 
 voltaic battery, to a wheel interrupter, in 
 such a manner that the E. M. F. will be 
 periodically cut off and applied. In all 
 cases, although the strength of the 
 E. M. F. varies at different times, yet at no 
 
ELECTRO-THERAPEUTICS. 
 
 113 
 
 time does it change direction, so that the 
 curved line lies wholly above the base 
 line. When an E. M. F. changes direc- 
 tion, as well as magnitude, it becomes 
 
 4-10— 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 4-5 - m 
 
 CO 
 
 i- 
 _i 
 
 
 
 
 
 
 
 
 3 
 
 £ . 
 
 
 SECOKOS 
 
 
 
 
 
 
 B 
 
 
 - -5-£ 
 
 
 
 
 
 
 
 
 t 
 
 
 -10- 
 
 A 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Fig. 43.— Alternating E. M. F. 
 
 alternating. Thus, in Fig. 43, the 
 E. M. F. is seen to alternate between 10 
 volts positive and 10 volts negative, the 
 transitions in this particular case being 
 made instantaneously. Such an E. M. F. 
 might be produced by connecting a 
 battery of .voltaic cells with a current 
 
114 ELECTRICITY IN 
 
 reverser, in such a manner, that by rotat- 
 ing the handle, the E. M. F. would be 
 periodically reversed without being with- 
 drawn from the circuit. 
 
 4100- 
 
 t 80- 
 
 A 
 
 
 
 
 4 60- 
 
 
 
 
 
 4 40- 
 
 
 
 
 
 tt+ 20- 
 
 7i n 
 
 
 \B 
 
 
 D / 
 
 o O 
 -.20- 
 
 SECONDS 
 
 1 \ 
 
 
 / 2 
 
 /lOO 
 
 - 40- 
 
 
 
 
 
 -60- 
 
 
 
 
 
 - 80- 
 
 
 
 >s *^iL^ 
 
 
 -100- 
 
 
 
 
 
 Fig. 44.— Symmetrical Alternating E. M. F. 
 
 It is not necessary that an alternating E. 
 M. F. should change abruptly from its 
 maximum positive to its maximum nega- 
 tive value. In most cases, in fact, the 
 change occurs more gradually, as shown 
 in Fig. 44, which represents a common 
 
# 
 
 ^W? & DIESEL ^ 
 
 '¥, 
 
 ELECTRO-THERJ 
 
 
 type of alternating E. M. l^'^Eigs. 45 and 
 46, represent the same alternating E. M. 
 F., although the graphical appearance of 
 the waves is changed, owing to the varia- 
 
 4100- 
 
 A 
 
 
 
 
 80- 
 
 
 
 
 
 £ 60- 
 
 
 
 
 
 -J 40- 
 
 
 
 
 
 9. 20- 
 
 
 \b 
 
 
 d/ 
 
 U 
 -20- 
 
 SECONDS 
 
 106\ 
 
 
 
 - 40- 
 
 
 
 
 
 - 60- 
 
 
 
 
 
 - 80- 
 
 
 
 ^-— Q— ^ 
 
 
 -100- 
 
 
 
 
 
 Fig. 45.— Symmetrical Alternating E. M. F. 
 
 tions of the scale of time along the base, and 
 the scale of E. M. F. alon^ the vertical. It 
 will be observed that in all representations 
 of alternating E. M. F. there is a motion in 
 one direction, in which the E. M. F., begin- 
 ning at the base line or zero, gradually in- 
 creases in value, and then gradually falls 
 
116 
 
 ELECTRICITY IN 
 
 until it again reaches zero, then changing 
 its direction and going through the same 
 
 100— 
 
 
 
 A 
 
 80- 
 
 /\ 
 
 VOLTS 
 
 1 1 
 
 \ 1 
 
 20— 
 
 / \ / 
 
 
 1 \B D/ „ 
 
 
 
 SECONDS I ° 
 
 -111 "If 
 
 "-20- 
 
 1 I 
 
 -40- 
 
 \ / 
 
 -60- 
 
 \ / 
 
 -80- 
 
 \<LS 
 
 ,-100- 
 
 
 G. 46.- 
 
 —Symmetrical Alternating E. M. F. 
 
 changes in the opposite direction. Thus 
 the E. M. F. commencing, in Fig. 44, at 
 zero, advances in the positive direction to 
 
ELECTRO-THERAPEUTICS. 117 
 
 its maximum or greatest value at A, then 
 diminishing, but still in the positive direc- 
 tion, to zero, at B, changing- direction and 
 increasing negatively to a maximum value 
 at C, and then regularly decreasing again 
 to zero where it again repeats the former 
 movement. Each of the waves OAB, or 
 BOB , is called an alternation ; so that it 
 will be seen, that in the cases considered 
 in Figs. 44, 45, and 46, an alternation lasts 
 l/100th of a second, or, there are 100 alter- 
 nations in a second. A complete to-and- 
 fro motion is called a cycle, and the time 
 required to complete a cycle is called a 
 period. The period in the cases here con- 
 sidered is l/50th second and it is evident 
 that the number of cycles in a second 
 will depend upon the value of the period. 
 For example,* if the period is 1/1 00th of 
 a second, there would then be 100 cycles 
 in a second. The number of cycles in a 
 
118 
 
 ELECTRICITY IN 
 
 second is called the frequency ; so that, in 
 the last case the frequency would be 100. 
 This is often written 100~. 
 
 Fig. 47.— Symmetrical Alternating E. M. F. 
 
 Alternating E. M. F.'s may be sym- 
 metrical, or dissymmetrical. A symmetri- 
 cal E. M. F. is one which is graphically 
 symmetrical about its zero ljne; that is 
 to say, the positive waves are the same as 
 the negative waves, except that they occur 
 in the opposite direction. Fig. 47, repre- 
 
ELECTRO-THERAPEUTICS. 119 
 
 sents a type of symmetrical wave of E. M. 
 F. having a frequency of 75 ~ ; and, conse- 
 quently, a period of 1/7 5th second. A dis- 
 symmetrical alternating E. M. F. is one in 
 
 Fig. 48.— Dissymmetrical Alternating E. M. F. m 
 
 which the positive wave differs from the 
 negative wave not merely in direction but 
 also in outline. A type of dissymmetrical 
 wave is shown in Fig. 48. 
 
 Symmetrical alternating E. M. F. waves 
 are produced by alternating -current dyna- 
 mos, or alternators. Dissymmetrical alter- 
 
120 ELECTRICITY IN 
 
 nating E. M. F. waves are j)roduced by 
 particular types of apparatus, such s&fara- 
 dic coils, the construction of which will be 
 explained hereafter. 
 
 G 
 
 Fig. 49.— Sinusoidal E. M. F. 
 
 It is evident from the preceding figures 
 that an E. M. F. becomes alternating if 
 it periodically changes its direction and 
 magnitude, and that considerable varia- 
 tion may exist in the manner in which 
 both of these changes may occur. A 
 wave of the form shown in Fig. 49, is 
 
ELECTRO-THERAPEUTICS. 121 
 
 called a sinusoidal wave, and an E. M. F. 
 alternating in this manner is called a sinu- 
 soidal R M. F. This type of E. M. F. 
 possesses important characteristics. 
 
 It is evident that the following distinct 
 types of E. M. F. exist ; namely, 
 
 ( Pulsating i Intermittent, 
 f Continuous \ < Non-Intermittent. 
 
 ( Steady 
 
 (Symmetrical... | Sinusoidal. 
 [Alternating-^ < Non-sinusoidal. 
 
 ( Dissymmetrical 
 
 A continuous E. M. F. acting in a closed 
 circuit produces in it a continuous current. 
 A pulsating E. M. F. similarly produces a 
 pulsating current. An' alternating E. M. 
 F. produces an alternating current. 
 
 • In fact, all the varieties of E. M. F. in 
 the above table, when acting in a circuit, 
 
122 ELECTRICITY. 
 
 produce their particular type of electric 
 current, although the graphic representa- 
 tion, of the current is not always the 
 exact counterpart of the graphic repre- 
 sentation of the E. M. F. The above 
 table may, therefore, be repeated for cur- 
 rents as well as for E. M. F.'s. 
 
 The character of the electric current is 
 dependent on the character of the E. M. F. 
 producing it. A continuous electric cur- 
 rent, like the continuous E. M. F. which 
 causes it to flow, does not vary in its 
 strength at different times, but flows 
 through the circuit like a steady flow of 
 water through a pipe or river channel. A 
 continuous electric current is sometimes 
 called a direct current, in contradistinc- 
 tion to an alternating electric current, 
 which, like the E. M. F. producing it," 
 changes its direction at every half cycle. 
 
ELECTRO-THERAPEUTICS. 123 
 
 All electric currents, however, are produced 
 by the action of some form of E. M. R, so 
 that the presence of current in any circuit 
 necessitates the existence of an E. M. F. 
 which has produced it. 
 
CHAPTER VI. 
 
 ELECTRIC WORK AND ACTIVITY. 
 
 An electric current is never produced 
 Avithout an expenditure of work. The 
 greater the strength of the current, the 
 greater is the amount of work done in a 
 given time. In mechanics, the amount of 
 work done by the action of any force is 
 measured by the distance through which 
 the force acts. A convenient unit of work 
 is the foot-pound, or the amount of work 
 done when one pound is raised, against the 
 force of gravity, through a vertical dis- 
 tance of one foot. Since to do work 
 requires the expenditure of energy, the 
 rate at which work is done represents 
 
 124 
 
ELECTRO-THERAPEUTICS. 125 
 
 the rate at which energy is expended. 
 Thus, a pound weight raised through a 
 foot, requires, as we have seen, the expen- 
 diture of a foot-pound of work. The same 
 amount of work would be done if a pound 
 weight were raised through a foot in a 
 minute, or in a second, but the rate at 
 which energy would be expended would 
 be sixty times greater in the second case 
 than in the first. 
 
 The rate of doing work, or expending 
 energy, is called activity. For the electric 
 unit of work the foot-pound might be 
 employed, but it is more convenient in 
 practice to employ a unit called the joule. 
 The joule is nearly equal to 0.738 foot- 
 pound ; that is to say, one foot-pound is an 
 expenditure of work equal to about 1.355 
 joules. In the same way the foot-pound- 
 per-second, the mechanical unit of activity, 
 
126 ELECTRICITY IN 
 
 might be employed as the electric unit 
 of activity, but it is more convenient to 
 employ the joule-per-second, or the watt, 
 of which 746 are equal to one horse-power, 
 of 550 foot-pounds per second. 
 
 An electromotive force or pressure is 
 always required to send a current through 
 a circuit ; that is to cause electricity to 
 flow, and, as in the case of mechanical 
 work, the amount of electrical work done 
 is equal to the amount of electricity set in 
 motion multiplied by the pressure which 
 causes it to move. When a unit of E. M. 
 R, or one volt, causes one coulomb of elec- 
 tricity to pass through a circuit, there is 
 one unit of work expended called the 
 volt-coulomb, or the joule. When a pres- 
 sure of one volt causes a coulomb-per- 
 second to pass through the circuit, that is 
 when a volt causes a current of one ampere 
 
to flow, electric work is abrffein the cir- * 
 
 cuit at the rate of one watt, so that if we 
 multiply the volts in the circuit by the 
 amperes, we have the activity in the circuit 
 expressed in watts. Thus, if an electric 
 pressure of 20 volts, measured between 
 electrodes, be applied to the human body, 
 and the current produced in the circuit 
 under these conditions be 25 milliamperes, 
 then the electric activity in the body will 
 
 be 20x ifro = wo = °- 5 watt ' or half a 
 
 joule each second; i. e. y 0.369 foot-pound 
 each second. 
 
 If a galvano-cautery be supplied with a 
 current of 20 amperes, under a pressure of 
 2 volts, the activity in the knife will be 
 2 x 20 = 40 watts = 40 joules-per-second = 
 29.52 foot-pounds-per-second ; and, if this 
 activity be sustained for two minutes, the 
 
128 ELECTRICITY IN 
 
 work done will amount to 40 X 120 = 4,800 
 joules = 3,546 foot-pounds; that is to say 
 to 1 pound lifted 3,546 feet, or to one ton 
 of 2,000 pounds lifted 1.723 feet. 
 
 The amount of activity expended in a 
 small incandescent lamp, of say 1/2 candle 
 power, frequently employed for exploring 
 cavities in the human body, may be found 
 from the fact that such a lamp only re- 
 quires a current of 1.4 amperes, at a pres- 
 sure of 3 volts at its terminals. This 
 represents an activity of 3 X 1.4 = 4.2 
 watts, or an expenditure of 8.4 watts per 
 candle. 
 
 Electric activity in a circuit has al- 
 ways to be provided by the source of the 
 driving E. M. F. Thus, when a voltaic 
 battery is supplying a pressure which 
 drives a current, it is the chemical energy 
 
ELECTRO-THERAPEUTICS. 129 
 
 in the battery which has to provide the 
 activity and the work done. In other 
 cases, where a dynamo is the source of 
 
 E. M. P., the power has to be supplied 
 from the engine which drives the dynamo. 
 When, therefore, an E. M. F. drives a cur- 
 rent, it does work on that current, and the 
 work must be supplied by the source of E. 
 M. F. On the other hand, when an E. M. 
 
 F. is driven by a current, that is to say 
 when a current passes in a circuit against 
 the action of an E. M. F., so that the cur- 
 rent overcomes the E. M. F., then work is 
 done upon the E. M. F. instead of by the 
 E. M. F., and work appears in the source 
 of E. M. F. For example, when a current 
 passes through a voltameter ; i. e., a vessel 
 containing acidulated water, and provided 
 with platinum electrodes, an E. M. F. is 
 set up at the surface of the immersed 
 electrodes, opposing the passage of the cur- 
 
130 ELECTRICITY IN 
 
 rent. Such an E. M. F. is called a counter 
 M M. F., and is abbreviated C. E. M. P. 
 
 Fig. 50, represents a form of voltameter. 
 The current passes between the binding 
 posts through the acidulated water con- 
 tained in the vessel A, being led in and 
 out by platinum plates or electrodes. If a 
 current of two amperes passes through 
 such an apparatus, and a C. E. M. F. of 
 2.5 volts be set up at the surface of the 
 electrodes, an activity will be expended of 
 2.5X2=5 watts, upon this E. M. F., and 
 this work will be expended chemically, 
 in decomposing the acidulated water and 
 liberating its constituent gases, oxygen and 
 hydrogen, which appear at the terminals 
 connected respectively with the positive 
 and negative poles of the battery. As- 
 suming that none of the liberated gas 
 is dissolved, it will accumulate in the 
 
ELECTRO-THERAPEUTICS. 
 
 131 
 
 Fig. 50. 
 
 collection tubes over the respective elec- 
 trodes, and, if allowed to enter into com- 
 bination at any subsequent period, will 
 liberate an amount of work in the ex- 
 
132 ELECTRICITY IN 
 
 plosion, equal to the work done by the 
 electric current in evolving it. A volta- 
 meter is used for measuring the strength 
 of a current by the rate of decomposition 
 of water. It is not, however, as convenient 
 for such purposes as an ammeter. 
 
 When an electric current passes through 
 a wire offering a resistance, a C. E. M. F. is 
 practically developed in the wire ; that is 
 to say, if a current of 5 amperes passes 
 through a resistance of 2 ohms, a C. E. M. 
 F. of 10 volts will be established in the 
 wire, for the reason that, by Ohm's law, 10 
 volts are necessary at the terminals of the 
 wire in order to send five amperes through 
 2 ohms resistance. The product of the C. 
 E. M. F. and the current, represents the ac- 
 tivity expended on the C. E. M. F., and this 
 work is always expended in heating the 
 wire. In the case assumed, 50 watts 
 
ELECTRO-THERAPEUTICS. 133 
 
 would be expended in the substance of 
 the wire as heat. The apparent C. E. M. 
 F., which is produced in a circuit by its 
 resistance when overcome by a current, 
 expends activity in heat ; whereas, the 
 C. E. M. F., which is due to chemical 
 decomposition, or to magnetic action, ex- 
 pends activity chemically or magnetically. 
 In other words, work done in a circuit 
 against the C. E. M. F. of resistance, is 
 work expended thermally ; and, therefore, 
 is practically irrecoverable, while work 
 done in a circuit against the C. E. M. F. 
 of chemical decompositions, or of magnetic 
 action, is capable of being partly or almost 
 entirely utilized. 
 
 Fig. 51, represents an electric calorim- 
 eter ; i. e., a device for measuring an 
 electric current by the amount of heat 
 produced in a wire which is immersed in a 
 
134 ELECTRICITY IN 
 
 known volume of water. In the form of 
 calorimeter shown in the figure, the cur- 
 rent enters and passes through the resist- 
 ance coil JVM, surrounded by a known 
 quantity of water. A thermometer T, is 
 
 Fig. 51. 
 
 provided for measuring the increase in 
 temperature. The amount of electric 
 energy, which must be expended as heat 
 in order to raise the temperature of one 
 cubic centimetre, or one gramme, of water 
 through 1° C, is called a water-gramme- 
 
^ iwR «^ 
 
 degree-centigrade, or a le^^^jalorie, and 
 is approximately equal to 4.2joules. Con- 
 sequently, a pound of water being 453.6 
 grammes, when raised through a tempera- 
 ture of 10° C, requires an expenditure of 
 energy equal to 4,536 water-gramme- 
 degrees, and 4,536 X 4.2 = 19,051 joules. 
 
 In the case of a current of 25 milli am- 
 peres passing through the human body, 
 under a pressure of 20 volts, and repre- 
 senting an activity of 0.5 joule-per-second, 
 or 0.5 watt, a certain amount of this C. E. 
 M. F., probably 2.5 volts, would be due to 
 electrolytic action, and the remainder, or 
 17.5 volts, due to the resistance of the body 
 as a conductor. The activity expended 
 electrolytically would, therefore, be 2.5 x 
 
 25 
 
 —^-r = 0.0625 joule-per-second, and the 
 
 remainder, or —^7^- = 0.4375 ioule-per- 
 1,000 J r 
 
136 ELECTRICITY IN 
 
 second, would be expended in warming the 
 conducting materials in the body. The 
 electrolytic work would be expended at 
 the surface of the metallic electrodes, while 
 the thermal activity would be expended 
 wherever the resistance was overcome ; 
 and, moreover, expended in proportion to 
 the amount of resistance. Assuming, 
 however, for the sake of illustration, that 
 the amount of heat so developed was 
 equally diffused throughout the whole 
 body, and that the capacity of the body 
 for heat was that of 100 lbs. of water, 
 then one joule, expended in the body 
 thermally, would raise its temperature 
 
 4.2xl00X453.6 = i9pl0 ° f a de ^ ee ^ 
 tigrade. 0.4375 joule would raise it 
 
 ' of a degree centigrade. The appli- 
 
ELECTRO-THERAPEUTICS. 137 
 
 cation of the current for 30 minutes, or 
 1,800 seconds, would raise its temperature 
 
 ^-^r of a degree centigrade, ap- 
 
 190,510 242 
 proximately. 
 
 It will therefore be evident that the 
 current strengths ordinarily employed in 
 electro-therapeutics cannot directly raise 
 the temperature of the body to any sensible 
 degree, although physiological activities 
 called into action thereby may do so. A 
 current of several amperes, however, passed 
 through the body, for a few seconds, will 
 raise the temperature appreciably. 
 
CHAPTER VII. 
 
 FRICTIONAL AND INFLUENCE MACHINES. 
 
 Our earliest notions concerning elec- 
 tricity were obtained from the electric 
 effects produced by the friction of one sub- 
 stance against another, such, for example, 
 as a piece of glass against a silk handker- 
 chief. Under these circumstances, both 
 the glass that is rubbed and the silk with 
 which it is rubbed, acquire electric ex- 
 citement, as manifested by their ability to 
 attract light bodies, such as shreds of 
 paper, brought near them. It can be 
 shown that all bodies possess the ability 
 of acquiring electric excitement by fric- 
 tion against other bodies, and that when- 
 
 138 
 
ELECTRO-THERAPEUTICS. 139 
 
 ever two dissimilar substances, or even two 
 dissimilar surfaces of the same substance, 
 are brought into contact, an E. M. F. 
 is produced at the contact surfaces, one 
 substance becoming positive and the other 
 negative. The friction of one substance 
 against another is, therefore, another 
 method of producing an E. M. F., and, as 
 in the case of the other sources referred 
 to, this E. M. F., if provided with a circuit, 
 is capable of setting electricity in motion. 
 
 The E. M. F.'s produced by friction, 
 are much higher than those produced by 
 either voltaic cells or dynamo-electric 
 machines; consequently, they are capable 
 of causing electricity to pass through a cir- 
 cuit even when separated by a small inter- 
 val or air-gap. It is found that an E. M. 
 F. of, approximately, 80,000 volts is re- 
 quired to produce a discharge or spark 
 
140 ELECTRICITY IN 
 
 across an air-gap one inch in length, be- 
 tween two slightly convex surfaces, and 
 that, roughly, 80,000 volts per inch of 
 sparking distance, is a fairly reliable esti- 
 mate of pressure for distances, lying 
 between 1/100" and 3". Beyond these 
 limits, the rule cannot be safely applied, 
 and, in fact, for very large sparking dis- 
 tances of more than one foot, a much 
 smaller E. M. F. than 80,000 volts per 
 inch appears to be needed. When the 
 opposed electrodes terminate in points, 
 instead of in blunt surfaces, a relatively 
 much smaller pressure is needed to effect 
 discharge. 
 
 Various devices have been employed in 
 order to produce electricity by the friction 
 of one substance against another. Such 
 devices are called frictional electric ma- 
 chines, and consist essentially of a plate 
 
ELECTRO-THERAPEUTICS. 141 
 
 or cylinder, generally of glass, so rotated 
 as to be rubbed against a rubber of 
 chamois skin, covered with an amalgam of 
 mercury and tin. By this friction, both 
 the rubber and the glass acquire an elec- 
 tric potential. A comb of points placed 
 near the glass and connected with an 
 insulated conductor, enables the conductor 
 to become charged. A smaller insulated 
 conductor, connected electrically with the 
 rubber, enables the conductor to take the 
 opposite or negative charge. 
 
 A well known form of frictional elec- 
 tric machine is shown in Fig. 52. Here 
 the glass plate PP, is rotated in a vertical 
 plane between two rubbers J?, i?, of 
 chamois leather, connected electrically 
 with the ground. In this form of 
 machine, two insulated conductors C, C, 
 are connected to separate pairs of combs 
 
142 
 
 ELECTRICITY IN 
 
 near the surface of the revolving glass 
 plate and thus receive a positive charge. 
 
 Fig. 52. 
 
 When an electric machine is properly 
 operated it has the power of sending a 
 torrent of minute sparks through a con- 
 siderable air-space. The E. M. F. pro- 
 
ELECTRO-THERAPEUTICS. 
 
 143 
 
 duced by this type of source is of the 
 pulsatory character, and is shown in Fig. 
 53, here represented at about 140,000 volts, 
 or 140 kilo volts. 
 
 100- 
 110- 
 
 m- 
 
 o I 
 
 d 
 
 * 40H 
 
 29- 
 
 /FWflWFTHITK 
 
 SECONDS. 
 
 Fig. 53. 
 
 When a pulsatory E. M. F. rises to such 
 an amount as will permit it to discharge 
 through an air-gap, it suddenly falls on 
 discharge to a minimum which is not 
 always the same. It then recovers and 
 again discharges, this action being carried 
 
144 ELECTRICITY IN 
 
 on in a pulsatory manner at frequent in- 
 tervals. E. M. F.'s of this character are 
 frequently employed in electro-therapeu- 
 tics under the name of Franklinic E. M. 
 F^s, after Benjamin Franklin. A fric- 
 tional electric machine, however, is too un- 
 certain in its action to be employed for 
 this purpose, being too much dependent 
 on the conditions of the weather, since, 
 during damp weather, the film of moisture 
 which settles upon the surface of the glass, 
 and on the supporting pillars, is often 
 sufficient to conduct away the electric 
 charges and prevent their formation. For 
 this reason, frictional machines have been 
 replaced by influence machines of which 
 there are a number of different designs. 
 
 In order to understand the operation of 
 an influence machine, it is necessary to 
 first investigate what occurs in the space 
 
ELECTRO-THERAPEUTICS. 
 
 145 
 
 surrounding an electrically charged body. 
 If we support a metallic sphere A, upon 
 a table in a room, B C D E, Fig, 54, 
 
 E I 
 
 Fig. 54. 
 
 and connect this sphere with an E. M. 
 F., the sphere will receive a charge. If 
 we connect the sphere by a wire to one 
 terminal of a voltaic cell, the sphere will 
 become charged, thereby, but so feebly, 
 
146 ELECTRICITY IN 
 
 that delicate apparatus will be necessary 
 in order to reveal the presence of the 
 charge. If, however, we connect the 
 sphere to one terminal of a battery of 
 10,000 cells, each having an E. M. F. of 
 one volt, an appreciable charge will be 
 communicated to the sphere, which will 
 now manifest distinct electrostatic proper- 
 ties. Finally, if we connect the sphere to 
 one terminal of an electrostatic machine, 
 having an E. M. F. of, perhaps, 200,000 
 volts, the charge acquired by the sphere 
 will be comparatively great. That is to 
 say, it will receive a comparatively large 
 quantity of electricity, which will be a cer- 
 tain fraction of a coulomb. It is common 
 to regard such an electric charge as being 
 situated on the surface of the body A. 
 Such, however, is not the fact, the charge 
 really resides in the air, or more strictly 
 in the air and the ether surrounding the 
 
ELECTRO-THERAPEUTICS. 147 
 
 body, and the function of the metallic 
 cylinder is merely to provide a surface 
 from which the charge can enter and 
 influence the ether around it. 
 
 If we assume, for the sake of illustra- 
 tion, that the charge communicated to the 
 
 sphere is -i aaa oonfo °^ a cou ^ om ^ or one 
 micro-coulomb, and that the E. M. F. at 
 which it was delivered to A, was 100,000 
 volts, or, in other words, that the potential 
 of A, is 100,000 volts, then the work de- 
 
 livered to A, is 100,000 x lf) ^ m = ^ 
 
 joule =0.0738 foot-pound. This energy is 
 received by the air and ether surrounding 
 the sphere, and is held there during the 
 maintenance of the charge. The energy is 
 distributed through all the ether in the 
 room, although not equally distributed. 
 
148 ELECTRICITY IN 
 
 A certain fraction of a. joule is thereby 
 charged in each cubic inch of space, the 
 greater amount being in the immediate 
 neighborhood of the sphere, and lessening 
 with distance from the same. The charge 
 is passed into the ether by an action which 
 is called electric displacement. Electric 
 displacement takes place along defined 
 lines or curves through the ether in the 
 room, or as it is sometimes stated, electro- 
 static flux proceeds from the charged body 
 through the ether in the room along lines 
 or paths called lines or curves of electro- 
 static flux. The shape of these lines de- 
 pends on the shape of the body, and on the 
 shape of the enclosure in which it is 
 placed, or of the bodies which may be in 
 its neighborhood. Fig. 54, gives a dia- 
 grammatic view of some of these displace- 
 ment curves, or curves of electrostatic flux. 
 The total amount of electrostatic flux 
 
ELECTRO-THERAPEUTICS. 
 
 149 
 
 is proportional to the charge on the 
 body. 
 
 Fig. 55, gives a graphic representation 
 of a mechanical model showing the action 
 
 Fig. 55. 
 
 which an electrified sphere exerts upon the 
 space surrounding it. Let j h I ?n, be a 
 diametral section of a spherical elastic 
 bag of rubber, and e f g h, the section of 
 another bag of rubber surrounding the first 
 concentrically. Moreover, suppose these 
 bags to have the space between them en- 
 tirely filled with water. If 
 
 now, air be 
 
150 ELECTRICITY IN 
 
 pumped into the interior of the inner bag 
 it will distend, and, in distending, will 
 cause the outer bag to also distend, al- 
 though to a lesser degree, since the water 
 between them is practically incompressible. 
 The inner bag corresponds to the metallic 
 sphere of Fig. 54, and the outer bag cor- 
 responds to the walls of the room, in 
 which the sphere A, is suspended. The 
 water filling the space between the bags 
 corresponds to the ether filling the space 
 between the sphere and the walls of the 
 room. The air pressure communicated to 
 the interior bag by pumping air into it, 
 corresponds to the electric pressure, or high 
 potential communicated to the sphere. 
 Under the influence of this pressure, the 
 inner bag expands or receives a charge, the 
 expansion causes liquid flux or streamings 
 through all the mass of liquid, which dis- 
 tends the outer sphere. This corresponds 
 
On 
 
 to tlie fact that the cltaiige communicated 
 to the sphere is imparte^^O'^he,, surround- 
 ing ether and passes, in the form of electro- 
 static flux, through the whole ether space 
 until intercepted by the walls of the room. 
 The total charge of the sphere may be re- 
 garded as being equal to the total dis- 
 placement and also numerically equal to 
 the total negative charge accumulated on 
 the walls of the room. 
 
 In order to show the effect of the shape 
 of a body on the direction of the flux 
 paths, a few examples may be noted. Fig. 
 56, shows two insulated concentric spheres 
 connected with an E. M. F., the inner 
 sphere being positive. Here the flux 
 issues from the inner sphere in radial 
 lines, and terminates on the surface of the 
 outer sphere. The total amount of dis- 
 placement which passes through any 
 
152 ELECTRICITY IN 
 
 spherical envelope, which could be drawn 
 between A and B, is equal to the charge 
 which resides on A or B. In other words, 
 the system behaves as though a certain 
 amount of ether had been liberated at the 
 surface of A, passed through the surround- 
 
 Fig. 56.— Electrostatic Flux Paths Between Con- 
 centric Spherical Conducting Surfaces, Insulated 
 prom Each Other. 
 
 ing space against elastic reaction, along 
 radial lines, until finally, a certain amount 
 is forced into the shell of the external 
 sphere at B. The quantity of charge 
 which thus enters the system depends 
 upon two things; namely, the magnitude 
 
ELECTRO-THERAPEUTICS. 153 
 
 of the E. M. F. employed, and, secondly, 
 the shape of the mass of ether brought 
 under the action of this force. If we 
 double the E. M. F., we double the 
 charge, and if we halve the E. M. F. we 
 halve the charge. Again, if we employ a 
 
 Fig. 57. — Electrostatic Flux Paths Between 
 Insulated Concentric Spheres. 
 
 thinner stratum of ether we shall reduce 
 its resiliency, and increase the charge for a 
 given E. M. F. employed. If, for example, 
 the inner sphere be larger, as shown in 
 Fig. 57 ? the same E. M. F. will now act 
 upon a thinner layer of air and ether, and 
 will produce a greater displacement or 
 
154 
 
 ELECTRICITY IN 
 
 charge ; in other words, the resiliency of the 
 mass of ether, enclosed between the ter- 
 
 -->-->—>--> 
 
 Fig. 58. — Electrostatic Flux Paths, Parallel 
 Plane Spheres. 
 
 minals connected with the E. M. F., has 
 been reduced. 
 
 Fig. 58, represents two parallel plane 
 surfaces connected with an E. M. F, 
 
ELECTRO-THERAPEUTICS. 155 
 
 Here the left-hand plane A, is considered 
 as positive ; i. e., the electrostatic flux is 
 assumed to emanate from A, pass through 
 the intervening space, and terminate at the 
 surface of B. There will be a positive 
 charge on A, an equal negative charge on 
 B, and the same charge represented in dis- 
 placement all through the mass of ether be- 
 tween the plates. The amount of charge 
 which will enter the system, will depend 
 upon the E. M. F. brought to bear upon 
 the plates A and B, the thickness of the 
 stratum of ether, and the area of the plates 
 or stratum. If the area of the opposed 
 plates be increased, the elastic resiliency of 
 the mass of ether between the plates is di- 
 minished, and a proportionally greater 
 charge enters the system. Similarly, if the 
 plates be approached, so that the stratum 
 of included air becomes thinner, its resili- 
 ency is diminished, and the E. M. F. will 
 
156 ELECTRICITY IN 
 
 force more flux through the system, and a 
 corresponding greater charge. In either 
 case, therefore, we have what might be re- 
 garded as an electrostatic circuit. 
 
 The amount of flux which will pass 
 through the circuit ; i. e., the amount of 
 charge which can be communicated to the 
 surface of the dielectric involved, will 
 depend upon the E. M. F., and also 
 upon the elastic resistance of the medium. 
 
 E M F 
 
 Electrostatic flux=-pn — ; ' . * — '- 
 
 Electrostatic resistance. 
 
 The greater the electrostatic resistance, the 
 less the flux, and vice versa. This corre- 
 sponds completely to Ohm's law for the 
 voltaic circuit, except that the electrostatic 
 resistance is not a resistance to the passage 
 of electric current but is the resistance to 
 the passage of electrostatic current or flux. 
 Moreover, the same rules apply to the 
 
ELECTRO-THERAPEUTICS. 157 
 
 resistance offered by a wire to the passage 
 of a current, and the resistance offered by 
 a dielectric mass to the passage of an 
 electrostatic flux. The longer the mass the 
 greater the resistance ; the greater its area 
 of cross-section, the smaller its resistance. 
 
 The displacement lines, or lines of 
 electrostatic flux, which may be drawn 
 for any completely specified electrostatic 
 system, and which can be experimentally 
 determined in most cases, represent lines 
 in the dielectric medium along which 
 stress exists, by virtue of the electrostatic 
 flux. This stress, which is developed in 
 the ether, is dependent upon the energy 
 absorbed by the ether during the existence 
 of the electric charge. Along these 
 curves, in fact, there is exerted a continual 
 tension, or, in other words, the displace- 
 ment lines are always tending to contract 
 
158 ELECTRICITY IN 
 
 and shorten. For example, the two 
 charged plates A and B, shown in Fig. 58, 
 being connected by a number of displace- 
 ment lines, tend to attract each other. 
 The real tendency is for the shortening of 
 the lines of stress, or flux lines. The ordi- 
 nary statement that positively and nega- 
 tively' electrified bodies tend to attract 
 each other, should more accurately be : 
 positively and negatively electrified bodies, 
 being connected by lines of electrostatic 
 flux, tend to come together by reason of 
 the contraction of the flux lines. 
 
 If, as in Fig. 59, a small spherical con- 
 ductor C, be introduced into the electrostatic 
 flux, the effect is twofold ; first, the electric 
 medium is thinned locally by the presence 
 of the conductor, so that its resiliency is 
 locally diminished, and a more powerful 
 flux will pass through the system in its 
 
ELECTRO-THERAPEUTICS. 159 
 
 neighborhood than elsewhere. Second, 
 the flux will be intercepted by the con- 
 ductor, which will form the termination 
 of the flux on the entering side and a new 
 starting point on the leaving side. A 
 negative charge will, therefore, appear on 
 the surface where the flux terminates, and 
 a positive charge on the surface where the 
 flux reissues. This appearance of posi- 
 tive and negative charges, on opposite 
 sides of an insulated body supported in 
 an electrostatic flux, is commonly called 
 electrostatic induction. It is merely a 
 consequence of the fact that the body 
 relieves from electrostatic stress the ether 
 which it displaces, and that, in conse- 
 quence of this relief, charges appear at the 
 surfaces where the flux enters and leaves. 
 
 If in Fig. 59, the small conductor C, is 
 charged by having been connected with a 
 
160 
 
 ELECTRICITY IN 
 
 suitable E. M. F. it will do more than 
 merely relieve ether of its duties; for, it 
 
 "A 
 + 
 
 Fig. 59.— Diagrammatic Representation of Electro- 
 static Flux Paths. 
 
 will add flux of its own to the flux in which 
 it is introduced. For example in Fig. 60, 
 two spheres + and — , are shown, at A, 
 
ELECTRO-THERAPEUTICS. 161 
 
 which have been connected with the posi- 
 tive and negative terminals of a high 
 E. M. F. The electrostatic flux passing 
 between them, through the surrounding 
 ether, is partly represented diagrammati- 
 cally by the dotted lines. These two 
 spheres evidently behave as though they 
 attracted each other, owing to the contract- 
 ing forces of all the flux paths between 
 them. If we suppose them fixed upon 
 suitable insulating pillars, so that they 
 cannot approach each other, and that a 
 smaller conducting sphere is introduced 
 between them, as shown, this smaller 
 sphere will acquire a positive charge from 
 the positive sphere, and will thus become 
 the recipient of a number of flux paths 
 which emerge from it, which tend to pull it 
 across toward the negative sphere. Under 
 the influence of these attractive forces, 
 the smaller sphere, if it be free to move, 
 
162 ELECTRICITY IN 
 
 will move to the right. When it is in 
 the position shown on the right, midway 
 between the two spheres, it will be seen 
 that many flux paths connect it with the 
 negative sphere, while no flux paths con- 
 nect it with the positive sphere. As soon 
 
 / - — -« 
 
 A 
 
 Fig. 60.— Effect of Charged Sphere. 
 
 as it reaches the negative sphere it will 
 deliver up its charge, and reduce the po- 
 tential of the negative sphere unless the 
 latter be connected with an electric source. 
 It will then acquire a negative charge and 
 new flux paths will enter its surface from 
 the positive sphere. Owing to the attrac- 
 tion of these flux lines it will again be 
 drawn to the left and thus a continual to- 
 
PROPERTY of' 
 
 ELECTRO-THER 
 
 and-fro motion will be set 
 difference of potential or E. M. 
 between the two large spheres. 
 
 Fig. 61, represents at A, the effect of 
 inserting, between the two large spheres, 
 
 ^W^ j 
 
 A B 
 
 Fig. 61.— Effect of Uncharged Sphere. 
 
 a small sphere in an uncharged condition. 
 It will be observed that the effect is to 
 intercept a larger number of flux paths, 
 and thus to relieve from duty the ether 
 contained within the space occupied by 
 the small sphere. In this case the attrac- 
 tions, on each side of the small sphere, are 
 balanced. If, however, the small sphere 
 be placed nearer one side than the other, 
 
164 ELECTRICITY IN 
 
 as shown at B, the stratum of ether 
 between it and the positive sphere will be 
 thinner than the stratum on the right ; and, 
 consequently, a greater electrostatic flux 
 will pass through the space on the left, 
 thereby entailing the introduction of a 
 greater number of flux lines, and a greater 
 electrostatic force urging the sphere to the 
 left. 
 
 It will be seen, from a consideration of 
 the preceding phenomena, that the follow- 
 ing may be generalized as the laws of 
 electrostatic charges, attractions and repul- 
 sions; namely, 
 
 (1) That every electrified body forms a 
 locus or place where electrostatic flux 
 enters or leaves a dielectric medium ; and, 
 conversely, that all conducting surfaces, 
 where lines of electrostatic flux terminate, 
 are said to be charged surfaces, the flux 
 
ELECTRO-THERAPEUTICS. 165 
 
 being assumed to leave at positively 
 charged surfaces aud to enter at negatively 
 charged surfaces. 
 
 (2) Lines of electrostatic flux are the 
 directions along which electrostatic stress 
 exists in the ether, and accompany the 
 temporary absorption of energy into the 
 ether. 
 
 (3) Dissimilarly charged bodies attract 
 one another, because lines of electrostatic 
 flux tend to contract. 
 
 (4) That similarly charged bodies ap- 
 pear to repel, owing to the fact that 
 no flux paths connect them, but that flux 
 paths connect each of them with neigh- 
 boring objects, so that they are drawn 
 to the neighboring bodies and are not 
 repelled from each other. 
 
 (5) Electrostatic induction accompanies 
 the introduction of a conductor into the 
 electrostatic flux, whereby charges are 
 
166 ELECTRICITY IN 
 
 caused to form upon orrposite sides of 
 the interposed conductor. 
 
 The principle of electrostatic induction 
 is employed in influence machines. A 
 
 Fig. 62.— Electrophorus. 
 
 simple form of influence machine is called 
 the elect rophoros, and is illustrated in 
 Fig. 62. A disc £, of vulcanite, resin, 
 or other suitable material, is vigorously 
 rubbed, say with a cat skin, and thereby 
 becomes negatively charged, under the 
 influence of the powerful E. M. F. set 
 
ELECTKO-THERAPEUTICS. 167 
 
 up. If such an electrified disc be laid on 
 a table, as shown in Fig. 63, so that its 
 electrified surface is uppermost, the flux 
 paths may be represented diagrammati- 
 cally b}^ the arrows. 
 
 Fig. 63. — Representing Action and Operation 
 of electrophoru8. 
 
 If now, an insulated metallic disc A, 
 Figs. 62, and 64, furnished with round 
 edges, be rested on the disc, there will be 
 no great change produced in the electro- 
 static system. There will only be a slight 
 reduction in the dielectric resistance of the 
 air, owing to the interposition of the con- 
 
168 ELECTRICITY IN 
 
 ducting disc across the electrostatic flux 
 paths. When, however, the disc is 
 touched with a finger, as shown in Fig. 
 62, or connected with the ground, as 
 shown in Fig. 65, all the flux paths are 
 shortened, until they exist only between 
 
 
 ' „- — v , 
 
 1 ; r<l 
 
 t /„-.. 
 
 1/ * i * 
 
 Fig. 64. — Representing Action and Operation of 
 Electrophorus. 
 
 the excited disc and the grounded metallic 
 plate. In other words, the length of the 
 electrostatic circuit has been reduced to 
 that of a thin film of air, and, con- 
 sequently, the electrostatic flux, set up 
 across this film, will be comparatively 
 powerful, and a powerful charge will be 
 
ELECTRO-THERAPEUTICS. 169 
 
 communicated to the plate at the point 
 where the flux emerges from it. If now, 
 the ground connection to the plate be 
 removed, and the plate lifted, as shown 
 in Figs. 66 and 67, the electrostatic 
 circuit will again be lengthened, and a 
 
 f 
 
 Fig. 65.— Electrophorus. 
 
 charge will be left in the plate as well 
 as on the disc, the disc being negative 
 and the plate positive. The metal plate 
 can be discharged and recharged many 
 times in succession. 
 
 An influence machine is, in reality, 
 a form of a revolving electrophorous. 
 A common form of influence machine, 
 
170 ELECTRICITY IN 
 
 called a ToepUr-Holte Machine is shown 
 in Y\v. 68. It will be understood that 
 since this apparatus operates by electro- 
 static induction, no friction is needed. An 
 initial charge is, however, required. The 
 apparatus consists essentially of three 
 
 /vmtHmtttttttttr^ 
 
 t HttttttHtttttntttt/ / 
 
 1 ■ —L_ 
 
 Fig. 66.— Electrophorus. 
 
 vertical glass plates of which the central 
 is the largest in diameter, and is fixed, 
 while the two outside plates are mounted 
 on a common shaft, and are capable of 
 being revolved in the same direction by 
 the aid of the handle h. The central plate 
 carries two metallic and papered surfaces 
 
# 
 
 ^^^SEtfj^ 
 
 ^L^CTUo-TmmAVT^dff^p r P 7^1 
 
 % 
 
 CF 
 
 Q> 
 
 e/ 
 
 A B Ctm&A' B C^%)f which A B 'c, 
 is positively electrified ik^^ev outset aqd 
 J.' ^' 0*, is negatively elec^Sisi^Bia^- 
 outside plates carry only metallic buttons 
 on their external surfaces, each button 
 
 Fig. 67. — Electrophorus. 
 
 consisting of a disc of tinfoil, with a small 
 brass cap in the centre. Six of these tin- 
 foil discs are represented as being carried 
 on each outside plate. 
 
 An electrostatic circuit will be set up 
 from the positively electrified to the nega- 
 
172 
 
 ELECTRICITY IN 
 
 tively electrified surface as shown at A, in 
 Fig. 69. The presence, however, of the 
 metallic rod i?i?, which is supported in 
 
 Fig. 68. 
 
 -Triple-Plate Toepler-Holtz Electrical 
 Machine. 
 
 such a manner that the combs at its ex- 
 tremities come in contact with the insu- 
 lated discs on the outside plates as they 
 revolve, limits the electrostatic circuit al- 
 most entirely to the space between the 
 
ELECTKO-THERAPEUTICS. 
 
 173 
 
 central and outside plates, as shown at B, 
 Fig. 69, so that the flux paths become 
 more numerous and terminate on the inner 
 
 
 VT4 vv^ 
 
 
 -^tt^ 
 
 ->*•• — *-^» 
 
 B + 
 
 
 a 
 
 bT- 
 
 
 za 
 
 Fig. 69.— Electrostatic Circuits of Influence 
 Machine. 
 
 surfaces of the insulated discs as they pass 
 by. Under these circumstances, a nega- 
 tive charge will form on the disc a?, and a 
 positive charge on the disc y. As soon as 
 
174 ELECTRICITY IN 
 
 the discs have been carried from beneath 
 the combs on the rod HH, they retain 
 these charges until they reach the opposite 
 side of the frame, when the disc a?, comes 
 in contact with the brush b', thereby com- 
 municating its charge to the already nega- 
 tively electrified surface A' B' C> on the 
 central plate, and, passing with the re- 
 mainder of its charge, delivers this re- 
 mainder to the comb of points attached to 
 the handle and main conductor IF. Simi- 
 larly, the disc y, which retains its positive 
 charge after quitting the comb on the 
 lower extremity of the rod MR, is carried 
 to the brush b, and communicates its charge 
 to the already positively electrified surface 
 A B C, the remainder of its charge being 
 collected by the comb on the handle H. 
 Consequently, during rotation, the half of 
 the rotating plates on one side of the rod 
 Hit, is positively, and the other half, nega- 
 
ELECTRO-THERAPEUTICS. 175 
 
 tively electrified. The charges on the 
 electrified surfaces A B Cand A! B (7/ auto- 
 matically increase, until a balance is main- 
 tained between the further accession of 
 charge, and the leakage which takes place 
 between them. This leakage limits, there- 
 fore, the maximum E. M. F. obtainable by 
 the machine. When the discharging rods 
 H, Hj are brought close together, the pres- 
 sure obtained is lower, owing to the fact 
 that a smaller E. M. F. is required to pro- 
 duce a spark discharge across the air-gap, 
 and a more rapid stream of discharges over 
 this air-gap and a lower pressure may, con- 
 sequently, be expected. On increasing the 
 distance between the discharging rods, the 
 pressure increases, but the frequency of 
 discharge usually diminishes. 
 
 A form of apparatus known as a con- 
 denser consists essentially of an electro- 
 
176 ELECTEICITY IN 
 
 static circuit of low resistance, that is to 
 say, of an electrostatic circuit of short 
 length and large cross-sectional area. A 
 condenser, therefore, offers a comparatively 
 small elastic resistance to displacement of 
 flux, and, under a given E. M. F., will re- 
 ceive a correspondingly large charge. 
 
 Fig. 70, shows a Ley den jar, which is 
 the usual form of condenser employed with 
 high E. M. F.'s. Here the active sur- 
 faces are formed of inner and outer coat- 
 ings of tin-foil, and the dielectric consists 
 of the glass walls of the jar. The length 
 of such an electrostatic circuit; i. e. y the 
 thickness of the glass, may be about l/8th 
 of an inch, and the cross-sectional area of 
 the electrostatic circuit ; i. e., the area of 
 the tin-foil surface, about a square foot. 
 Moreover, glass offers less electrostatic 
 resistance than air, and, therefore, the glass 
 
ELECTRO-THERAPEUTICS. 
 
 177 
 
 Leyden jar makes a better condenser than 
 an imaginary air jar of the same dimen- 
 sions. The relative value of the glass de- 
 
 II 
 
 Fig. 70.— Leydes Jar. 
 
 pends upon its quality, but it may readily 
 offer five times less electrostatic resistance 
 than air ; consequently, the* capacity of a 
 
178 ELECTRICITY IN 
 
 Leyden jar condenser may be five times 
 greater than that of a similar air con- 
 denser. Two small Leyden jars are shown 
 in Fig. 68, having their inner coatings con- 
 nected with the main terminals i7 and H\ 
 and their outer coatings connected by a 
 metallic strip not shown in the figure. 
 The effect of these jars is to diminish the 
 electrostatic resistance between the termi- 
 nals, and, therefore, enables a given E. M. 
 F. to accumulate -a greater electrostatic 
 flux or charge between the terminals. 
 
 The electric energy obtained from the 
 discharge of an influence machine through 
 an external circuit is supplied, mechanic- 
 ally, in the effort necessary to revolve the 
 machine against electrostatic forces. One 
 electrostatic machine acting as a gener- 
 ator, may readily be made to cause an- 
 other electrostatic machine to run back- 
 
Fig. 71.— Holtz Influence Machine. 
 
 wards, as a motor. The hand has, there- 
 fore, to be applied with greater force to 
 drive the influence machine, owing to the 
 
180 
 
 ELECTRICITY IN 
 
 fact that it is operating so as to furnish 
 current to the circuit connected to it. 
 
 Fig. 72. — Bonette Electrostatic Influence Machine. 
 
 A number of forms of influence ma- 
 chines are in existence. The principal 
 difficulty in operating such machines is to 
 maintain their insulation during all condi- 
 
ELECTRO-THERAPEUTICS. 181 
 
 Fig. 73. — Wimshurst Electrical Machine. 
 
182 ELECTRICITY IN 
 
 tions of weather, so that their charge shall 
 not be lost. For this purpose they are 
 often enclosed in glass chambers in 
 which the air is kept dry by some hygro- 
 scopic substance, such as calcium chloride. 
 Such a form of machine driven ■ by a small 
 electric motor is shown at Fig. 71. 
 
 In some forms of influence machines, in 
 order to ensure the presence of a small 
 charge, a small frictional attachment is 
 supplied, so that the proper charge shall 
 be ensured by the friction set up. 
 
 Glass plates are not invariably used in 
 these machines. Sometimes plates of 
 hard rubber are employed as shown in 
 Fig. 72. 
 
 Another convenient form of influence 
 machine is shown in Fig. 73 called the 
 
ELECTRO-THERAPEUTICS. 183 
 
 Wimshurst machine. In this case two 
 glass plates, supporting a number of small, 
 separate tin-foil conductors, are rapidly 
 driven in opposite directions. The action 
 of the machine differs only in detail from 
 that already described. 
 
 In conclusion, we may observe that all 
 electrostatic influence machines depend for 
 their operation upon the principle of the 
 electrophorus. The electrostatic circuit in 
 such machines is periodically lengthened 
 and shortened, and the charges so induced 
 are separated and accumulated. 
 
CHAPTER VIII. 
 
 MAGNETISM. 
 
 Magnetism is the science which treats 
 of the properties and laws of magnets 
 whether artificial or natural. Although 
 the nature of magnetism is not known, yet 
 a certain relationship unquestionably ex- 
 ists between magnetism and electricity, so 
 that a knowledge of the nature of one 
 must inevitably determine a knowledge 
 of the nature of the other. Both are 
 believed to be active conditions of the 
 universal ether and are so related that the 
 following laws appear to hold generally ; 
 viz., 
 
 (1) A motion of electricity invariably 
 produces magnetism. 
 
 184 
 
ELECTRO THERAPEUTICS. 
 
 185 
 
 (2) A motion of magnetism invariably 
 produces an E. M. F. 
 
 The nature of the action which exists 
 between electricity and magnetism may 
 
 Fig. 74.— Hydraulic Analogy op Relation Between 
 Electricity and Magnetism. 
 
 be illustrated by the following hydraulic 
 experiment. Suppose that a large cy- 
 lindrical tank, represented in Fig. 74, be 
 completely filled with water, and that a 
 plunger JP, is provided with a rod t, pass- 
 
186 ELECTRICITY IN 
 
 ing through water-tight packing in the 
 centre of one circular end. It is evident 
 that if the rod t, be moved forward, the 
 plunger jP, will advance into the tank. In 
 so doing it will displace the water in front 
 of it, which will flow round to the back of 
 the plunger in vortical paths, formed sym- 
 metrically around the face of the plunger. 
 These vortical paths, passing from the 
 front to the back* of the plunger, are illus-. 
 trated diagrammatically at A, Fig. 74. 
 The vortical movement of the water will 
 clearly be most marked in the immediate 
 neighborhood of the eds*es of the advanc- 
 ing plunger, gradually decreasing from 
 the edges to the sides of the tank. Imagi- 
 nary lines in the mass of the water, drawn 
 so as to represent the intensity of the 
 vortical movement, form circles, concentric 
 to the axis of the plunger, and at right 
 angles to the direction of its motion, as 
 
ELECTRO-THERAPEUTICS. 187 
 
 represented at B 1 Fig. 74. Circles are 
 marked with long arrows near the edge of 
 the plunger where the vortical motion is 
 most intense, and with shorter and shorter 
 arrows at greater distances from it. If 
 now, we remove the plunger from the tank, 
 and artificially cause a system of electric 
 currents to be produced in the mass of 
 quiescent water, such as is represented at 
 B, in Fig. 74, then accompanying this 
 system of electric currents, would be pro- 
 duced a magnetic distribution throughout 
 the water, such as is represented by the 
 stream lines at A, at right angles to the 
 direction of the electric current. In other 
 words, the relation of magnetic distribu- 
 tion, to electric current distribution, in any 
 space, is identical with the relation be- 
 tween the stream lines of motion in a 
 liquid, and the vortical distribution of 
 motion accompanying the same. 
 
188 ELECTRICITY IN 
 
 It follows from the preceding that if 
 the all-pervading ether were a non-com- 
 pressible fluid, like water, and if electric 
 currents consisted of vortices or whirls in 
 this fluid, that magnetism would consist of 
 a streaming motion in the ether. The prop- 
 erties of the ether are not yet thoroughly- 
 known, and it is by no means certain that 
 electric currents are vortices therein. All 
 that can be safely asserted is the existence 
 of a relationship between electric activity 
 and magnetic activity in the universal 
 ether, of the general nature- we have here 
 pointed out ; so that, if we should at any 
 time, discover the nature of either electric- 
 ity or magnetism, the nature of the other 
 would be immediately deduced. 
 
 Magnetism may be produced in two 
 ways ; viz., 
 
 (1) By permanent magnets of iron or 
 
ELECTRO-THERAPEUTICS. 189 
 
 steel ; or, in a lesser degree, by other mag- 
 netic metals such as nickel or cobalt ; and, 
 (2) By electric currents. 
 
 Magnetism appears to be an inherent 
 property of the molecules, or ultimate 
 particles, of iron or steel. In other 
 words, if we could isolate and perceive a 
 single molecule of iron, it is believed that 
 we should find that it naturally and per- 
 manently possessed magnetism, as a prop- 
 erty inherent in it. If the ultimate par- 
 ticles of iron are essentially magnetic, 
 the question naturally arises, why all iron 
 does not manifest magnetic properties? 
 The reason is believed to be found in the 
 fact that in iron, which is apparently un- 
 magnetized, the molecules lack a definite 
 arrangement of direction, and, pointing 
 irregularly, mask or neutralize each other's 
 magnetic influence. Such an undirected 
 
190 ELECTRICITY IN 
 
 system would, therefore, necessarily pos- 
 sess no appreciable external magnetism. 
 When a bar of iron is magnetized, it is 
 subjected to a process whereby its molec- 
 ular magnets are aligned, or similarly 
 directed, and, acting in concert, are 
 thereby enabled to manifest external mag- 
 netic effects. 
 
 The region surrounding a magnet is 
 filled with what is called magnetic flux or 
 magnetism, which is most powerful in the 
 immediate neighborhood of the poles. If 
 we assume, as a working hypothesis, that 
 magnetism consists of a streaming motion 
 of the ether, in accordance with the 
 hydraulic analogue of Fig. 74, then we 
 may regard a magnet as a device for pro- 
 ducing such a streaming motion in the 
 ether. The magnetic flux ; i. <?., the stream- 
 ing ether, would issue from one pole of the 
 
ELECTRO-THERAPEUTICS. 191 
 
 magnet, and, after having passed in. ex- 
 panding curved paths through the space 
 surrounding the magnet, would re-enter it 
 at the other pole. The pole from which 
 the magnetic flux is conventionally as- 
 sumed to issue is called the north pole / 
 i. e., the pole of the magnet, which, if 
 the magnet were freely suspended, would 
 tend to point toward the geographical 
 north.; and the pole at which the flux 
 enters the magnet, after having passed 
 through the region outside it, is called the 
 south pole. The flux, after entering the 
 magnet, passes through the body of the 
 magnet to the north pole, where it again 
 emerges. Magnetic flux, therefore, com- 
 pletes a closed path or circuit called a 
 magnetic circuit, and the convention em- 
 ployed as to its direction in this circuit, is 
 similar to the convention employed as to 
 the direction of electrostatic flux in an 
 
192 ELECTRICITY IN 
 
 electrostatic circuit, or to the direction of 
 a current in a voltaic circuit. 
 
 When an electric current passes through 
 a conductor, the conductor temporarily 
 
 Urn*. i;m>: 
 
 
 
 / 
 ^ 
 
 Fig. 75.— Magnetic Flux Paths Surrounding a 
 Straight Active Conductor. 
 
 acquires magnetic properties, magnetic 
 flux encircling the conductor in concentric 
 paths. The direction of magnetic flux 
 around an active conductor, depends on 
 the direction of the current in the con- 
 ductor. This is shown in Fig. 75, where, 
 at J5, the current is supposed to be passing 
 
ELECTRO-THERAPEUTICS. 193 
 
 through the wire, in a direction from the 
 observer. Here the circles surrounding 
 the wire show that the magnetic flux is 
 passing in concentric circles in the direction 
 of motion of the hands of a clock ; while at 
 A 1 where the current passes through the 
 wire in a direction towards the observer, 
 the direction of the magnetic flux around 
 the wire is opposite to the direction of 
 motion of the hands of a clock, or counter- 
 clockwise. A suspended magnetic needle, 
 introduced into the neighborhood of the 
 active wire ; i. e., into the influence of its 
 circular magnetic flux, is deflected thereby, 
 and tends to set itself parallel to the mag- 
 netic flux, or at right angles to the direc- 
 tion of the current, its north pole pointing 
 in the direction of motion of the flux. 
 
 The power possessed by an active con- 
 ductor of deflecting a magnetic needle is 
 
194 ELECTRICITY IN 
 
 utilized in a number of ammeters, in which 
 a magnetic needle is deflected by the pas- 
 sage of a current through a number of 
 turns of wire placed in its vicinity. When 
 a wire carrying an electric current is bent 
 into a turn or loop, all the magnetic flux 
 linked with the wire enters this loop at 
 one face and leaves it at the other face. 
 Consequently, that face of the loop from 
 which the flux emerges must correspond 
 to the north magnetic pole, and that at 
 which it enters, to the south magnetic pole, 
 of an ordinary bar magnet. This is illus- 
 trated in Fig. 76, both in the case of a 
 permanent steel magnet, and of an active 
 coil. In the case of a magnet, the flux is 
 represented as coming out of the north 
 pole, as indicated by the arrows, traversing 
 the region or space outside of the magnet, 
 re-entering the magnet at its south pole, 
 and continuing through the body of the 
 
*s* 
 
 ^U? & DIESfL f %/ 
 
 magnet to its north pole,^^s completing 
 the magnetic circuit. Simil§rfe?4n r ^ e 
 case of an active loop, as showi 
 current circulates around this loop clock- 
 wise, as viewed by an observer at A, then 
 
 C'R r 
 
 
 S-^-lN 
 
 -Z" B 
 
 Fig. 76. — Diagram of Flux Produced by Permanent 
 Magnet and by Coil of Active Conductor. 
 
 the flux will enter at A, and emerge at j&, 
 so that the face B, becomes a north pole, 
 and A, a south pole, corresponding to the 
 permanent magnet. In the case of an 
 active loop, the flux paths form closed 
 magnetic circuits as in the case of the mag- 
 
196 ELECTRICITY IN 
 
 net, although these are not shown in the 
 figure. When the current in the conduct- 
 ing loop ceases, the magnetic flux linked 
 with the loop entirely disappears. 
 
 Magnetic circuits are of three kinds; 
 namely, 
 
 (1) Those in which all parts of the path 
 of the flux are completed through air, or 
 other non-magnetic material, such as wood, 
 copper, glass, etc. Such a circuit is called 
 a non-ferric magnetic' circuit. 
 
 (2) Those in which all portions of the 
 path of the flux are completed through 
 iron or steel. Such a circuit is called a 
 ferric circuit. 
 
 (3) Those in which parts of the circuital 
 path of the flux are completed through 
 iron or steel, and parts through air or other 
 non-conducting material. This is called 
 an aero-ferric circuit 
 
ELECTRO-THERAPEUTICS. 
 
 197 
 
 Non-magnetic circuits are formed by 
 active conductors, such as wires, loops or 
 coils carrying electric currents in the ab- 
 
 Fig. 77.— Ferric Magnetic Circuit. 
 
 sence of iron. An example of such a cir- 
 cuit is represented by the active coil 
 shown in Fig. 77. Ferric magnetic cir- 
 cuits are less frequently met with, from 
 
198 ELECTRICITY IN 
 
 the fact that the object of sending a mag- 
 netic flux through a circuit is to employ 
 such flux in the operation of some mechan- 
 ism, generally placed in a gap in the circuit 
 itself. There is, however, a compara- 
 tively large class of apparatus called alter- 
 nating-current transformers, which will be 
 briefly explained later, and which almost 
 always employ ferric circuits. 
 
 An iron ring, or core, wrapped with a 
 coil of wire connected to the terminals of a 
 battery, is an example of a ferric magnetic 
 circuit. Such a magnetic circuit is shown 
 in Fig. 77. Here all the flux due to the 
 active conductors is entirely confined to 
 the iron ring. A practical form of a ferric 
 circuit is represented in Fig. 78, which 
 represents an alternating-current trans- 
 former. The coil of active conductor is 
 shown at AA, linked with a laminated or 
 
ELECTRO-THERAPEUTICS. 199 
 
 sheet iron core BB. Aero-ferric magnetic 
 circuits are commonly observed in the case 
 
 1 
 
 Fig. 78.— Alternating-Current Transformer, 
 Ferric Magnetic Circuit. 
 
 of permanent magnets. Thus the bar 
 magnet shown in Fig. 76, has its magnetic 
 circuit completed partly through the bar 
 and partly through the air outside the bar. 
 
200 ELECTRICITY IN 
 
 A very common type of aero-ferric 
 magnetic circuit is represented in Fig. 79, 
 which shows an electromagnet, consist- 
 ing essentially of a bar of soft iron AB, 
 wound usually with a large number of 
 
 Fig. 79.— Bar Electromagnet. 
 
 turns of active conductor. Here the pres- 
 ence of the iron core causes the flux pro- 
 duced by the current passing through the 
 coil, to be more powerful than that 
 which the coil alone would produce. The 
 polarity of the iron core AB, will, of 
 course, depend on the direction of the 
 
ELECTRO-THERAPEUTICS. 201 
 
 current in the wire. If this direction be 
 such that the flux enters the core at the 
 end I>, and leaves at the end A, then the 
 north and south poles of the electromagnet 
 so formed will be as marked in the figure. 
 The introduction of the iron core, has not, 
 therefore, altered the polarity produced by 
 the helix, but it has greatly increased the 
 quantity of magnetic flux, so that the 
 magnet exerts a greater influence at a dis- 
 tance, and also a greater attractive power 
 at its poles. Moreover, when the core is 
 absent, the cessation of the magnetizing 
 current is accompanied by a complete loss 
 of the magnetic properties of the coil ; that 
 is to say, the copper wire forming the coil 
 possesses no permanent magnetism. If, 
 however, a core be present, the magnet 
 does not immediately lose its magnetism 
 on the cessation of the current. A certain 
 amount of flux called residual flux, or 
 
202 ELECTRICITY IN 
 
 ■remanent flux remains in the circuit. 
 When the core of iron is very soft and 
 carefully annealed, the amount of this 
 residual magnetism is very small. When, 
 however, the bar is formed of hard iron, 
 a considerable portion remains on the 
 cessation of the magnetizing current. In 
 the case of a bar electromagnet, the 
 magnetic circuit is largely formed of air, 
 less than half of the circuit existing in the 
 iron or steel. 
 
 If we bend the bar shown in the pre- 
 ceding figure, so as to bring the two poles 
 nearer together, we get a form of electro- 
 magnet called the horse-shoe electromagnet 
 in which the length of the air path is con- 
 siderably reduced. Instead of actually 
 bending the bar, it may, for purposes of 
 convenience, be made in three separate 
 parts as shown in Fig. 80, which is the 
 
ELECTRO-THERAPEUTICS. 203 
 
 form ordinarily given to an electromagnet. 
 Here the magnet consists of two separate 
 iron cores, connected together at the ends 
 by a bar of soft iron called a yoke. Each 
 of the two cores is provided with a 
 magnetizing coil. 
 
 Fig. 80.— Electromagnet. 
 
 The value of the electromagnet depends 
 largely on the fact that its core being 
 made of very soft iron, possesses the prop- 
 erty not only of greatly increasing the 
 strength of the flux produced by the mag- 
 netizing coils, but also of readily losing 
 nearly all its magnetism on the cessation of 
 the magnetizing current; so that such a 
 
204 ELECTRICITY IN 
 
 magnet when small can readily acquire and 
 lose its magnetism, many times in a second. 
 
 Various forms are given to electromag- 
 nets according to the purposes for which 
 they are designed. Where it is desired 
 that an electromagnet should possess the 
 power of attracting or repelling magnetic 
 bodies at a considerable distance from its 
 poles, the circuit is necessarily of the aero- 
 ferric type, since the flux must pass for a 
 considerable distance through air; but 
 where it is desired that the magnet shall 
 possess the power of holding heavy weights 
 attached to its armature, — the name given 
 to the bar of iron completing the magnetic 
 circuit, — this circuit approaches more nearly 
 to the ferric type. 
 
 A form of magnet capable of producing 
 very powerful magnetic flux in the space 
 
ELECTRO-THERAPEUTICS. 
 
 205 
 
 between the poles is shown in Fig. 81. 
 It consists of two powerful magnetizing 
 
 yiA 
 
 BH^HB 
 
 
 
 flJ£ 
 
 
 
 
 
 Fig. 81. — Electromagnet. 
 
 coils M, M, wound on iron cores, iron 
 yoke Y y and iron pole pieces P, P. The 
 
206 ELECTRICITY IN 
 
 power of an active coil to produce a mag- 
 netic flux is called its magnetomotive force, 
 generally contracted M. M. F. This mag- 
 netomotive force is proportional to the 
 number of turns of wire, and also to the 
 current strength passing through the same. 
 Consequently, if we add more turns to a 
 coil, or increase the current strength pass- 
 ing through it, we will increase its M. M. 
 F., and produce a greater magnetic flux 
 through the circuit. 
 
 In every magnetic circuit the strength 
 of the magnetic flux depends on two 
 quantities; namely, the resistance oppos- 
 ing the magnetic flux, called the magnetic 
 resistance or reluctance. A similar resist- 
 ance to the passage of electrostatic and 
 electric flux exists in the case of both the 
 electrostatic and electric circuits. The 
 value of the magnetic flux, like that of the 
 
ELECTRO-THERAPEUTICS. 207 
 
 electric flux may be expressed by the 
 
 formula of Ohm's law, as follows; namely, 
 
 _* Magnetomotive Force 
 
 Magnetic Flux = ° p 1 — 7 , 
 
 & Keluctance 
 
 so that if we know the magnetomotive 
 
 force in a magnetic circuit, and the value 
 
 of the magnetic resistance or reluctance, 
 
 by dividing the former by the latter, -we 
 
 obtain the value of the magnetic flux. 
 
 As in the case of the electric circuit, 
 special names are given to the unit values 
 of these quantities. The units of magneto- 
 motive force are called the arrupere-t'ivrn, and 
 the gilbert, the ampere-turn being greater 
 than the gilbert in the ratio of approxi- 
 mately 5 to 4. By an ampere-turn is 
 meant the amount of magnetomotive force, 
 which is produced by a turn of wire carry- 
 ing a current of one ampere ; that is to say, 
 if the magnetizing coils shown in Fig. 80 
 
208 ELECTRICITY IN 
 
 coDsisted of 200 turns in each spool, and if a 
 
 current of five amperes passes successively 
 
 through these spools, then the total M. 
 
 M. F., urging the magnetic flux through the 
 
 circuit, is 5 amperes x 400 turns = 2,000 
 
 2,000X5 - k „ .,, 
 ampere-turns = - — = 2,o00 gilberts, 
 
 approximately. The amount of magnetic 
 flux produced in the circuit by this M. M. F. 
 will depend entirely upon the reluctance of 
 the circuit. If the air-gap is large ; i. e. 9 if 
 the magnetic circuit contains a long air path, 
 the magnetic resistance, or reluctance, of the 
 circuit will be great, and the magnetic flux 
 produced by the magnetomotive force will 
 be comparatively small. If, on the other 
 hand, the length of the air-path is small, 
 the reluctance will be very small, and the 
 amount of flux produced will be corre- 
 spondingly great. This is for the reason 
 that the reluctance of iron is very small as 
 
ELECTRO-THERAPEUTICS. 209 
 
 compared with that of air, provided that 
 the iron is not saturated; i. e., is not 
 already conducting a large amount of flux 
 per square inch or per square centimetre 
 of cross-sectional area. 
 
 As in the electric circuit, the resistance 
 of a wire depends upon its length and 
 cross-sectional area, as well as on the 
 nature of the material of which it is com- 
 posed, so, in the magnetic circuit,, the 
 reluctance depends upon the length and 
 area of cross-section of the circuit, and on 
 the nature of the substance through which 
 the flux is passing. In order to decrease 
 the resistance of a wire, we may either 
 decrease its length, or increase its area of 
 cross-section. In the same way, in order 
 to decrease the reluctance of a magnetic 
 circuit, we may decrease its length or 
 increase its area of cross-section. The 
 
210 ELECTRICITY IN 
 
 resistivity, or resistance in a unit cube, 
 varies markedly with the nature of the 
 substance, but does not vary with the 
 current strength passing through the 
 material, provided the temperature is con- 
 sidered as remaining the same. In the 
 magnetic circuit, the reluctivity, or reluc- 
 tance in a unit cube (reluctance in one 
 cubic centimetre measured between par- 
 allel faces) is practically the same for all 
 substances other than the magnetic metals ; 
 in which the reluctivity is much lower. 
 
 Unlike the case of the electric circuit, 
 the reluctivity varies markedly with the 
 strength of the magnetic flax passing 
 through the circuit. When the magnetic 
 flux passing through iron is feeble, the 
 reluctivity may be a thousand times less 
 than that of air, while iron magnetically 
 saturated ; *. e., carrying a very dense 
 
ELECTRO-THER Jp^JTKJSrt L F L £M Y (, f 
 
 magnetic flux, has a ikffetivity prac- 
 tically equal to that of air/V3£&^fiWtn!*rff 
 reluctance is called the oersted, and is eqtlaf" 
 to the reluctance offered by a cubic centi- 
 metre of air, or more strictly of air-pump 
 vacuum, measured between parallel faces, 
 and is, therefore, nearly equal to the reluc- 
 tance of a cube of glass, air, wood, copper, 
 etc., measured between parallel faces. The 
 reluctivity of air is, therefore, taken as 
 unity. 
 
 The unit of magnetic flux is called the 
 weber. One weber will flow in a magnetic 
 circuit under a M. M. F. of one gilbert, 
 through a reluctance of one oersted. If 
 the reluctance of the magnetic circuit 
 represented in Fig. 80, be 0.5 oersted, 
 then since its M. M. F. has been assumed 
 at 2,500 gilberts, the flux through the cir- 
 cuit will be ^— - — 5,000 webers. 
 
 0.D 
 
212 ELECTRICITY IN 
 
 There are but two ways of varying the 
 M. M. F. in a circuit ; L e., by increasing 
 the number of turns, or by increasing the 
 current strength circulating in them ; or, 
 briefly, by increasing the number of am- 
 pere-turns. 
 
 It is necessaiy to draw a distinction 
 between the total flux in a circuit meas- 
 ured in webers, and the intensity of the 
 flux per unit of cross-sectional area ; /. e., 
 per square inch, or per square centimetre ; 
 just as it is necessaiy to distinguish 
 between the total current strength in an 
 electric circuit, as measured in amperes, 
 and the density of that current, as meas- 
 ured in amperes-per-square-inch, or per- 
 square-centimetre, of cross-sectional area in 
 the wire conveying the current. Since, in 
 the case of the magnetic circuit, the intro- 
 duction of iron is invariably attended by 
 
ELECTRO-THERAPEUTICS. 213 
 
 an increase in the magnetic flux, it is 
 evident that by the introduction of a suf- 
 ficiently great amount of iron, the amount 
 of magnetic flux can be increased almost 
 to any extent. Although this would neces- 
 sitate a marked increase in the area of 
 cross-section of the iron, yet the flux den- 
 sity per square inch, or square centimetre, 
 may not be increased. Since soft iron 
 practically saturates at an intensity of 
 19,000 webers-per-square-centimetre, and 
 its reluctance near saturation rapidly 
 increases, it is difficult to obtain at any 
 portion of the magnetic circuit, intensi- 
 ties higher than .19,000 webers-per-square 
 centimetre ; i. e., 19,000 gausses, the gauss 
 being the wn/it of magnetic intensity, or 
 the density of one weber-per-square-centi- 
 metre of perpendicular area of cross- 
 section. The intensity of the earth's mag- 
 netic flux is, approximately, half a gauss, 
 
214 ELECTRICITY IN 
 
 while the highest experimental intensity 
 on record is 45,350 gausses. 
 
 It does not appear that magnetic flux 
 produces any apparent physiological ef- 
 fects on the human body. The human 
 body, containing in its composition no 
 appreciable quantity of magnetizable ma- 
 terial, has practically the same reluc- 
 tivity as ordinary air; that is to say, 
 the interposition of the human body in 
 a magnetic circuit does not appreciably 
 affect the distribution of the magnetic 
 flux. For example, if a delicately sus- 
 pended magnetic needle be deflected by 
 a magnet placed at a certain distance from 
 it, the direct interposition of the body of 
 a person between the magnet and needle 
 is not found to produce any appreciable 
 effect, although if the same person wears, 
 for example, an iron rimmed pair of 
 
ELECTRO-THERAPEUTICS. 215 
 
 spectacles, or carries a key in his pocket, 
 the effect on the needle may be very 
 marked. This is because the magnetic 
 ilux, acting on the needle, passes through 
 the body of the person as readily as 
 through the previously intervening air, 
 but the magnetic influence of the iron 
 rimmed spectacles, or the key, may have 
 a powerful influence on a delicately sus- 
 pended needle even though twenty feet 
 away from it. 
 
 Not only is the reluctivity of the human 
 body practically the same as that of other 
 non-magnetic materials, but portions of 
 the body subjected to powerful magnetic 
 fluxes do not appear to have produced in 
 them any appreciable physiological effects. 
 Although experiments are still wanting 
 concerning the physiological influence 
 which long sustained powerful magnetic 
 
216 ELECTRICITY IN 
 
 flux may exert, yet it has been shown that 
 human beings and dogs subjected for 
 many minutes to intensities of magnetic 
 flux of about 2,500 gausses, and, therefore, 
 about 5,000 times that of the earth's 
 magnetic flux, have not experienced any 
 influence that could be observed. Simi- 
 larly, experiments made both with con- 
 tinuous and rapidly alternating magnetic 
 fluxes have not shown any effect produced 
 upon the circulation of the blood due to 
 the iron it contains, upon ciliary or proto- 
 plasmic movements, upon sensory or motor 
 nerves, or upon the brain. 
 
 It has been positively asserted that in 
 a perfectly dark room certain individuals 
 possess the power of observing faint 
 luminous phenomena, around the poles of 
 permanent or electro-magnets ; that is, that 
 these persons actually possess the power 
 
ELECTRO-THERAPEUTICS. 217 
 
 of being visually affected by magnetic 
 flux. Investigations, however, have not 
 only thrown doubt upon the original ex- 
 periments, but repetitions of these experi- 
 ments, with powerful electromagnets, have 
 entirely failed to confirm the statements. 
 
 So far, therefore, as we know at the 
 present time, it would appear that mag- 
 netic flux is absolutely without influence 
 either upon the human body, or on any of 
 its physiological processes, and that, conse- 
 quently, if any therapeutic effects do 
 attend the use of magnets, the causes must 
 be of a psychic rather than of a physio- 
 logical nature. It is to be remembered, 
 however, that carefully conducted re- 
 searches with very powerful magnetic 
 fluxes may yet show lesser residual influ- 
 ences, which the experiments up to the 
 present time have failed to bring to light. 
 
218 ELECTRICITY IN 
 
 But up to the present time experiments 
 made on human beings have failed to 
 establish any physiological effects what- 
 ever, even when such a delicate organ as 
 the brain is placed in the direct passage of 
 a powerful magnetic flux. When, for ex- 
 ample, a person is placed with his head 
 between the poles of a powerful dynamo- 
 electric machine, from which the armature 
 has, been removed, so that the flux passes 
 directly through the head, even prolonged 
 exposure has failed to produce any ob- 
 served effect either on the pulse or respira- 
 tion, whether the magnetic flux was inter- 
 mittent or was steadily maintained. 
 
 Or, take the case of a powerful electro- 
 magnet, made by wrapping an iron cannon 
 with a suitable magnetizing coil, and pro- 
 ducing a flux sufficiently great to cause 
 heavy iron bars or bolts to be sustained on 
 
ELECTRO-T Fl ER APEUTICS 
 
 219 
 
 the person of a soldier standing before the 
 gun, as shown in Fig. 82. Under these 
 
 Fig. 82. — Magnetic Gun Attraction through a 
 Soldier's Body. 
 
 circumstances, no sensations were experi- 
 enced by the soldier other than those of 
 pressure from the attracted masses of iron. 
 
220 ELECTRICITY. 
 
 It would appear evident from the pre- 
 ceding observations that very little cre- 
 dence can be placed on the extravagant 
 claims as to the curative power possessed 
 by small magnets carried or worn on the 
 body. The magnetic flux produced by 
 such magnets is necessarily comparatively 
 feeble, and if the more powerful fluxes 
 before referred to failed to produce any 
 appreciable physiological effects, there are 
 no reasons for believing that these feeble 
 fluxes can produce any marked effects 
 unless that due to a feeble influence, long 
 sustained. In the case, however, of most 
 of the magnetic nostrums, for which cura- 
 tive effects are claimed, even the weak flux 
 they produce usually fails to be properly 
 directed, does not pass through any por- 
 tion of the body, and can, therefore, have 
 no physiological effect, except through the 
 medium of the imagination. 
 
CHAPTER IX. 
 
 INDUCTION OF E. M. F. BY MAGNETIC FLUX. 
 
 When a conducting loop is filled with, 
 or emptied of, magnetic flux, electromotive 
 forces are thereby set up or induced in the 
 loop. This is called the induction of M 
 M. F. by magnetic flux. Four cases of 
 such induction may arise ; namely, 
 
 (1) Self induction. 
 
 (2) Mutual induction. 
 
 (3) Electro-magnetic induction. 
 
 (4) Magneto-electric induction. 
 
 We have seen that when an electric 
 current circulates through a coil or loop, 
 
 221 
 
222 ELECTRICITY IN 
 
 all the flux produced by the current is 
 caused to enter the loop at one face, and to 
 emerge at the opposite face. When a cir- 
 cuit is closed, so that the electric source 
 begins to force electric currents through 
 the circuit connected with it, some little 
 time is required before the full current 
 strength is established ; so that, during 
 this time, the magnetic flux that is passing 
 through the loop is increasing in strength. 
 Also, when the circuit is opened, some 
 time is required for the current to entirely 
 cease flowing through the circuit, and, dur- 
 ing this time, the magnetic flux passing 
 through the loop is decreasing. Therefore, 
 both at the moment of making and break- 
 ing an electric circuit, a tendency will 
 exist, if the circuit contains coils or con- 
 ducting loops, for electromotive forces to 
 be induced in the circuit. These E. M. Fs. 
 continue only while the current strength is 
 
ELECTRO-THERAPEUTICS. 223 
 
 varying; as soon as the current strength 
 in the circuit becomes constant, they dis- 
 appear. 
 
 The amount of the E. M. F. induced 
 at any moment of time in a conducting 
 loop, by filling or emptying it with flux, 
 depends upon the rate at which the loop 
 is filled with, or emptied of flux. Sup- 
 pose, for example, that 100,000 webers are 
 passed through a loop, in, say two seconds 
 of time: then if the rate at which this 
 flux enters the loop is uniform, the E. M. 
 F. generated in the 'loop will be main- 
 tained during the entire two seconds, and 
 will be equal to the rate of entry in 
 
 i , 200,000 
 webers-per-second, or 1 = 100,000 
 
 webers-per-second = 100,000 units of E. M. 
 F. The unit of E. M. F., the volt, has been 
 so chosen that 100,000,000 webers, passing 
 
224 ELECTRICITY IN 
 
 through the loop per second, generate one 
 volt; so that this E. M. F. is 100,000 H- 
 
 100,000,000 =_L-volt. If, however, 
 
 the 200,000 webers, above mentioned, 
 entered the loop in say 0.01 of a 
 second, the E. M. F. induced in the loop 
 
 would be 200 times greater, or 1 — = 
 
 8 0.01 
 
 20,000,000 units of E. M. F. = 0.2 volts, 
 but this E. M. F. would only last for the 
 1/1 00th of a second. When, therefore, a 
 loop is filled with and emptied of a given 
 number of webers of flux, the E. M. F. 
 which will be produced in the loop de- 
 pends entirely upon the time in which the 
 filling and emptying takes place. If the 
 filling takes place very suddenly, the E. 
 M. F. will be powerful, but of very short 
 duration. On the other hand, if the fill- 
 ing or emptying takes place slowly, the 
 
ELECTRO-THERAPEUTICS. 225 
 
 E. M. F. will be correspondingly weaker, 
 but longer sustained. 
 
 The direction of the E. M. F. induced 
 by filling a conducting loop with flux, is 
 opposite to that induced by emptying the 
 same loop of flux. The direction of the 
 E. M. F. induced by filling a conducting 
 loop with flux, is readily remembered by 
 the following rule : 
 
 Regarding the loop as the face of a 
 watch, held in front of the observer, then 
 if the flux passes through the loop in the 
 same direction as the light passing from 
 the face of the watch to the observer's 
 eye, the E. M. F. induced in the loop will 
 have the same direction as that of the 
 hands of the watch. 
 
 Some general idea concerning the 
 manner in which E. M. F. is generated in 
 
226 
 
 ELECTRICITY IN 
 
 a loop by the passage of magnetic flux 
 through it, may, perhaps, be obtained from 
 
 Fig. 83.— Mechanical Model Having Analogies with 
 Electric Circuit. 
 
 the mechanical model shown in Fig. 83. 
 A cylinder AB, pivoted upon a vertical 
 axis CD, mechanically represents a con- 
 ducting loop of wire. The cylinder is con- 
 
ELECTRO-THERAPEUTICS. 227 
 
 nected with the axis by a number of radial 
 spokes in the form of fan-blades, so that, 
 if a stream of liquid, such as water, be 
 poured through the cylinder or loop from 
 above, the impact of the water on the 
 blades will cause the cylinder to rotate in 
 a direction opposite to that of the hands of 
 a watch. If, however, the water be forced 
 upward through the loop, its impact will 
 cause the cylinder to rotate in the oppo- 
 site direction. If a given number of gal- 
 lons of water be passed through the 
 cylinder, the driving impulse communicated 
 to it will depend upon the time during 
 which the water passes. If the water be 
 delivered in a brief time, its rate of pas- 
 sage through the loop will be great, and 
 the driving impulse communicated to the 
 cylinder will be great, though of brief 
 duration. If, on the other hand, the time 
 during which the water passes through the 
 
228 ELECTRICITY IN 
 
 loop be considerable, the driving impulse 
 exerted on the cylinder will be prolonged, 
 but correspondingly feeble. It will be 
 observed that the driving impulse or force 
 in this mechanical analogue stands for 
 electromotive force in the electrical case. 
 When the water is first poured through 
 the cylinder, the inertia of the cylinder will 
 prevent it from being immediately set in 
 motion. Similarly, when the water has 
 ceased to pass, the motion of the cylinder, 
 owing to inertia, does not immediately- 
 cease. In the electric circuit, this corre- 
 sponds to the effect of self-induction ; for, 
 the effect of pouring flux into a loop is to 
 induce in the loop an E. M. F., and the 
 effect of the current so set up, is to pro- 
 duce in the loop a flux opposite to that 
 of the inducing flux, producing thereby 
 a C. E. M. F. retarding the development 
 of the electric current. On the other 
 
ELECTRO-THERlt^BUTICSn C P ijffif Y f l 
 
 hand, when the flux has \ffifecl the loop, 
 the current does not imniMi^^^ftcea^e r > m n 
 flowing, being prolonged by the ac^iofTTS^ 
 the flux set up by the current ; in other 
 words, the loop acts as though it possessed 
 electrical inertia. 
 
 When a number of turns are connected 
 in series, as, for example, in the case of the 
 coil of conducting wire shown in Fig. 77, 
 the effects produced by each turn are 
 added, so that the coil has induced in it an 
 E. M. F. proportional to the number of its 
 turns. When a conducting coil, contain- 
 ing many turns, has its terminals connected 
 to a voltaic battery, some little time 
 elapses before the full current strength is 
 established in the circuit. The reason is 
 to be found in the C. K M. F. of self 
 induction of the coil. Similarly, when the 
 circuit of this coil is opened, the current 
 
230 ELECTRICITY IN 
 
 does not instantly cease flowing through 
 it, -since the emptying of the coil of the 
 flux, produces in it an E. M. F. which es- 
 tablishes in the coil a current in the same 
 direction as that sent through it by the 
 battery. In other words, the E. M. F. 
 produced in the coil by self-induction, at 
 the moment of making, tends to oppose the 
 establishment of the current, and that pro- 
 duced at the moment of breaking, tends to 
 aid the passage of the current. 
 
 When the circuit of an electric source, 
 such as a voltaic battery, is opened, a 
 minute spark is frequently visible at the 
 point of opening. If, however, a coil of 
 many turns of wire be contained in the cir- 
 cuit, the spark upon opening the circuit 
 will, probably, be much greater, and a dis- 
 tinct shock may be felt under favorable 
 conditions by the person opening the cir- 
 
ELECTRO-THERAPEUTICS. 231 
 
 cuit. This spark is due to the self-induc- 
 tion of the circuit. The current which has 
 passed through the circuit has produced a 
 magnetic flux linked with the turns ; i. e., 
 the loops in the coil or coils of wire. On 
 the opening of the circuit, this current is 
 suddenly interrupted, and the flux, rapidly 
 disappearing from the coils ; i. e. pouring 
 out of them, induces a brief but powerful 
 E. M. F., which, actiug in the same direc- 
 tion as the current, tends to prolong it. 
 
 If two coils A and £, Fig. 84, connected 
 in separate circuits, are placed side by side, 
 and an electric current be sent through 
 one, say A, the passage of this current will 
 produce a magnetic flux, part of which will 
 pass through B. During the process of 
 filling J3, with this portion of A'a flux, an 
 E. M. F. will be set up in each turn of £, 
 equal, at any moment, to the rate at which 
 
232 
 
 ELECTRICITY IN 
 
 the flux is entering in webers-per-second ; 
 or, expressed in volts, to the rate at which 
 the flux is entering in hundred millions of 
 webers-per-second. As soon as the 
 
 cur 
 
 Fig. 84.— Diagram Illustrating Mutual Induction. 
 
 rent in A, becomes stationary, the flux 
 through B, due to this current, is also 
 stationary, and, consequently, no further 
 E. M. F. is induced in B. If, however, the 
 current strength in A, diminishes, its flux 
 through B, will be correspondingly dimin- 
 ished, and an E. M. F. will be induced in 
 
ELECTRO-THERAPEUTICS. 233 
 
 B, in the opposite direction to that origin- 
 ally produced, and equal in volts to the 
 rate of emptying in millions of webers-per- 
 second. When the flux through J3, due to 
 the current in A, has entirely disappeared, 
 the E. M. F. induced in jB, has also disap- 
 peared. If there be 100 turns of wire in 
 the coil B, the E. M. F. induced in the coil 
 will be 100 times as great as if it consisted 
 of a single turn, assuming that the same 
 quantity of A 1 a flux passes through all of 
 B's turns alike. This inductive influence 
 extending from one coil to another, whereby 
 a current in one circuit induces an E. M. F. 
 in another circuit, is called mutual induction. 
 An example of mutual induction can be 
 shown by means of the apparatus repre- 
 sented in Fig. 85, in which A, represents 
 the inducing coil ; L e., the coil in which 
 the current flows; and B, the coil in which 
 the current is induced. Or, as they are 
 
234 
 
 ELECTRICITY IN 
 
 generally called, A, is the primary coilaxiA 
 JB, is the secondary coil. If the terminals 
 of the primary coil A, be connected with 
 
 Fig. 85.— Mutual Induction. 
 
 the voltaic cell C\ as shown in the figure, 
 and the terminals of the secondary coil B, 
 be connected with an ammeter, or galva- 
 nometer, G) then, as soon as the current 
 
ELECTRO-THERAPEUTICS. 235 
 
 is established in A, no current will be 
 induced in B, as long as A, remains at 
 rest. If, however, A, be moved either 
 toward or from B, currents will be pro- 
 duced in the secondary coil, as will be indi- 
 cated by the galvanometer, the current 
 passing iu one direction, when A, is moved 
 toward B, and in the opposite direction 
 when A, is moved from B. It can be 
 shown that the current induced in a 
 secondary coil is induced in the opposite 
 direction to that in its primary, on the 
 approach of A to B, and in the same di- 
 rection, on its withdrawal from B. The 
 two circuits A and B y although electrically 
 disconnected, are connected magnetically 
 by the flux permeating the space between 
 them, and the E. M. F. of mutual induction 
 is caused by the flux proceeding from the 
 primary coil being carried toward or from 
 the secondary coil, during its motion, so as 
 
236 ELECTRICITY IN 
 
 to cause the secondary coil to be filled 
 with more or less flux. 
 
 That mutual induction may take place 
 between stationary primary and secondary 
 coils, may be experimentally demonstrated. 
 For example, if as in Fig. 86, the primary 
 coil A, is fixed at a constant distance from 
 £ y then on completing the circuit of the 
 primary coil, by closing the switch S, 
 while the current is increasing in the pri- 
 mary, the magnetic flux produced by it 
 passes through the conducting loops on the 
 secondary coil C, thereby inducing an 
 E. M. F. and establishing a current, as is 
 shown by the galvanometer G. 
 
 The distinction between electro-magnetic 
 and magneto-electric induction is seen in 
 Fig. 87, where the motion of the magnet 
 M, into or out of the coil of wire, pro- 
 
ELECTRO-THERAPEUTICS. 237 
 
 duces electromotive forces in the coil C, 
 as shown by the galvanometer G. When 
 the magnet is thrust into the coil, the 
 
 Fig. 86.— Mutual Induction. 
 
 galvanometer indicates a temporary cur- 
 rent in one direction, and, on its with- 
 drawal from the coil, it shows a current 
 in the opposite direction. The introduc- 
 
238 ELECTRICITY IN 
 
 tion of the south pole into the coil pro- 
 duces the same direction of current as 
 the withdrawal of the north pole. Here, 
 as in the other instances, E. M. Fs. are 
 
 Fig. 87. — Magneto-Electric Induction. 
 
 induced by the passage of magnetic flux 
 through the coil ; the flux produced by the 
 magnet, being advanced or moved so as to 
 pass through, or link with, the turns in the 
 secondary coil. 
 
 A form of apparatus for producing 
 E. M. Fs. by magneto-electric induction is 
 
ELECTRO-THERAPEUTICS. 
 
 239 
 
 represented in Fig. 88. A permanent 
 horse-shoe magnet, MM, is supported in 
 
 - Elee. World 
 
 Fig. 88.— Magneto-Electric Generator. 
 
 a vertical position, and two coils of fine 
 insulated wire GG, are supported on a 
 horizontal axis, in such a manner as to be 
 capable of rotation by the turning of the 
 
240 ELECTRICITY IN 
 
 handle JT, the rotary speed of the coils be- 
 ing made greater than the rotary speed of 
 the handle, by the interposition of suitable 
 multiplying gear. The coils are wound 
 on cores of soft iron, which are connected 
 by a soft iron yoke y. The ends of the 
 cores revolve in close proximity to the 
 poles of a permanent magnet, leaving a 
 small air gap or clearance of compara- 
 tively small reluctance. When the two 
 coils stand vertically, the flux from the 
 magnet passes through the air gap, the 
 cores and their connecting yoke, thereby 
 filling all the turns of wire wound upon 
 the core. An E. M. F. will be induced in 
 each turn equal in volts to the rate of fill- 
 ing it with flux, in hundred millions of 
 webers-per-second ; and, since all the turns 
 in each coil, and the two coils themselves, 
 are connected in series, the total E. M. F. 
 will be correspondingly multiplied. 
 
ELECTRO-THERAPEUTICS. 
 
 241 
 
 The condition of affairs in the preceding 
 machine, is represented in Fig. 89, where 
 
 Fig. 89.— Diagram Representing Changes in the Mag- 
 netic Circuit of Magneto-Electric Generator. 
 
 the coils are shown at A, as being imme- 
 diately opposite to the magnet poles, and 
 
242 ELECTRICITY IK 
 
 in such a position as to be filled with flux, 
 so that they cannot receive any further 
 increase of flux by a further rotation in 
 either direction. 
 
 At £, the coils are leaving the pole 
 pieces, so that the reluctance in the 
 magnetic circuit is increasing, and the 
 magnetic flux, which passes through the 
 cores of the coils, is diminishing. In 
 other words, the coils are becoming 
 emptied of the flux they contain. An 
 E. M. F. is, therefore, induced in them. 
 
 At C % the coils are completely emptied 
 of magnetic flux, and, therefore, have no 
 E. M. F. At Z>, the coils are being filled 
 with flux, but in the opposite direction 
 to that which exists at A. Consequently, 
 the E. M. F. induced has the opposite 
 direction to that induced at B. 
 
ELECTRO-THEKAPEUTICS. 243 
 
 At jS 7 , the coils are full of flux in the 
 opposite direction to that at A. t and the 
 E. M. F. in the coils will have ceased. 
 
 It will, therefore, be evident that dur- 
 ing any half revolution, as from A to E, 
 the E. M. F. induced in the coils has made 
 a single alternation or reversal ; and that 
 during the next succeeding half revolution, 
 in which the coil returns to the position A, 
 the E. M. F. induced will be of the same 
 magnitude as above pointed out, but in the 
 opposite direction. 
 
 The revolving coils, therefore, generate 
 alternating currents in the circuit con- 
 nected with them, the E. M. F. being 
 alternately in opposite directions, during 
 successive half revolutions. One com- 
 plete revolution of the coils produces one 
 complete double alternation, or cycle of 
 
244 ELECTRICITY IN 
 
 the E. M. F. and electric current, con- 
 sequently, the frequency of the alternat- 
 ing currents produced; i. e., the number 
 of complete double alternations, or cycles 
 per second, is equal to the number of 
 
 Fig. 90.— Diagram of a Possible Wave Form of Mag- 
 neto-Electric Generator E. M. F. 
 
 revolutions made by the coils per second. 
 The alternating E. M. F. produced by 
 this machine might be represented dia- 
 grammatical ly in Fig. 90. The exact wave 
 form, in each case, would depend upon the 
 shape of the poles and of the iron cores. 
 If, however, a commutator be employed on 
 the armature, as shown in Fig. 91, whereby 
 
^ 
 
 {* 
 
 ^ & WBFtflS 
 
 ELECTRO-THER; 
 
 "WC/fifty U 
 
 at each half revolution flj|te6£onnections of 
 
 v*. 
 
 is c 
 
 the coils with the extern 
 
 versed, the current produced 
 
 nating E. M. F. will be unidirectional in 
 
 the external circuit. 
 
 is re- ^9. 
 
 Fig. 91.— Diagram op Two-Part Commutator. 
 
 Fig. 92, represents the corresponding 
 form of pulsating E. M. F. wave produced 
 in the external circuit when the commu- 
 tator is employed. It will be seen that 
 the E. M. F. is now always above the line. 
 If the E. M. F. were reversed, the waves 
 
246 ELECTRICITY IN 
 
 might be represented as being entirely 
 below the line. 
 
 Fig. 93, represents a form of magneto- 
 electric machine, the current from which 
 is capable of lighting a small incan- 
 
 Fig. 92.— Diagram of a Possible Wave Form of Mag- 
 neto-Electric Generator E. M. F. (when a Com- 
 mutator is Employed). 
 
 descent lamp. If an alternating mag- 
 neto-electric generator, that is a magneto- 
 electric generator not employing a commu- 
 tator, is connected to the body of a patient 
 by suitable electrodes, alternating electric 
 currents will pass through the body. If, 
 however, a commutator be employed, and 
 the currents be of the wave form shown in 
 Fig. 92, the physiological effects will be 
 
ELECTRO-THERAPEUTICS. 247 
 
 somewhat different. The type of current 
 of Fig. 93, possesses polar properties ; i. e. y 
 possesses the characteristics of unidirec- 
 
 Fig. 93. — Magneto-Electric Generator. 
 
 tional currents, while symmetrical alterna- 
 ting currents do not possess these proper- 
 ties, since the polar effects produced by one 
 wave, are neutralized by the following 
 wave in the opposite direction. 
 
CHAPTER X. 
 
 THE MEDICAL INDUCTION COIL. 
 
 The medical induct ton coil, generally 
 called the faradic coil, is very frequently 
 employed in electro-therapeutics. It con- 
 sists essentially of means whereby E. M. Fs. 
 are induced by mutual induction, and, con- 
 sequently, of a primary and a secondary cir- 
 cuit. A simple form of induction coil is rep- 
 resented in Fig. 94, where the terminals of 
 the primary circuit are shown at P, P y and 
 those of the secondary circuit at S, S. Fig. 
 95, shows a similar coil in longitudinal sec- 
 tion. An inspection of the latter figure will 
 show that the primary coil P, consists of a 
 comparatively short length of fairly coarse 
 
 248 
 
ELECTRO-THERAPEUTICS. 249 
 
 wire, wrapped around a hollow bobbin. 
 The secondary circuit generally consists of a 
 greater length of finer fire wrapped either 
 directly over the secondary, or on a hollow 
 bobbin capable of being moved over the 
 
 Fig. 94.— Simple Form of Induction Coil. 
 
 primary. When the current strength in the 
 primary circuit is varied ; i. e., when either 
 an alternating or a pulsatory current is sent 
 through the primary, an alternating E. M. 
 F. is induced in the secondary circuit by 
 the influence of mutual induction. 
 
 The amount of E. M. F. induced in the 
 secondary circuit depends upon the num- 
 ber of turns in its coil and the rate at 
 
250 ELECTRICITY IN 
 
 which the magnetic flux fills and empties 
 these turns. The induced E. M. F. does 
 not depend upon the number of yards or 
 feet of wire in the secondary coil, except 
 in so far as a greater length of wire pro- 
 
 's * 
 
 Fig. 95.— Section of Simple Form of Induction Coil. 
 
 vides a greater number of turns in the coil. 
 If we double the number of turns in the 
 coil without altering in any way the 
 amount of flux which passes through each 
 turn, we double the number of volts in- 
 duced therein ; whereas, if we double the 
 number of feet or yards in the secondary 
 coil, we do not necessarily, and in point of 
 fact very rarely, double the number of 
 
ELECTRO-THERAPEUTICS. 251 
 
 turns, and, therefore, the number of volts, 
 because the average length of turn in each 
 successive layer increases. If we double 
 the rate at which the flux threads or links 
 with the turns of the secondary coil, we 
 double the E. M. F. induced in the coil. 
 
 An increased rate of filling and emptying 
 conducting loops or turns with flux can 
 be obtained in one or both of two ways ; 
 viz., 
 
 (1) By causing the same flux to fill and 
 empty the loop a greater number of times 
 per second ; i. e., increasing the frequency 
 of oscillation of the flux in the magnetic 
 circuit; and, 
 
 (2) By increasing the amount of mag- 
 netic flux in the circuit without increasing 
 the frequency of oscillation, so that more 
 flux enters or fills the coils at each alterna- 
 tion. 
 
ELECTRICITY IN 
 
 Consequently, for a given primary and 
 secondary circuit, with a given geometrical 
 relationship between them, we can only 
 increase the E. M. F. in the secondary 
 circuit either by increasing the magnetic 
 flux, or by increasing the frequency of flux 
 oscillation, or both. 
 
 In order to increase the frequency of 
 flux oscillation, we require to increase the 
 frequency of the primary current. On the 
 other hand, in order to increase the total 
 amount of flux we must either increase the 
 M. M. F. of the primary circuit, or diminish 
 the reluctance of the magnetic circuit ; 
 that is to say, we must either employ 
 more ampere-turns at the same frequency, 
 or employ such a form of iron core in the 
 primary coil as will increase the magnetic 
 flux from a given M. M. F. by diminish- 
 ing the magnetic resistance of its circuit. 
 
ELECTRO-THERAPEUTICS. 
 
 253 
 
 The frequency required for the primary 
 circuit may be obtained either by an al- 
 ternating, or by a continuous, but pulsat- 
 ing current. The ordinary farad ic coil 
 
 Fig. 96.— Primary Connections of Medical Induction 
 Coil. 
 
 only employs the latter, the connections 
 being represented in Fig. 96. As soon as 
 the circuit is closed at the switch W, the 
 current flows through the primary coil of 
 the instrument, into the spring p, through 
 
254 
 
 ELECTRICITY IN 
 
 the contact c, and the screw stud S, in the 
 support T. The M. M. F. of this current 
 produces a magnetic flux passing through 
 the core, and through the air outside, in 
 circuital paths. This flux being produced 
 within the magnetic circuit, sets up a C. 
 E. M. F. of self-induction, tending to re- 
 tard the development of both flux and 
 current in the primary coil, so that the 
 primary current does not instantly reach 
 its full strength, but rises comparatively 
 slowly to a maximum. As soon as suf- 
 ficient magnetic flux has been produced in 
 the magnetic circuit, to move, by magnetic 
 attraction, a soft iron armature A, sup- 
 ported at the extremities of the springy, 
 the spring is forced to leave the contact c. 
 and thus open the circuit. As soon as the 
 circuit opens, the current strength would 
 immediately fall to zero, but for the fact 
 that the magnetic flux in the circuit, being 
 
ELECTRO-THER^EUTlJ 3.V/ I L flMPfy Q f 
 
 unsupported by M. M/SEfo»M?id]y disap- ^ ^ 
 pears, so that the primary l8^pg4^£8»pidly; XN ^-^ 
 emptied of flux and become the seat of an 7 
 E. M. F. tending to prolong the current. 
 This E. M. F. is comparatively powerful, 
 owing to the rapid rate at which the flux 
 is emptied; in fact, it may be sufficiently 
 great to cause a spark to form between the 
 spring and the contact c. In this manner 
 both the making and the breaking of the 
 primary circuit produce E. M. Fs. in the cir- 
 cuit ; that on making, tending to oppose the 
 establishment of the current, and that on 
 breaking, tending to oppose its cessation. 
 
 On the closing of the primary circuit, 
 the time occupied in producing the full 
 flux is comparatively great ; that is to 
 say, it may amount to, perhaps, the one 
 hundredth of a second ; the loops do not, 
 therefore, fill so rapidly ; but when the cir- 
 
256 ELECTRICITY IN 
 
 cuit is opened at the contact c, the flux is 
 necessarily withdrawn with great rapidity, 
 and the E. M. F. induced at breaking, 
 owing to this greater rate, is much in 
 excess of the E. M. F. induced at making. 
 The value of the E. M. F. on breaking will, 
 therefore, be increased by any cause which 
 will tend to diminish the time required for 
 the complete cessation of the primary cur- 
 rent. The spark which bridges the space 
 between the contact point and the spring 
 has, therefore, the effect of prolonging the 
 current, since it provides a path, of heated 
 air, through which the current may flow, 
 even when the contact is broken. Conse- 
 quently, any device which will stop the 
 spark will result in the production of a 
 higher E. M. F. of self-induction in the 
 primary coil. This is sometimes effected 
 by introducing a condenser into the pri- 
 mary circuit, with its terminals connected 
 
ELECTRO-THERAPEUTICS. 257 
 
 in shunt to the contact. As soon as the 
 contact is broken, the current instead of 
 following through the air in a spark, rushes 
 into the condenser and charges it. As 
 soon as the condenser is charged, the cur- 
 rent ceases very suddenly, and is then 
 unable to jump across the interval of air 
 which has become interposed between the 
 contact point and the spring. The result 
 is, therefore, that the current in the pri- 
 mary coil, being very suddenly arrested in 
 the condenser, the rate at which the loops 
 are emptied of flux is greatly increased, 
 and the E. M. F. induced in the primary 
 circuit is correspondingly increased. 
 
 So far we have only considered the pri- 
 mary coil as though the secondary coil 
 were entirely removed from it. We may 
 now consider the effects that are produced 
 in the superposed secondary coil. If the 
 
258 ELECTRICITY IN 
 
 terminals of the secondary coil be opened 
 so that its external resistance is practically 
 infinite, it can, therefore, send no current. 
 The E. M. F. induced in the secondary 
 coil is the exact counterpart of that which 
 is induced in the primary coil, except, 
 that having more turns, the secondary 
 E. M. F. is correspondingly greater. This 
 is on the assumption that all of the mag- 
 netic flux threading through the primary 
 coil, also threads through the secondary. 
 On closing the primary circuit, the en- 
 trance of magnetic flux through the pri- 
 mary and secondary loops together causes 
 E. M. Fs. to be induced in both coils. 
 This E. M. F. is a C. E. M. F. in the pri- 
 mary circuit, since it acts against the 
 E. M. F. impressed upon it, but it is the 
 only E. M. F. which appears in the 
 secondary circuit. If, for example, the 
 primary coil consists of 100 turns of wire, 
 
ELECTRO-THERAPEUTICS. 
 
 259 
 
 and has induced in it a C. E. M. F. com- 
 mencing at, say one volt, as shown in Fig. 
 97, this C. E. M. F. dying away, along the 
 curved line bed; then, if the secondary 
 
 — i 
 
 -3- 
 
 -4- 
 — 5-J 
 
 Fig. 97. — Diagram of Primary Induced E. M. Fs. 
 
 coil consists of 5,000 turns, the E. M. F. 
 induced in this secondary coil will be 
 represented by the same curve on a scale 
 50 times as great; that is to say, com- 
 mencing at 50 volts, instead of at 1 volt. 
 
260 ELECTRICITY IN 
 
 The moment that the primary circuit is 
 broken at the contact <?, the E. M. F. 
 induced in the primary circuit, instead of 
 being 1 volt reducing to almost zero in 
 the hundredth of a second, may be 5 volts, 
 
 reducing to zero in, perhaps, ■ nrtn ^ n °f a 
 
 second, as is represented in the Fig. 97, by 
 the curve def. Similarly in the second- 
 ary circuit, the E. M. F. induced will be 
 represented by the same curve on a scale 
 50 times as great, and will commence at 
 500 volts, reducing rapidly to zero. The 
 E. M. F. of such a faradic coil is, therefore, 
 obviously alternating, but dissymmetrical in 
 character, the wave at breaking being much 
 greater in amplitude, but correspondingly 
 more brief than the wave at making. 
 
 If the secondary coil be moved away 
 from the primary coil, as in instruments 
 

 .if Jb Ticp: n C F Pj^T V r c 
 
 of the Dubois-Raymond t^ib$9§ne of which 
 
 is represented in Fig. 9o^&ffafyjtexi, r , y$$R* 
 
 secondary coils are shown with 
 
 mary partly inserted in one of them, 
 
 Fig. 98.— Induction Coil, Dubois-Raymond Type. 
 
 the E. M. F. of self-induction, in the 
 primary circuit, will not be influenced, 
 but the E. M. F. of mutual induction 
 in the secondary coil, will be reduced, 
 because the amount of flux linked 
 with the secondary turns is reduced, 
 
262 ELECTRICITY IN 
 
 and finally, when the coils are entirely 
 separated, the amount of flux from 
 the primary coil, which remains linked 
 with the secondary coil, is so small, 
 that its filling and emptying produces an 
 inappreciable induced secondary E. M. F. 
 Consequently, in the use of the Dubois- 
 Raymond type of coil, when it is desired 
 to increase the secondary E. M. F., the 
 primary coil is pushed further into 
 secondary coi], or vice versa, so that the 
 flux produced in the magnetic circuit may 
 link with more and more turns in the 
 secondary coil. In no case, can the E. M. 
 F. in a secondary coil be other than 
 increased by bringing the primary and 
 secondary coils closer together, so long at 
 least as the secondary circuit is open. 
 
 We have hitherto considered the second- 
 ary coil as being open, and have seen that 
 
ELECTRO-THERAPEUTICS. 263 
 
 in such cases, although E. M. Fs. are pro- 
 duced in the secondary, its presence has no 
 appreciable effect upon the nature of the 
 phenomena that occur iii the primary cir- 
 cuit. As soou, however, as the terminals 
 of the secondary coil are connected to an 
 external circuit, so that a secondary cur- 
 rent can be produced in such circuit, then 
 effects are produced, which are superposed 
 upon those already existing; namely, 
 secondary currents flow. These second- 
 ary currents produce a M. M. F. in the 
 secondary coil, and a magnetic flux through 
 the secondary coil, part of which passes 
 through the primary coil. This secondary 
 flux, filling and emptying the secondary 
 loops, sets up a C. E. M. F of self-induc- 
 tion in the secondary circuit tending to 
 oppose both the development and the 
 extinguishment of each current wave. 
 Moreover, the flux from the secondary cir- 
 
264 ELECTRICITY IN 
 
 cuit, passing through the primary coil, 
 induces in it an E. M. F. by mutual 
 induction, and disturbs the flow of current 
 strength through the primary coil. It is 
 to this disturbance, and to the flow of 
 primary current against the mutually 
 induced C. E. M. R, that the energy is 
 obtained from the primary circuit which 
 appears in the secondary circuit. The 
 amount of reaction taking place from 
 the secondary back into the primary 
 circuit, depends upon the strength of 
 the secondary currents, which in its turn 
 depends upon the resistance of the second- 
 ary circuit as composed partly of the 
 resistance in the secondary coil, and 
 partly in the exterual resistance, and also 
 upon the self-induction, or, as it is usually 
 called, the inductance of the secondary coil 
 already mentioned. The greater the cur- 
 rent strength in the secondary circuit, 
 
ELECTRO-THERAPEUTICS. 265 
 
 the greater the mutually induced C. E. M. 
 F. in the primary circuit, and the greater 
 the reactive disturbance in magnetic flux 
 and in any current strength there existing. 
 
 Although, when the secondaiy circuit is 
 open, its E. M. F. at the peak of the wave 
 on breaking, may measure 200 or 300 
 volts, yet on closing the secondary circuit, 
 through a comparatively low resistance, 
 the E. M. F. at the secondary terminals 
 may only be 2 or 3 volts. It is evident, 
 that if the secondaiy terminals be short 
 circuited by a stout piece of wire having 
 negligible resistance, the E. M. F. at 
 secondary terminals would be volts, but 
 even when the secondary external resist- 
 ance is say, 1,000 ohms, the E. M. F. 
 available at secondary terminals is always 
 very small in the medical induction coil. 
 This is both for the reason that, owing to 
 
ELECTRICITY IN 
 
 the reaction of the secondary M. M. F. of 
 the primary circuit, the E. M. F. in the 
 secondary coil is reduced, and because 
 such E. M. F. as remains, so soon as it 
 tends to produce such a current in the ex- 
 ternal circuit, as would flow in accordance 
 with Ohm's law, is met by a powerful 
 C. E. M. F. due to its self-induction. 
 This C. E. M. F. is most powerful when 
 the wave of induced E. M. F. is most 
 abrupt, since, at such times, the inrush of 
 magnetic flux into the secondary coil from 
 its own rising current is greatest. The re- 
 sult is that the drop of pressure in the coil 
 is always very great, owing to its large 
 resistance and large inductance ; i. e. y its 
 large number of turns and capability of 
 producing C. E. M. F. by self-induc- 
 tion. The current which can flow in 
 the secondary circuit is, therefore, not 
 only comparatively feeble, but is also 
 
ELECTRO-THERAPEUTICS. 26^ 
 
 much less abrupt in wave character than 
 the induced E. M. F. would lead one to 
 expect. 
 
 The core employed in the medical induc- 
 tion coil almost invariably consists of a 
 bundle of soft iron wires. A solid rod of 
 soft iron cannot be efficiently employed, 
 because, under the influence of the rapidly 
 oscillating magnetic flux through such a 
 rod, electric currents would be produced 
 around it in such a manner as to oppose 
 the development of the magnetization ; 
 that is to say, the external shells of the 
 rod would act as closed secoudary coils, 
 tending to check and react upon the pri- 
 mary current and wasteful ly expend 
 energy. By employing a bundle of fine, 
 soft iron wires, all these induced eddy 
 currents are reduced to negligibly small 
 strengths 
 
268 ELECTRICITY IN 
 
 If the core, instead of being limited to 
 the interior of the coil, be brought around, 
 so that its extremities shall meet, and the 
 core thus entirely surround the coil, the 
 magnetic flux, produced in the magnetic 
 circuit by a given primary M. M. F., will 
 be much greater. It might, therefore, at 
 first sight be supposed that such an 
 arrangement would increase the induced 
 secondary E. M. F.; but while the flux 
 would certainly be increased, the residual 
 magnetism in the ring of iron thus formed 
 would be so considerable, that the rate at 
 which flux could be caused to oscillate in 
 the ring would be reduced in a greater 
 proportion than the total flux could be 
 increased, so that the employment of such 
 a ring would be detrimental. If the pri- 
 mary current, instead of being pulsatory 
 or unidirectional, were alternating, its in- 
 fluence in eliminating the residual magnet- 
 
ELECTRO-THERAPEUTICS. 269 
 
 tism, at each reversal, would be much 
 greater, and the benefit of a completely 
 ferric circuit would be proportionate. 
 
 The rate of vibration of the spring con- 
 tact has an important bearing upon the 
 action of the apparatus. The frequency 
 of the primary pulsating current depends 
 upon the natural frequency of vibration 
 of the spring, which in turn depends upon 
 its length, breadth, thickness, elasticity and 
 the weight of the armature it supports 
 at its' free end. The lowest tone, which 
 the spring vibrator will emit, corresponds 
 to the slowest natural rate of vibration 
 at which the spring will vibrate as a 
 whole. By causing the contact to ap- 
 proach the core, so as to reduce the range 
 of vibration, the spring is aided in produc- 
 ing overtones / i. e., vibrations set up from 
 the contact point as a node, and in some 
 
270 ELECTRICITY IN 
 
 cases the vibration of the spring is com- 
 posite, being partly performed as a Whole, 
 and partly in segments, having the con- 
 tact point as node. 
 
 The effect of increasing the frequency of 
 vibration in the first instance is to increase 
 the E. M. F. induced in the primary and 
 secondary coils by self and mutual induc- 
 tion owing to the greater rapidity with 
 which the flux is compelled to enter and 
 leave the coils. On the other hand, when 
 the rapidity of vibration exceeds a certain 
 value, depending upon all the electric con- 
 ditions of the circuit, the time in each 
 pulsation, during which contact is main- 
 tained at the vibrating spring, is so short 
 that the entering primary current is greatly 
 reduced in strength, since it lias not time 
 to overcome the C. E. M. F. of self induc- 
 tion. On this account the primary cur- 
 
ELECTRO-THERAPEUTICS. 271 
 
 rent waves are reduced in amplitude, and 
 the M. M. F. is correspondingly reduced, 
 thereby restricting the development of 
 flux. When, therefore, this particular 
 speed of vibration has been obtained, 
 the gain in secondary E. M. F., by 
 reason of the higher frequency, is offset 
 and neutralized by the loss of total flux 
 produced in the magnetic circuit. More- 
 over, the greater the rapidity with which 
 the secondary waves of E. M. F. are in- 
 duced, the more rapid will be the waves 
 of M. M. F. and flux in the secondary coil, 
 and the greater the secondary C. E. M. F. 
 of self-induction tending to oppose the 
 development of abrupt changes in the 
 secondary current strengths. Although 
 the effect of these separate influences de- 
 pends upon the particular proportions of 
 each induction coil, yet, broadly speaking, 
 the effect of increasing the frequency of 
 
272 ELECTRICITY IN 
 
 vibration in the contact spring may be 
 expressed as follows. In all cases the fre- 
 quency in the secondary circuit increases 
 in conformity with the increase in the 
 primary. Up to a certain frequency both 
 the E. M. F. and the current strength in 
 the secondary circuit are increased ; beyond 
 this frequency, the E. M. F. in the second- 
 ary circuit is not increased, but the wave 
 type of secondary current becomes modi- 
 fied. The more rapid the vibration, the 
 smoother and less abrupt the current 
 waves produced, particularly, when long 
 fine-wire secondary coils are employed, 
 owing to the powerful C. E. M. F. set up 
 by rapid fluctuations, and the throttling 
 effect thus produced upon all abrupt varia- 
 tions of current strength. 
 
 In order, therefore, to produce in the 
 secondary circuit abrupt waves of the type 
 
ELECTR0-TH1 
 
 diagrammatically slio^n^p Fig. 97, the 
 secondary circuit snoukiN^a?^ ^ompara- ■ O o^^ 
 tively small number of t rrmn^nf- yn ytr^*^ 
 placed as close as possible to the primary 
 coil, so that the throttling effect of self-in- 
 duction in its own turns may be avoided, 
 
 Fig. 99.— Type of Secondary Induced E. M. P. at 
 High Frequencies Under Load. 
 
 and the frequency of vibration compara- 
 tively small. On the other hand, in order 
 to produce in the secondary circuit, 
 smooth, less abrupt current waves such as 
 those shown in Fig. 99, long, fine-wire 
 coils, with considerable inductance, should 
 be employed. Moreover, it is not necessary 
 that all the turns should be linked with 
 
274 ELECTRICITY Itt 
 
 the primary coil ; that is to say, the second- 
 ary E. M. F., produced by mutual induc- 
 tion, can be induced in part of the second- 
 ary coil. The remainder of the second- 
 ary coil, being unacted on, will serve to 
 choke or throttle the sudden variations in 
 the secondary current by its inductance. 
 This effect will be enhanced by making 
 the frequency conveniently great. It is to 
 be remembered, however, that in no case 
 can either the E. M. F. wave, or the 
 current wave, in the secondary circuit be 
 made symmetrical, the wave on breaking 
 being always steeper than that on making. 
 
 The usual method of altering the fre- 
 quency in the primary circuit, consists in 
 advancing the spring contact, so as to in- 
 crease the tension of the vibrating spring, 
 and reduce its range of vibration. The 
 range of frequency obtained in this way is 
 
ELECTRO-THERAPEUTICS. 
 
 275 
 
 comparatively small, as it is rarely that the 
 frequency can be doubled in this manner. 
 A device sometimes employed in connection 
 
 &AITZBX 
 
 Fig 100.— Adjustable Vibrator for Faradic Coils. 
 
 with larger induction coils is shown in 
 Fig. 100, where the screw S, and the nut JV, 
 serve to tighten the vibrating spring and 
 thus vary its tension. The frequency ob- 
 tained from an ordinary spring vibrator is 
 
276 ELECTRICITY IN 
 
 from 150 to 300- ; i. e., 150 to 300 com- 
 plete periods per second. For higher fre- 
 quencies a ribbon vibrato?* is sometimes 
 employed, such as shown in Fig. 101. 
 
 Fig. 101.— Induction Coil with Ribbon Vibrator. 
 
 In a ribbon vibrator the contact screw 
 (7, presses upon a horizontal steel ribbon, 
 at a point which is about one quarter of 
 the length of the ribbon from the fixed 
 
ELECTRO-THERAPEUTICS. 277 
 
 support on the left. The ribbon can be 
 tightened by the thumbscrew on the right 
 hand pillar or support. By varying the 
 tension on the ribbon, a considerable 
 range of frequency of vibration can be ob- 
 tained. The induced secondary currents 
 obtained from such an apparatus, with a 
 long, thin- wire coil, have considerable fre- 
 quency, small strength, and comparatively 
 smooth, wave character. For abrupt, 
 powerful, secondary currents, a slow speed 
 vibrator is represented at V, in the same 
 figure. It consists of a vertical electro- 
 magnet, which attracts an armature of 
 soft iron, attached to a horizontal steel 
 spring, and is loaded with a spherical 
 weight, capable of being clamped at vary- 
 ing distances along the rod attached to the 
 moving system. By clamping the weight 
 near the free extremity of the rod, the 
 slowest vibrations are obtained ; while by 
 
27a 
 
 ELECTRICITY IN 
 
 clamping it near to the armature, the 
 speed of vibration is increased. The 
 secondary coil employed with the slow 
 vibrator is usually of fewer turns and 
 
 
 ^■IeS. ^~?H* f £3f J ? 
 
 f'^wr~^^^^A H* 
 
 1 
 
 1 
 
 
 
 Kki^ g^ 
 
 rKJh 3 ^ 
 
 
 
 
 T ^r T?__* 
 
 T 7 * «J5 *? s ^ ~ ^ " ' ~ * |S, 
 
 
 • 
 
 
 Fig. 102.— Induction Coil with Rapid Interrupter. 
 
 coarser wire, offering a smaller resistance 
 and a considerably smaller inductance. 
 
 Fig. 102, represents a type of medical 
 induction coil in which the frequency is 
 varied by means of a small electromagnetic 
 motor MM. This motor drives three 
 
ELECTRO-THERA 
 
 wheels 1, 2 and 3, upon 
 which are a series of 
 which are pressed upon by spring col 
 By the insertion of one of these wheels in 
 the primary circuit of the induction coil, 
 the frequency of pulsation of the primary 
 current can be varied within wide limits, 
 and a high frequency attained. The 
 secondary coil C, can be moved toward or 
 from the primary coil within it, by turn- 
 ing the screw S. 
 
 Another method of varying the induced 
 E. M. F. in a secondary coil, without vary- 
 ing the frequency, is by the use of a 
 metallic tube inserted between the core 
 and the primary coil, between the primary 
 and secondary coils, or over the secondary 
 coil. The second method is represented in 
 Fig. 103, where the handle H y is attached 
 to one extremity of a brass tube, inserted 
 
280 ELECTRICITY IN 
 
 between the primary and secondary coils 
 of the medical induction coil, which in this 
 
 Fig. 103. — Induction Coil with Dry Cell and 
 Internal Damping Tube. 
 
 instance is supplied by a dry voltaic cell, 
 situated in the base of the apparatus. An 
 example of the third method is represented 
 in Fig. 104, where the tube T, is advanced 
 
ELECTRO-THERAPEUTICS. 
 
 281 
 
 over the surface of the secondary coil. 
 The action of the tube is in all cases the 
 
 Fig. 104.— Induction Coil with External Damping 
 Tube. 
 
 same. It forms, in reality, an independent 
 secondary coil, having a very low resist- 
 
282 ELECTRICITY IN 
 
 ance, consisting as it does of but a single 
 turn of conductor. When the magnetic 
 flux from the primary coil and core, fills 
 and empties this tube, it sets up around 
 it, comparatively powerful induced current 
 strengths, owing to its low resistance. 
 Although the induced E. M. F. may be but 
 feeble, yet with the presence of a single 
 turn the effect of this comparatively 
 powerful secondary current is to produce 
 an M. M. F. and magnetic flux with such 
 reactive power upon the primary circuit, 
 that the resultant magnetic flux, linked 
 with the secondary coil, is very greatly 
 enfeebled. The further the tube is pushed 
 into or over the coil, the greater will be 
 its reactive influence upon the primary cir- 
 cuit, and the smaller will be the induced 
 E. M. F. in the secondary coil. 
 
 The Dubois-Eaymond method of vary- 
 
ELECTRO-THERAPEUTICS. 283 
 
 ing the secondary E. M. F. consists, there- 
 fore, of varying the mutual inductive 
 power of the primary and secoodary coils, 
 by varying their mean distance from each 
 other. The shield method consists in 
 leaving the primary and secondary coils 
 at a fixed distance, but so distributing 
 the magnetic flux through the coils, 
 under the influence of powerful shield 
 secondary currents, that the resultant mag- 
 netic flux through the secondary circuit is 
 reduced. 
 
 Notwithstanding the great variety of 
 medical induction coils, and their seeming 
 differences of construction, electrically they 
 invariably consist of a primary and second- 
 ary coil in mutual inductive relation to 
 each other, and an interrupter, to vary the 
 frequency of the primary current. The 
 primary coil may possess a greater or less 
 
284 ELECTRICITY IN 
 
 resistance and inductance, and the two 
 coils may be so placed as to have a greater 
 or less mutual inductance. The number 
 of turns in the primary and secondary coils, 
 the number of cells to be employed to 
 operate the instrument, and the manner 
 in which the mutual inductive influence 
 between the coils is varied, are merely, 
 from an electric standpoint, incidental 
 to the main purpose for which the coil 
 is intended; namely, to produce at the 
 secondary terminals an E. M. F. of a given 
 frequency, value, and wave type, when 
 the external resistance is assigned. That 
 is to say, if the resistance of the external 
 circuit comprising the body of a patient, 
 and the electrodes connected therewith, 
 amounts, to say 2,000 ohms, then the func- 
 tion of the apparatus is to produce at its 
 secondary terminals a certain effective volt- 
 age, such as would be indicated by a suit- 
 
ELECTRO-THERAPEUTICS. 285 
 
 able voltmeter, a certain frequency, and a 
 certain wave character of E. M. F. If the 
 effective pressure under these circumstances 
 at the secondary terminals be, say 6 volts, 
 then the effective current through the 
 external circuit, provided that the induct- 
 ance of the external circuit is small ; i. e., 
 that there are no coils of many turns of 
 wire in the circuit, will by Ohm's law, be 
 
 %m = im = 3 miiiiam P eres - 
 
 In order to compare the relative electric 
 effectiveness of different medical induc- 
 tion coils, it is only necessary to measure 
 the effective E. M. F. maintained at the 
 terminals of the secondary, when con- 
 nected through different external induc- 
 tionless resistances, the frequency of the 
 interrupted and the wave type of E. M. F. 
 being at the same time observed. For 
 
286 ELECTRICITY IN 
 
 some apparatus a high frequency, and a 
 smooth type of wave are the desiderata, 
 while for others, a low frequency, and a 
 rough wave type are desired. The coil 
 which will supply the requisite number of 
 volts at its secondaiy terminals, under all 
 variations of load, and will, at the same 
 time, supply these frequencies and wave 
 type with the maximum economy, sim- 
 plicity, durability and convenience, will 
 be the most effective coil, from an electric 
 standpoint, and no structural variations in 
 an induction coil, whether obtained by 
 altering its magnetic circuit, its winding, 
 or its various parts, can do anything 
 except to alter the frequency, the wave 
 type and the magnitude of the E. M. F. at 
 its secondary terminals. 
 
 It might be supposed that when a 
 medical induction coil, of the Dubois- Ray- 
 
ELECTRO-THERAPEUTICS. 287 
 
 mond type, has its secondary coil as far 
 over the primary coil as it will go, that 
 the current strength in the secondary cir- 
 cuit will be a maximum. This, however, 
 is not always the case, since the M. M. F. 
 of the current in the secondaiy coil may 
 so greatly react upon the primary flux and 
 current, as to cause the secondary to act 
 like a metallic shield. In other words, 
 the secondary coil may overload the 
 primary. For this reason, it is possible 
 that the maximum current strength in the 
 secondary circuit may be obtained when 
 the coil is only partly covering the pri- 
 mary, say one half or three quarters. 
 
 In a continuous-current circuit, since the 
 E. M. F. is always acting, there is no diffi- 
 culty in determining its value ; but since, in 
 an alternating-current circuit, the value of 
 the E. M. F. reaches a maximum twice in 
 
288 ELECTRICITY IN 
 
 every cycle, varying on each side of zero, it 
 is more difficult to define what is the value 
 of the E. M. F. The E. M. F. is always 
 defined as the effective thermal F. M. F. or 
 simply the effective F. M. F. That is to 
 say the value of an alternating or pulsat- 
 ing E. M. F. is taken as being equal to 
 that of the continuous E. M. F., which is 
 capable of producing the same heating ef- 
 fect in an inductanceless resistance. Simi- 
 larly, when the electric current pulsates 
 or varies, its effective thermal value, com- 
 monly called its effective current strength, 
 is equal to that strength of continuous cur- 
 rent which would produce in a given re- 
 sistance the same heating effect. If, there- 
 fore, the effective E. M. F. at the terminals 
 of a medical induction coil be expressed or 
 measured as five volts effective, we mean 
 that the E. M. F. it produces, although 
 varying between, perhaps, 20 volts and 
 
^ 
 
 j^T& OlESf IfljSj 
 
 electro-tiim^pelPI^^. p r ri8f f L 
 
 zero, produces in a sr»$£ resistance, the 
 same amount of heat l^fe^uvr.^p,^ 
 a minute or five minutes, 
 of continuous-current pressure. Conse- 
 quently, the statement of the effective 
 current strength or effective E. M. F. at 
 the terminals of an induction coil does not 
 express the range of current or of E. M. F. 
 in each wave, unless the shape of the 
 wave be known. 
 
 The connections of the medical induc- 
 tion coil are commonly so arranged that 
 the E. M. F. induced in either the primary 
 or the secondary circuit can be externally 
 employed. We have seen that on break- 
 ing, an E. M. F. of self-induction appears 
 in the primary circuit, as well as an E. M. 
 F. of mutual induction in the secondary 
 circuit, but the E. M. F. of mutual induc- 
 tion is practically always greater than in 
 
290 
 
 ELECTRICITY IN 
 
 the primary circuit owing to the greater 
 number of turns in the secondary coil. If 
 P lf P 2 , Fig. 105, represent the terminals of 
 the primary coil, and /Si, S% the terminals of 
 
 Pl P 3 P 2 S, s, 
 
 Fig. 105. — Connections op Medical Induction Coil. 
 
 the secondary coil, while Pi, is connected 
 to the vibrating spring, then it will be 
 seen that when the spring breaks contact, 
 the induced E. M. F. in the primary coil 
 will be developed between the terminals 
 P% and P t , while the mutually induced E. 
 
ELECTRO-THERAPEUTICS. . 291 
 
 M. F. will be developed between $, $». 
 There is this difference, however, between 
 the E. M. F. of the primary and secondary 
 circuits, that w T hereas, S\ and $, will 
 supply an E. M. F. both at making and at 
 breaking, in opposite directions, although 
 the E. M. F. at breaking is, as we have 
 seen, much stronger than the E. M. F. at 
 making ; yet, on the other hand, between 
 P, and P 3y the E. M. F. on making is always 
 very small, and must be less than the E. 
 M. F. between P x and P* ; that is to say, 
 it must be less than the E. M. F. of the 
 batteiy. Under ordinary circumstances, 
 therefore, when the primary coil has com- 
 paratively few turns of coarse wire, the 
 current obtainable between P 2 and P 3 , is 
 practically zero at making, and rises sud- 
 denly and abruptly on breaking, or acts 
 exactly in the same manner as a secondary 
 coil of the same number of turns. Such a 
 
292 ELECTRICITY IN 
 
 type of medical induction coil is repre- 
 sented in Fig. 106. 
 
 In Fig. 107, a common form of connec- 
 tion of the circuits of a medical induction 
 
 Fig 106.— Induction Coil with Handle Electrodes. 
 
 coil is shown. Here one end of the sec- 
 ondary coil is brought into connection with 
 one end of the primary coil. By this means 
 we obtain between P* and K, the self- 
 induced E. M. F. of the primary, and be- 
 
ELECTRO-THERAPEUTICS. 
 
 293 
 
 tween /Si and /&, the mutually induced E. 
 M. F. of the secondary coil. Between P* 
 and &, the total E. M. F. in both coils is 
 obtained. The effect in fact is to add to 
 the secondary coil another layer which is 
 
 Fig. 107. — Inter-connection op Primary and Second- 
 ary Windings in Medical Induction Coil, 
 
 actually serving, before breaking contact, 
 as a primary. At making, however, the 
 effect due to the secondary coil is greater 
 than the effect due to the primary coil. 
 It is to be remembered that when any 
 
294 ELECTRICITY IN 
 
 combination takes place, by the superposi- 
 tion of a secondary current upon a primary 
 current, or of either upon a continuous 
 current in a circuit, the result will be the 
 sum of all the influences acting independ- 
 ently, and will consist electrically, of a 
 pulsating or alternating wave of current, 
 having a wave type which may be modi- 
 fied by the simultaneous influence of the 
 various components. 
 
 Fig. 108, shows a form of apparatus con- 
 sisting of a medical induction coil and of 
 a battery of silver chloride cells. The in- 
 duction coil can be operated from a few of 
 the cells in this battery so as to supply 
 dissymmetrical alternating currents of, per- 
 haps, 200~, while the battery, which will 
 probably have an E. M. F. of 50 volts, 
 will be capable of supplying a continuous 
 current. 
 
ELECTRO-THERAPEUTICS. 
 
 295 
 
 We have already seen that both the E. 
 M. F. and the current strength at making, 
 
 Fig. 108. — Induction Coil with Battery of Chloride 
 of Silver Cells. 
 
 are smaller than the E. M. F. and current 
 strength on breaking. It is to be remem- 
 bered, however, that in all cases, the total 
 quantity of electricity which passes through 
 
296 ELECTRICITY IN 
 
 the circuit is the same in each alternating- 
 current wave. In the secondary circuit of 
 an induction coil, no matter how great may- 
 be the dissymmetry of wave type, that is 
 to say, no matter if the current strength is 
 much greater on making than on break- 
 ing, it will be correspondingly briefer in 
 duration. In, for example, Fig. 97, which 
 represents dissymmetrical alternating cur- 
 rents produced in the secondary circuit of 
 an induction coil, the wave ((bed, in one 
 direction, or above the line, being produced 
 at making contact, and def, the greater 
 wave, being produced at breaking contact, 
 then the area of the wave abed as rep- 
 resented graphically, will be equal to the 
 area def, under all circumstances. This 
 area also represents the total quantity of 
 electricity, measured in coulombs, which 
 will pass through the circuit. Conse- 
 quently, all physiological effects, which 
 
ELECTRO-THERAPEUTICS. 297 
 
 depend upon current strength, will be dis- 
 symmetrical or polar in the secondary cir- 
 cuit ; that is to say, the effect produced at 
 making will be different from the effect 
 produced on breaking and, consequently, 
 the effects at one pole will be different 
 from those at the other; but all physio- 
 logical effects, which depend only on the 
 quantity of electricity, will be the same 
 both on making and on breaking. Thus, 
 it is well known that muscular excitability 
 is a function of the current strength, so 
 that the muscular excitation produced by 
 the making and breaking currents will be 
 different. On the other hand, all the elec- 
 trolytic effects, which are dependent upon 
 the quantity of electricity which passes, 
 will be alternately produced in equal 
 amounts at each wave. Consequently, 
 either electrolysis will not appear at 
 all, or the products of electrolysis will 
 
298 ELECTRICITY. 
 
 appear in equal quantities at each elec- 
 trode. 
 
 To sum up, the medical induction coil 
 gives discharges possessing the following 
 characteristics : 
 
 It produces dissymmetrical alternating- 
 current waves of fairly high frequency, 
 but intermittent, the waves being sepa- 
 rated by intervals of no current. The 
 waves on breaking are steep and abrupt, 
 except in the case of very long, fine wire 
 coils, with large self inductance, and chok- 
 ing effect. The E. M. F. induced in the 
 secondary coil may be hundreds of volts, 
 but the E. M. F. available at the termi- 
 nals, under any ordinary load, will be not 
 usually more than 15 volts effective. 
 
CHAPTER XL 
 
 A foem of electric source, occasionally 
 used in electro-therapeutics, is to be found 
 in the dynamo-electric generator. Of the 
 sources already discussed, the voltaic bat- 
 tery produces a continuous E. M. F. ; the 
 influence machine, a pulsatory E. M. F. ; 
 and the induction coil an alternating E. M. 
 F. Dynamos, on the contrary, can be em- 
 ployed to produce either continuous or 
 alternating E. M. Fs., according to their 
 construction. Those producing continuous 
 E. M. Fs. are generally termed simply 
 dynamos, or generators; while^those which 
 produce alternating E. M. Fs., are usually 
 called alternators. 
 
300 ELECTRICITY IN 
 
 The fundamental principle employed in 
 all dynamos and alternators, is the induc- 
 tion of E. M. F. by filling and emptying 
 conducting loops with magnetic flux. 
 Dynamos and alternators generally consist 
 of a fixed and a rotary part, by the mutual 
 action of which conducting loops become 
 filled and emptied with magnetic flux 
 during rotation. The E. M. Fs. so gene- 
 rated will be alternating, unless a com- 
 mutator be employed to render them con- 
 tinuously directed in the external circuit. 
 Any dynamo, therefore, which employs no 
 commutator is necessarily an alternator. 
 This has been already illustrated in the case 
 of the hand magneto-generator described in 
 Chapter VIII. 
 
 The E. M. F. obtained from a continu- 
 ous-current generator is always slightly 
 pulsating, as represented in Fig. 40. The 
 
ELECTRO-THERAPEUTICS. 301 
 
 departure from a continuous E. M. F., such 
 as is produced by a voltaic battery, is less 
 marked when the number of commutator 
 bars is great. When, however, few bars 
 are employed, the pulsation may be more 
 evident, as shown in Fig. 41. If it were 
 possible to employ an indefinitely great 
 number of commutator bars, the E. M. F. 
 would be unvarying. This deviation from 
 continuity occurs at each passage of a com- 
 mutator bar or segment beneath the brush. 
 
 A dynamo, whether continuous or al- 
 ternating, usually employs electromagnets 
 to supply the flux with which its loops of 
 wire are filled and emptied. In the case 
 of the magneto-generator illustrated in Fig. 
 93, a permanent magnet is employed for 
 this purpose. Electromagnets require to 
 be supplied with a continuous current for 
 their excitation. This exciting current is 
 
302 ELECTRICITY IN 
 
 either supplied from the armature of the 
 dynamo itself, or from some separate 
 source. In the former case the machine is 
 said to be self -excited, and in the latter, 
 separately-excited. When the external cir- 
 cuit of a continuous or alternating-current 
 generator is opened, the machine requires 
 no more power for its operation than that 
 necessary to overcome its frictions, to- 
 gether with the small amount of electrical 
 power supplied for the excitation of the 
 field magnets. When, however, the ma- 
 chine furnishes a current to an external 
 circuit, the power which has to be supplied 
 to drive the dynamo is increased by the 
 amount of electric power in the external 
 circuit. If, for example, a machine ex- 
 pends 50 watts, or volts-amperes, in its 
 external circuit, the power applied to 
 drive it is increased by something more 
 than 50 watts, since some loss of power 
 
ELECTRO-THERAPEUTICS. 303 
 
 occurs in the machine in order to furnish 
 the external work. 
 
 Continuous-current generators are not 
 very extensively used in electro-thera- 
 peutics, since other sources of continuous 
 E. M. F. are available. When generators 
 are used, they are generally driven by elec- 
 tric motors, taking their power from neigh- 
 boring electric circuits. For example, if it 
 be required to obtain a low continuous 
 pressure, of say 5 volts, in order to oper- 
 ate an electric cautery, and an electric 
 circuit of higher pressure, be available in 
 the neighborhood, say of 110 or 220 volts 
 pressure, as commonly used in electric 
 lighting, a small motor, operated from the 
 lighting mains, may be employed to drive 
 a dynamo constructed to give the neces- 
 sary E. M. F. and current required for 
 the cautery knife. 
 
304 ELECTRICITY IN 
 
 Electromagnetic motors are operated 
 either from continuous or alternating-cur- 
 rent circuits. A motor, however, which 
 is arranged to work on a continuous-cur- 
 rent circuit is not usually capable of oper- 
 ating on an alternating-current circuit, and 
 vice versa. Moreover, in either case a 
 motor will only operate effectively at the 
 E» M. F. for which it is designed. 
 
 A particular form of small motor, oper- 
 ated from 110 volt continuous-current pres- 
 sure, is represented in Fig. 109. 31, M y are 
 the field magnets, A, the armature, CO, 
 the commutator, B, one of the brushes rest- 
 ing upon the commutator, and P, the 
 pulley. 
 
 Another form of small motor intended to 
 be driven by a primary or storage battery 
 is shown in Fisr. 110. A form of small 
 
ELECTRO-THERAPEUTICS. 
 
 305 
 
 Fig. 109.— Electromagnetic Motor. 
 
306 -ELECTRICITY IN 
 
 alternator designed for electro-therapeutic 
 purposes is represented in Fig. 111. The 
 coils O, C, C, on the field frame, are wound 
 
 Fig. 110. — Electromagnetic Motor. 
 
 with two circuits ; a coarse wire circuit 
 excited by a continuous current from a 
 pair of binding posts P, and a fine wire cir- 
 cuit connected with the binding posts S. 
 The armature A A, which is rotated by a 
 small pulley at one end of the shaft, is con- 
 
ELECTRO-T] 
 
 
 structed of sheets of \o% iron, and carries 
 teeth Tj T, in such a mSijn^ ^alternately 
 to open and close the inM^i^t^rw? 7 >if n 
 
 Fig. 111.— Electro-Therapeutic Alternator. 
 
 the coils C, C, C. There are 12 poles in 
 the field frame, so that each revolution of 
 the armature produces 12 complete periods, 
 or 24 alternations. As soon as the teeth 
 
308 ELECTRICITY IN 
 
 bridge across adjacent poles, magnetic flux* 
 is poured through the secondary circuits, or 
 fine wire circuits, inducing in them an E. 
 M. F. in one direction, and as soon as the 
 teeth pass beyond this position, the mag- 
 netic circuits are opened, and the secondary 
 coils are emptied of flux, thus inducing an 
 oppositely directed E. M. F. The advan- 
 tage of such an alternator is that it fur- 
 nishes alternating E. M. Fs. of approxi- 
 mately sinusoidal type, and at a frequency 
 which, within certain limits, is under con- 
 trol. At the high speed of 4,800 revolu- 
 tions per minute, or 80 revolutions per 
 second, the frequency of alternation will 
 be 80X12=960 complete cycles, or 1,920 
 alternations per second; while at lower 
 speeds the frequency mil be correspond- 
 ingly reduced. The E. M. F. obtainable 
 from such a machine is about 50 volts, 
 The E. M. F. at terminals is, however. 
 
ELECTRO-THERAPEUTICS. 309 
 
 considerably less than this when ordinary 
 loads are applied. 
 
 Alternating currents are frequently sup- 
 plied from electric lighting stations to con- 
 sumption circuits and buildings, at a com- 
 paratively high pressure, 1,000 or 2,000 
 volts effective being the pressure com- 
 monly employed. As this is a danger- 
 ously high pressure to handle, it is never 
 permitted to enter a house, the pressure 
 being reduced at some point outside the 
 house, by an apparatus called a step-dotvn 
 transformer, which is a form of induction 
 coil in which the primary wire contains a 
 greater number of turns than the second-, 
 ary. Alternating currents generated by 
 large alternators, placed in the central sta- 
 tion, are sent through the primary coils of 
 the transformer, usually by overhead wires. 
 The secondary coils of the transformers 
 
310 ELECTRICITY IN 
 
 generate a pressure of 50, 100 or 200 volts, 
 according to circumstances, and wires from 
 
 Fig. 112.— Alternating Current Transformer. 
 
 the secondary coil enter the building 
 to be supplied. A form of transformer 
 is shown in Fig. 112, P, P, being the 
 
ELECTRO-THERAPEUTICS. 311 
 
 primary, and S, S, the secondary wires. 
 The ratio of the secondary to the primary 
 pressure is called the ratio of transforma- 
 tion. Thus, if the primary pressure be- 
 tween the wires P, P, be 1,000 volts effec- 
 tive, and the secondary pressure between 
 the wires S, S f 50 volts effective, the ratio 
 of transformation is 1 : 20, and this will be 
 approximately the ratio of the number of 
 turns in the secondary coil to the number 
 of turns in the primary coil of the trans- 
 former. The frequency of alternation 
 employed in alternating-current electric 
 lighting is not higher than 140~, or 280 
 alternations per second, and usually varies 
 between this and 125~ or 250 cycles. In 
 some cases, however, the frequency may 
 be 60 ~ and even as low 25 ~ per second. 
 
 A particular form of transformer, de- 
 signed for supplying alternating electric 
 
312 
 
 ELECTRICITY IN 
 
 currents at the pressure required to 
 operate a cautery knife, is shown in Fig. 
 113. PP, is the primary coil wound upon 
 
 Fig. 113. — Alternating-Current Transformer for 
 Cautery. 
 
 an iron core, consisting of a bundle of 
 straight iron wires and resembling, there- 
 fore, in general form, the primary coil of 
 a medical induction coil. The secondary 
 coil S, consists of a short coil of thick wire, 
 which, having a low resistance, enables 
 
ELECTRO-THERAPEUTICS. 313 
 
 comparatively powerful currents of say 5 
 to 30 amperes to be produced in the 
 secondary circuit. The apparatus is, 
 therefore, a step-down transformer. The 
 primary coil is wound for an effective 
 alternating pressure of 50 or 100 volts, 
 according to the pressure employed in the 
 lighting circuits of the building. In order 
 to regulate the E. M. F. and current in the 
 secondary circuit, the secondary coil S, is 
 moved from or towards the centre of the 
 primary coil P, by the screw S, after the 
 manner of the Dubois-Rayinond type of 
 adjustment in the medical induction coil. 
 The contact C\ is so arranged that by the 
 closing of the box, the primary circuit is 
 opened. 
 
 In some cases, where continuous cur- 
 rents are supplied to a building for light- 
 ing purposes, at 110 volts pressure, it is 
 
314 
 
 ELECTRICITY IN 
 
 possible to dispense entirely with the use 
 of batteries for the operation of a medical 
 i lid notion coil, or for the production of 
 
 Fig. 114. — Adapter for Continuous Current 
 Circuits. 
 
 feeble continuous currents in electro- thera- 
 peutic work. An apparatus for this pur- 
 pose called an adapter is shown in Fig. 
 114. It consists essentially of a rheostat, 
 
ELECTRO-THERAPEUTICS. 315 
 
 placed in the circuit of the electric light- 
 ing; mains, in such a manner as to reduce 
 the current required to the right strength 
 without danger. A long cylinder RR SSS, 
 of hard rubber, contains at the end RR, a 
 number of resistances, which are connected 
 with brass contact strips above. The 
 sliding contact C, makes connection with 
 one of these brass strips, so as to include 
 any desired number of resistances in the 
 circuit. At the end of these resistances, 
 and connected with thein, is a long spiral 
 of fine German-silver wire, wound in a fine 
 groove on the cylinder, SS, so that the 
 contact (7, in sliding over the cylinder to 
 the right, makes contact in succession with 
 each turn of German-silver wire, thus cut- 
 ting out the resistance very gradually over 
 this portion of the circuit. M, is a milliam- 
 metre, and I, an induction coil, whose pri- 
 mary circuit is operated by a current from 
 
316 
 
 ELECTRICITY IN 
 
 the electric lighting mains through the 
 lamps Z, Z, Z. 
 
 The connections of the adapter are 
 shown in Fig. 115. It will be seen that 
 
 Fig. 115. — Connections of Adapter. 
 
 the current through the mains AB 1 passes 
 through the lamps Zi, through the circuit 
 of the patient at P, and through the ad- 
 justable resistance and milliammetre. By 
 connecting the middle lamp Z, in the cir- 
 
ELECTRO-THERAPEUTICS. 317 
 
 cuit, the pressure connected with the 
 patient can be considerably reduced. It 
 will be observed, that in no case can the 
 circuit of the patient be connected to the 
 mains without the interposition of two 
 110-volt electric lamps. The primary of 
 the induction coil can be thrown into cir- 
 cuit by the use of the switch S. 
 
 The fact that an incandescent electric 
 lamp can be entirely enclosed in a non- 
 conducting air-tight glass chamber, renders 
 it suitable for introduction into the cavi- 
 ties of the body. Two miniature incan- 
 descent lamps, suitable for such explora- 
 tory purposes,-- are shown in Fig. 116. 
 These give about half a candle, and are 
 operated at pressures of between 2 and 
 4 volts, with a current strength of from 1 
 to 1 1/2 amperes. Care must be taken in 
 the operation of such lamps, that the pres- 
 
318 ELECTRICITY IN 
 
 sure shall not exceed that for which they 
 are designed, as otherwise the lamps will 
 be destroyed. They can be supplied with 
 either an alternating or a continuous cur- 
 rent, but are usually operated by a 
 battery. Since a 1/2 candle-power lamp 
 
 i 
 
 Fig. 116.— Incandescent Electric Lamps for 
 Exploration. 
 
 requires an activity of about 3 1/2 watts, 
 or at the rate of 7 watts per candle, 
 while a lamp of 8 candle-power requires to 
 be supplied with about 30 watts, or at the 
 rate of about 3 1/2 watts per candle, it is 
 evident, that when the lamp has consider- 
 able illuminating power, the heat it liber- 
 
* PRCFCEJY (/ 
 
 [cs. 319 v ' f 
 
 ELECTRO-THER\^EUTICS 
 
 ates may be inconvenientlj^gd^f,., .Conse- 
 quently, when the candle-power o^iamps^ 
 for exploratory purposes exceeds a certain 
 amount, it is customary to enclose the 
 globe in a second glass chamber, through 
 which water is circulated, so as to carry 
 off the surplus heat. 
 
 The heating power of the electric cur- 
 rent is often applied in surgery for 
 cauterizing purposes. Electric cautery 
 knives consist essentially of suitably 
 shaped platinum wires, heated by the elec- 
 tric current. Fig. 117, shows several forms 
 of such cautery knives. The amount of 
 activity required to render the cautery 
 knives white hot, depends upon the surface 
 of hot platinum which they expose to the 
 air. A broad, flat knife requires more 
 activity than narrow blades. Either alter- 
 nating or continuous currents are suitable 
 
320 
 
 ELECTRICITY IN 
 
 for cautery knives. Either primary or 
 secondary cells are frequently employed 
 for this purpose. For the broadest knife 
 
 Fig. 117.— Electric Cautery Knives. 
 
 in the figure, 25 or even 30 amperes, at 
 a pressure of approximately one volt, may 
 be required, representing an activity in the 
 knife of from 25 to 30 watts. In the 
 platinum snare cautery, a growth or part is 
 
ELECTRO-THERAPEUTICS. 321 
 
 removed by causing a length of wire to en- 
 circle the part and then drawing the loop 
 tight, so that the glowing wire is pulled 
 through the part to be removed. Here, 
 owing to the length of wire which has to 
 be heated, though the total activity may 
 be comparatively small, yet the E. M. F. 
 necessary to send the required current 
 through the length of platinum wire may 
 be considerably greater than that for a 
 cautery knife. 
 
 We have seen, that in accordance with 
 Ohm's law, the current strength in any 
 circuit may be altered, either by varying 
 the E. M. F., or by varying the resistance. 
 Both of these methods are employed 
 in electro-therapeutics. Instruments for 
 varying the resistance in a circuit are 
 called rheostats. They consist essentially 
 of resisting paths whose length or area of 
 
322 ELECTRICITY IN 
 
 cross-section may be adjusted, or varied at 
 will. In most forms of rheostat, it is the 
 length of resisting path and not the area of 
 cross-section, which is varied. 
 
 The form given to the resisting paths 
 depends upon the strength of the current 
 which has to be regulated. Currents for 
 cauterizing, which may be as high as 20 or 
 25 amperes, require comparatively coarse 
 wire coils ; for, each ohm through which a 
 current of 25 amperes passes, liberates heat 
 at the rate of 625 watts, or nearly one 
 H. P., and, consequent!} 7 , if this one ohm 
 consisted of fine wire of comparatively 
 short length and, therefore, possessing a 
 very limited radiating surface, the wire, 
 being unable to dissipate this heat, would 
 acquire a temperature, probably, sufficient 
 to melt it. The comparatively feeble cur- 
 rents generally employed in electro- 
 
ELECTRO-THERAPEUTICS. 323 
 
 therapeutics do not require an extensive 
 radiating surface, and the rheostats through 
 which they pass may be composed of fine 
 wire or of water or of carbon. 
 
 Fig. 118. — Carbon Rheostat. 
 
 One of the simplest forms of rheostat for 
 very feeble currents is shown in Fig. 118. 
 Here the resisting column consists of a thin 
 layer of graphite obtained by rubbing a soft 
 graphite pencil in a circular path around 
 the rim of a slate slab, CCC. By this 
 
324 ELECTRICITY IN 
 
 means a layer of fairly high resisting car- 
 bon is obtained, and the length of this 
 path in a circuit, determines the amount 
 of resistance included. This length is ad- 
 justed by altering the position of the brush 
 B, attached to the milled-headed screw M. 
 It becomes necessary in practice, to occa- 
 sionally renew the carbon layer. Its resist- 
 ance can be varied by rubbing more or 
 less graphite over the surface. 
 
 Another form of carbon rheostat is 
 shown in Fig. 119. Here the resisting 
 path is composed of pulverized carbona- 
 ceous material pressed into a groove in 
 an insulating plate. A number of brass 
 studs, CCO, pass through the surface of 
 the insulating plate and make contact with 
 the carbon column in the groove beneath. 
 The length of the carbon column inserted 
 between the terminals, T, T, can be varied 
 
ELECTRO-THERAPEUTICS. 
 
 325 
 
 by turning the handle H, so as to make 
 contact with the brass studs at different 
 portions of the circumference. 
 
 Fig. 119.— Carbon Rheostat. 
 
 Fig. 120, shows another- form of carbon 
 rheostat depending upon a somewhat dif- 
 ferent principle. Here powdered carbon 
 is placed in a chamber provided with elas- 
 tic sides CO. The resistance between the 
 top and bottom surfaces of this mass of 
 
ELECTRICITY IN 
 
 carbon depends upon the pressure which 
 is brought to bear upon the layer. When 
 the pressure is very light the carbon par- 
 
 Fig. 120. — Carbon Pressure Rheostat. 
 
 tides do not make good electric contact 
 with each other and interpose a compara- 
 tively great resistance to the passage of 
 the current from one to another. When, 
 
ELECTRO-THERAPEUTICS. 
 
 327 
 
 however, the pressure is considerable, the 
 particles are brought into more intimate 
 electric contact and the resistance of the 
 
 Fig. 121. — Water Rheostat. 
 
 mass is thereby greatly reduced. The 
 pressure in this instrument is varied by 
 turning the milled-headed screw M. 
 
 Fig*. 121 represents a form of water 
 
328 ELECTRICITY. 
 
 rheostat. Here the column of resisting 
 material is composed of water, Avhich, as 
 we have seen, possesses a high resistivity. 
 The binding posts bb y constituting the 
 terminals of the instrument, are connected 
 each to a triangular mass of carbon, CO, 
 armed at its extremity with the small 
 sponge S. In order to vary the resist- 
 ance, the milled head, M, is turned, which 
 by means of a worm gear rotates the car- 
 bon plates so as to move them into or out 
 of the liquid, and thus vary both the 
 length and cross-section of the liquid 
 column between them. 
 
CHAPTER XII. 
 
 HIGH FREQUENCY DISCHARGES. 
 
 All the electric sources we have de- 
 scribed produce E. M. Fs., and all 
 E. M. Fs., when permitted to do so, pro- 
 duce electric discharges or currents. 
 The type and magnitude of E. M. F. 
 determine the type and magnitude of the 
 electric current. It is, therefore, to be 
 remembered that, however different may 
 be the appearance of the machine which 
 produces an electric discharge, or however 
 different may be the appearance of the 
 discharge itself, the difference electrically 
 is simply one of frequency, magnitude and 
 wave type of E. M. F. 
 
330 ELECTRICITY IN 
 
 A high E. M. R, no matter how pro- 
 duced, may discharge in three ways. 
 
 (1) Convectively. 
 
 (2) Conductively. 
 
 (3) Disruptively. 
 
 Either of the two last mentioned 
 methods may be oscillatory or non-oscilla- 
 tory. 
 
 A convective discharge is the discharge 
 which occurs in the neighborhood of 
 points connected with a source of high 
 electric pressure. A pressure of 20,000 
 volts, or upwards, will produce convective 
 effects. Such a pressure is furnished by 
 an electrostatic, or influence machine, so 
 that if an upright metallic rod S, furnished 
 with a sharp pivot point, be attached, as 
 shown in Fig. 122, to the prime conductor 
 of a machine, a wheel, formed of a number 
 
ELECTRO-THERAPEUTICS. 
 
 331 
 
 of radially pointed spokes, supported on 
 the pivot, will be set into rapid rotation by 
 the reaction of the convective discharge of 
 electrified air particles, that are thrown oif 
 
 Fig. 122.- 
 
 -Rotation Produced by Convective 
 Discharge. 
 
 from the points. This motion of the air 
 produces a breeze, called a static or electric 
 breeze, which is sometimes employed elec- 
 tro-therapeutically. 
 
 If a damp cord be made to connect the 
 main terminals of a high-pressure ma- 
 chine, a silent or conductive discharge .will 
 
332' ELECTRICITY Iff 
 
 pass through it, the resistance offered by- 
 such a cord being a very great number of 
 ohms. It might be supposed, that if a me- 
 tallic wire were employed instead of a 
 string, that the discharge would pass more 
 readily than it would through a conduct- 
 ing string; but, curiously enough, this is 
 not the case, owing to the fact that the low 
 resistance of the wire causes an enormous 
 current strength to tend to flow through 
 it under a high pressure at its termi- 
 nals. Under the influence of this enor- 
 mous, rush of current, the inductance or 
 self-induction of even a short length of 
 straight wire, is sufficient to produce a 
 C. E. M. F. so great, that a disruptive dis- 
 charge may take place across a consider- 
 able air-gap, before any appreciable quan- 
 tity can escape through the wire. 
 
 When a knuckle of the hand is ap- 
 
ELECTRO-THERAPEUTICS. 333 
 
 proached to the rounded prime conductor 
 of a high -pressure machine, a disruptive 
 discharge or spark will pass through the 
 air-gap, between the hand and the con- 
 ductor. This appears to consist of a single 
 discharge, but generally consists, in reality, 
 of a number of separate discharges to-and- 
 fro between the machine and the hand. 
 In other words, the discharge is oscillatory, 
 and the current oscillating. The difference 
 between a quiet steady discharge, of a 
 given quantity of electricity at high pres- 
 sure, as compared with an oscillatory dis- 
 charge of the same quantity, is shown in 
 Fig. 123. At A, a steady discharge, com- 
 mencing at say 100,000 volts pressure, falls 
 steadily to zero ; that at B, starting at an 
 equal voltage, falls more rapidly to zero 
 and is slightly oscillatory ; that at C, 
 rapidly changes direction and becomes 
 oscillatory. The current strength in the 
 
334 
 
 ELECTRICITY IN 
 
 circuit has the same graphic type in each 
 case. The frequency of oscillation of these 
 discharges is often exceedingly high, reach- 
 ing sometimes hundreds of millions of 
 cycles per second. The total number of 
 
 a 
 \ A 
 
 id 
 
 \ B 
 
 Ve 
 
 \ j ° 
 
 \ \ \ C\ n p 
 
 
 / 
 
 \ / Wo e 
 1 1 \J m 
 
 \\ k 
 
 i 
 
 Fig. 123.— Oscillatory Discharge. 
 
 oscillations, however, in any discharge is 
 not very great, usually varying from 2 
 or 3 to 20 or 30, according to the condi- 
 tions of the circuit. The entire discharge, 
 therefore, is usually completed in a small 
 fraction of a second. 
 
ELECTRO-THERAPEUTICS. 
 
 335 
 
 If a steel spring, such as is represented 
 in Fig. 124, be clamped at its upper ex- 
 tremity, while its lower end is loaded with 
 
 Fig. 124.— Mechanical Vibrator, Side and End View. 
 
 a weight W, and also carries a vane V, 
 movable in a viscous liquid, then, if the 
 spring be drawn aside from its position 
 of rest, to the position S' W V , and then 
 released, it will after a number of vibra- 
 
336 ELECTRICITY IN 
 
 tions or oscillations return to rest, in a 
 manner which will depend upon the fric- 
 tional resistance offered by the liquid, upon 
 the elasticity of the spring, and upon the 
 weight with which it is loaded. 
 
 If the friction al resistance of the liquid 
 is very great, relatively to the elasticity of 
 the spring, such, for example, as might be 
 offered by impulses to tbe motion of a 
 large vane in molasses, then the spring 
 will not oscillate, but will slowly return 
 towards its position of rest. If, on the 
 other hand, all frictional resistance could 
 be withdrawn, not only in the vessel of 
 liquid, but also in the air and in the mo- 
 lecular structure of the spring, then the 
 spring would perform oscillations which 
 would continue for ever, as there would 
 then be no means for dissipating the 
 energy of the vibrating system. In a con- 
 
ELECTRO-THERAPEUTICS. 337 
 
 dition intermediate between the preced- 
 ing, that is to say, when the frictional 
 resistance offered to the motion is appre- 
 ciable, but not excessive, the spring will 
 execute, by reason of its elasticity, a cer- 
 tain number of oscillations of successively 
 diminishing amplitude before it comes to 
 rest. 
 
 The frequency of the oscillations ex- 
 ecuted by the spring depends upon its 
 elasticity, and the weight it carries. The 
 weaker the spring ; i. <?., the less its elastic 
 force, the slower the vibrations ; the greater 
 the load, the slower the vibrations. In 
 order, therefore, to produce a high fre- 
 quency, we require a very stiff spring ; 
 i. e. y a short thick spring, and a very small 
 weight. On the contrary, for very low 
 frequency vibrations, we require a long 
 and thin or weak spring, with a heavy 
 
338 ELECTRICITY IN 
 
 load. Provided the frictional resistance of 
 the liquid is not sufficiently great to check 
 all vibration, the amount of friction will 
 have only a very small influence upon the 
 frequency, and will affect only the number 
 of oscillations performed, before the system 
 comes to rest. In other words, if the spring 
 oscillates, the friction can only damp the 
 system, but if the friction exceeds a cer- 
 tain quantity, depending upon the size of 
 the spring, its elasticity and load, then 
 oscillation will be impossible. 
 
 Any electric circuit, in which a discharge 
 suddenly takes place, obeys laws which 
 are precisely parallel to those we have 
 above indicated in relation to the disturbed 
 spring. The frictional resistance of the 
 liquid corresponds to the resistance of the 
 electric circuit in ohms. The weakness of 
 the spring corresponds to the electrostatic 
 
ELECTRO-THERAPEUTICS. 339 
 
 capacity of the circuit, or its capability of 
 behaving as a condenser, and the load or 
 weight added to the spring, corresponds to 
 the inductance of the circuit ; so that instead 
 of mechanical inertia, in the electric circuit 
 we meet electromagnetic inertia. When, 
 therefore, a discharge takes place in an 
 electric circuit, this discharge will be 
 oscillatory or non-oscillatory, according to 
 the amount of resistance in the circuit 
 relative to its capacity and inductance. 
 The greater the resistance, the less the 
 probability of obtaining an oscillatory dis- 
 charge, and when the resistance is very 
 high, the discharge takes place slowly and 
 without oscillation. If, however, the re- 
 sistance is sufficiently small to permit oscil- 
 lations or alternations of current to take 
 place in the circuit, then the resistance will 
 have very little effect upon the frequency 
 of alternation, but will affect only the 
 
340 ELECTRICITY IN 
 
 damping out of the vibrations. The less 
 the resistance, the more slowly the oscilla- 
 tions will die out, and the greater the 
 number that will be performed before 
 extinction. In the same way, a circuit of 
 large electrostatic capacity behaves like a 
 weak spring of great length, and a circuit 
 of small electrostatic capacity, like a small 
 or short, stiff spring. 
 
 In order to produce very rapid oscilla- 
 tion or alternations in the discharge of a 
 circuit, it is necessary to have a small con- 
 denser, and a small inductance in the cir- 
 cuit. On the other hand, a large con- 
 denser, discharging through a circuit of 
 many loops of wire, and having, therefore, 
 a large inductance, will perform slow oscil- 
 lations or oscillations of low frequency. 
 Unfortunately, however, for the production 
 of very high frequency oscillations, a very 
 
ELECTRO-THtfKAPEl 
 
 ^pWsa^ 
 
 small condenser onJ ^contains ' a^ k striaB. 
 
 &, 
 
 quantity of electricity f^£^given pressui 
 applied, and, consequentIj7^e^a%plitude^ 
 of the current strength in the oscillations 
 t is feeble, and the total amount of energy 
 comparatively small. On the other hand, 
 a large jar, which will hold a large quan- 
 tity of electricity, and give rise to power- 
 ful oscillations, can only produce compara- 
 tively low frequency currents. The fre- 
 quency of alternating-current discharges, 
 when of an oscillating character, and from 
 an ordinary Leyden jar of pint size, is 
 roughly about 15,000,000 of periods per 
 second, when only a short length of wire 
 is employed to connect the external and 
 internal coatings. 
 
 In Pig. 125, a condenser or Leyden jar 
 J is represented as being about to dis- 
 charge through an air-gap in its circuit 
 
342 
 
 ELECTRICITY IN 
 
 Jabc. The discharge will be oscillatory 
 if the resistance for the circuit be suffi- 
 ciently small. Similarly, if as shown at B, 
 the condenser C y be charged by turning the 
 switch S, to a, the charge from the E. M. % 
 F. will be oscillatory if the resistance 
 
 «U 
 
 Fig. 125. — Circuit for the Production of Alter- 
 nating or Oscillatory Currents. 
 
 of the charging circuit be sufficiently low. 
 The condenser may receive a non-oscilla- 
 tory charge and give an oscillatory dis- 
 charge, or vice versa, by properly propor- 
 tioning the resistance of the circuits. 
 
 The E. M. F. of the discharge will, of 
 course, be greater, the greater the distance 
 through which the spark discharge passes, 
 
ELECTRO-THREEAPUTICS. 343 
 
 but whatever the E. M. F., the frequency 
 of oscillation in the circuit will be the 
 same, and only the amplitude of the 
 waves will be affected. It will be evi- 
 dent, that if a very low E. M. F. be em- 
 ployed, the waves will be very feeble and 
 a high E. M. F., such as supplied by an 
 influence machine, is necessary for power 
 ful oscillations. 
 
 The frequency of oscillation in a given 
 circuit is not readily computed with any 
 degree of accuracy, when the circuit is 
 very short, owing to the fact that even a 
 straight wire offers an amount of induc- 
 tance appreciable to rapid discharges, and, 
 although the inductance of a coil of many 
 turns of wire can be measured or calcu- 
 lated, that of a short length of bent wire 
 is difficult to estimate. The frequency of 
 oscillation of spark discharges has been 
 
344 ELECTRICITY IN 
 
 experimentally observed, in a number of 
 cases, by observing or photographing the 
 succession of sparks in a discharge with 
 the aid of a rapidly rotating mirror. 
 
 It is possible, therefore, to render the 
 charge or discharge of a circuit oscillatory 
 by suitably regulating its capacity, induc- 
 tance and resistance. Such discharges are 
 frequently used in electro- therapeutics 
 under the title of static induced currents, 
 an unfortunately misleading term. The 
 usual connections employed in such a cir- 
 cuit are shown in Fig. 126, where A and 
 B, the main terminals of an influence 
 machine, are maintained at a high pressure 
 until they discharge through an interven- 
 ing air-gap. Before discharge occurs, the 
 Leyden jars j, j, become charged by this 
 pressure, and the discharge of the jars 
 occurs as an impulsive discharge, that is to 
 
ELECTRO-THERAPEUTICS. 
 
 345 
 
 say, an oscillatory discharge. The number 
 of oscillations in the discharge will depend 
 upon the resistance in the circuit, both of 
 
 Fig. 126. — Oscillatory Current Circuit of Influence 
 Machine. 
 
 the spark gap G, and the resistance H, of 
 the patient between A and B. The resis- 
 tance of the spark gap is not definitely 
 known, but . from the observed dampen- 
 ing effect in experimental circuits, its resis- 
 
346 ELECTRICITY IN 
 
 tance appears to be only a few oliins. The 
 frequency of oscillations depends upon the 
 capacity of the jars and the inductance of 
 their circuit. Under ordinary conditions, 
 the frequency is several hundred thousand 
 periods per second. 
 
 In order that oscillations shall be set up 
 in the circuit CD, it is not essential that 
 two Leyden jars should be used, although 
 their presence assists in maintaining the 
 insulation of the prime conductors of the 
 machine. The spark discharge, which 
 occurs at the air-gap G, will always be 
 oscillatory, provided that sufficient capac- 
 ity exists connected with the machine, 
 relative to the resistance and inductance of 
 the discharging circuit. Two equal Ley- 
 den jars, in series, are shown in the figure, 
 connected as a single Leyden jar, and 
 equivalent to half the capacity of either, 
 
ELECTRO-THERAPEUTICS. 347 
 
 As already stated on page 81, the pass- 
 age of one coulomb of electricity through 
 a circuit in one second of time means a 
 rate of flow, or current strength, of one 
 ampere, on the average, during that time. 
 If this coulomb passed through the circuit 
 in one thousandth of a second, the mean 
 current strength would be 1,000 amperes, 
 and if in the millionth of a second, the 
 mean current strength would be 1,000,000 
 amperes. For this reason, although the 
 total quantity of electricity in a pair of 
 Leyden jars, such as represented in Fig. 
 122, even when charged at a pressure of 
 thousands of volts, is very small, yet, 
 owing to the great frequency, or rapidity 
 with which this charge is jmssed through 
 the circuit, the current strength during 
 that time may be very considerable. The 
 patient placed in the circuit between A 
 and B, may, therefore, be traversed by an 
 
348 ELECTRICITY IN 
 
 alternating current of much greater 
 strength than he would be able to receive 
 without pain under ordinary conditions. 
 For reasons which are yet not thor- 
 oughly understood, but which are 
 believed to be physiological rather than 
 physical, as soon as a certain frequency of 
 alternation, in an alternating current is 
 attained, the sensory effect of the current 
 almost disappears, as though it required a 
 certain interval of time to elapse between 
 successive alternating currents for a nerve 
 under the influence of this current to register 
 sensory effects in the brain. It would seem 
 probable, therefore, when such discharges 
 of high frequency pass through the body, 
 that owing to their high current strength, 
 as well as to their high frequency, power- 
 ful physiological effects may be produced. 
 
 So far as is at present known, no cur- 
 
ELECTRO-THERAPEUTICS. 349 
 
 rent can pass through such a mass of 
 materials as that of which the human 
 body is composed, without effecting electro- 
 lytic decomposition or, in other words, that 
 the only medium of conduction in such 
 a mass is chemical decomposition or elec- 
 trolysis. According to this view, rapidly 
 alternating currents produce electrolytic 
 effects, not only in the medium immediately 
 surrounding the poles or electrodes, but 
 also in the intrapolar or intervening tract. 
 
 It has been suggested that alternating 
 currents of such high frequency would be 
 unable, in traversing the human body, to 
 penetrate more than a very moderate dis- 
 tance below its surface, and that, therefore, 
 only superficial portions of the body could 
 be directly affected by the discharge. 
 Owing, however, to the feeble electric 
 conductivity of the materials in the body, 
 
350 ELECTRICITY II* 
 
 this shin effect, or tendency of the current 
 to seek the outer layers to the exclusion of 
 the inner layers, is comparatively small at 
 the frequencies which can be practically 
 produced, for, while the depth to which 
 such currents would penetrate in good con- 
 ductors, such as copper wires, is very 
 small, yet in the case of comparatively 
 high resisting materials, such as those con- 
 stituting the human body, the penetration 
 would probably extend practically through 
 the entire mass. High frequency alternat- 
 ing currents are, therefore, powerful but 
 painless currents, and are, probably, at- 
 tended by electrolytic effects in the entire 
 mass, although, as in the case of all alter- 
 nating currents, little if any accumulation 
 of electrolytic materials can take place. 
 
 It is not necessary to employ an in- 
 fluence machine for purposes of obtain- 
 
ELECTRO-THERAPEUTICS. 351 
 
 ing high-frequency alternating currents. 
 Alternating-current dynamos ; i. e., alter- 
 nators, can be employed to produce 
 directly frequencies up to 10,000 periods 
 per second, although such machines are 
 expensive and troublesome to operate, re- 
 quiring high speeds and special construc- 
 tion. Powerful induction coils, charging 
 condensers, are also capable of producing 
 high pressures through the discharging 
 circuit, in the same manner as influence 
 machines. 
 
 The general method employed for pro- 
 ducing high-frequency alternating cur- 
 rents, is illustrated in Fig. 127. Here S S, 
 is the secondary winding of a powerful 
 induction coil, provided with a spark gap 
 at G. S8, is preferably excited by a 
 low-frequency alternating current, passing 
 through its primary coil, say for example. 
 
352 
 
 ELECTRICITY IN 
 
 from an alternating electric lighting cir- 
 cuit. The function of s s' y is to produce 
 high pressures which charge the condenser 
 
 mwmmm 
 
 ^ Q o 
 
 6s 
 
 6s 
 
 Fig. 127.— Apparatus for High Frequency Alternat- 
 ing Currents. 
 
 O, with a quantity of electricity propor- 
 tional to this pressure. The spark gap G, 
 is so adjusted that the pressure is able 
 to discharge across it and in such dis- 
 charge to permit the condenser C, to 
 
ELECTRO-THERAPEUTICS. 353 
 
 empty its charge through the circuit 
 OppGC, with oscillations depending for 
 their frequency upon the capacity of C\ 
 and the inductance in the circuit compris- 
 ing the coil pp. This coil pp, consists of 
 comparatively few turns of well-insulated 
 wire, and serves as the primary winding of 
 an induction coil, whose secondary SS f has 
 also comparatively few turns, although 
 more than the primary pp 1 but the turns 
 are very carefully insulated from each 
 other. The effect of passing these very 
 rapidly alternating currents through the 
 primary pp, is to set up, by mutual induc- 
 tion, very powerful induced E. M. Fs. 
 in SS, of the same frequency, which 
 E. M Fs. may be utilized directly. These 
 very powerful induced E. M. Fs. are 
 capable under suitable conditions, of giv- 
 ing sparks several feet in length, and these 
 discharges, representing the surgings of 
 
354 
 
 ELECTRICITY IN 
 
 hundreds of thousands of volts, are, never- 
 theless, almost painless, owing to their 
 high frequency. One of the methods 
 
 (WMumm 
 
 Fig. 128. 
 
 -Apparatus for Creating Rapidly Oscil- 
 lating Magnetic Field. 
 
 which have been applied electro-therapeu- 
 ticallv, in connection with high-frequency 
 currents, is represented in Fig. 128. Here 
 the secondary winding SS, of a poweiTul 
 induction coil, charges and discharges the 
 
ELECTRO-THERAPEUTICS. 355 
 
 condenser C, through a large solenoid or 
 open coil F y of comparatively few turns, 
 upon a vertical cylindrical frame about 
 six feet high. The patient is introduced 
 into this frame. His body acting as a 
 secondary circuit, induced alternating cur- 
 rents circulate around it generally parallel 
 to those in the primary coil. 
 
 When the best results are desired from 
 high frequency discharge, it is essential 
 that the knobs between which the spark 
 discharges take place, shall be brightly 
 polished. If this precaution is not taken, 
 the discharges across the air-gap are apt to 
 assume a convective rather than a disrup- 
 tive character. 
 
CHAPTER XIII. 
 
 ELECTROLYSIS AND CATAPHORESIS. 
 
 The more modern theory of electrolysis 
 regards the conduction of electric currents 
 through all substances except metals as 
 a convective action, in which only free 
 atoms or radicals can take part ; that is to 
 say, a molecule of any substance is in- 
 capable of conducting electricity, except 
 in the case of metals. Where molecules, 
 however, are dissociated into their atomic 
 constituents ; i. e., into free atoms or radi- 
 cals, these constituents are capable of re- 
 ceiving and conveying electric charges, 
 and so become the medium of transport in 
 an electric current. As a consequence of 
 
 356 
 
jR^ffE^ics. 357 
 
 this, the atoms, after u^ving oeliv'ei'ed! * 
 their electric charges, acwrtalate at the 
 electrode to which they are oSfcspflf^ojs 
 
 A molecule invariably consists of two dis- 
 tinct parts called ions, or radicals, named re- 
 spectively the electro-positive and the electro- 
 negative ion or radical. When electrolytic 
 decomposition of the molecule occurs, it is 
 the electro-positive radical or ion which 
 appears at the negative electrode, called 
 the cathode, or the upway, and the electro- 
 negative radical or ion which appears at 
 the positive electrode, called the anode, or 
 the downway. When, for example, a cur- 
 rent is led between platinum electrodes, 
 through hydrochloric acid, it is supposed 
 that there exists in the liquid besides the 
 hydrochloric acid molecules proper, a con- 
 siderable number of atoms or radicals of 
 hydrogen, and of chlorine, in an uncom- 
 
358 ELECTRICITY IN 
 
 bined state, resulting from the decomposi- 
 tion of some of the molecules. When an 
 E. M. F. is connected to the electrodes 
 these atoms receive electric charges, the 
 hydrogen, or positively charged atoms, 
 being attracted to the negative electrode, 
 and the chlorine, or negatively charged 
 atoms, to the positive electrode, so that the 
 passage of the current is accompanied by 
 two streams of ions moving in opposite 
 directions through the liquid. 
 
 When a quantity of electricity passes 
 through a liquid, the products of electro- 
 lytic decomposition, collected at the elec- 
 trodes, are found to be in strict proportion 
 to the quantity of electricity which has 
 passed. Every coulomb of electricity, in 
 its passage through the solution, leaves at 
 the electrodes a definite number of ions 
 differing in different liquids. Thus, when 
 
ELECTRO-THERAPEUTICS. 359 
 
 hydrogen is liberated, this quantity is 
 0.01038 railligranime-per-coulomb ; so 
 that one ampere; i. e., one coulomb-per- 
 second, passing through a solution and 
 liberating hydrogen, will liberate 0.01038 
 milligramme-per-second. 
 
 A certain critical value of the E. M. F. 
 is required between the electrodes, in a 
 liquid, before electrolysis can take place. 
 In other words, a liquid offers a certain 
 C. E. M. F. having a definite minimum 
 value, and this C. E. M. F. must be over- 
 come, in addition to the C. E. M. F. due to 
 ohmic resistance, which the liquid possesses 
 by virtue of its resistivity and geometrical 
 proportions, before the current will pass 
 through the liquid. 
 
 When liquids that are capable of mix- 
 ing, are placed in a vessel, in compart- 
 
360 ELECTRICITY IN 
 
 ments separated from each other by 
 porous partitions, an unequal mixing of 
 the two takes place through the pores of 
 the partition or septum. This unequal 
 mixing through the pores of the intercept- 
 ing medium is called osmose. Under its 
 influence, the level of the liquids on oppo- 
 site sides of the septum will be changed. 
 In the case, for example, of sugar and water, 
 placed on one side of the septum, formed of 
 say a piece of hog's bladder, and pure water 
 placed at the same height on the other side, 
 the liquid current from the pure water is 
 stronger than the current from the sugar 
 and water, so that the level of the liquid 
 rises on the side of the sugar and water. 
 The two currents are sometimes distin- 
 guished as follows ; viz., the one towards 
 the higher level is called the endosmotic 
 current, and the one toward the lower 
 level, the exosmotic current. 
 
ELECTRO-THERAPEUTICS. 361 
 
 When an electric current is sent 
 through a porous septum separating either 
 two different liquids, or two portions of 
 the same liquid, some of the liquid is 
 transported bodily through the septum, 
 almost always in the direction of the elec- 
 tric current ; that is to say, from the posi- 
 tive pole or anode toward the negative 
 pole or cathode. This is called electric 
 osmose or cataphoresis. This electric os- 
 mose takes place independently of ordinary 
 osmose, and since its direction varies with 
 the current, it may be made to either aid 
 or oppose ordinary osmose. The quantity 
 of liquid transported depends both on the 
 nature of the liquid and on the nature of 
 the porous diaphragm, but in every case 
 is directly proportional to the quantity of 
 electricity which passes. The quantity 
 transported in a given time is, therefore, 
 proportional to the current strength. The 
 
362 ELECTRICITY IN 
 
 thickness and area of the porous dia- 
 phragm have no effect upon the amount of 
 liquid transported, provided the current 
 strength is constant, but it is evident, that 
 a thick septum or diaphragm with a small 
 active surface, will add a greater resist- 
 ance to the circuit than a thin diaphragm, 
 of large active surface, and, consequently, 
 will tend to restrict the current strength, 
 and, therefore, the amount of liquid trans- 
 ferred. With any given porous membrane 
 the rate of transfer, or the quantity of 
 liquid transferred per-coulomb of electricity, 
 is directly proportional to the resistivity of 
 the liquid ; the higher the specific resist- 
 ance or resistivity of the liquid, the greater 
 will be the amount of liquid transferred. 
 
 For the above reason, a very dilute so- 
 lution of a salt in water is much more 
 rapidly transferred through a porous dia- 
 
ELECTRO-THERAPEUTICS. 363 
 
 phragm by electric osmose or cataphoresis, 
 than a dense or nearly saturated solution, 
 since the dilute solution has a greater re- 
 sistivity, or smaller conducting power, but 
 the total quantity of salt transferred in a 
 given mass of dilute solution will be less 
 than that transported in the same quantity 
 of concentrated solution, so that the 
 advantage of employing a dilute and 
 rapidly transported solution frequently dis- 
 appears. 
 
 Since the human skin, from a physical 
 point of view, is a porous diaphragm, it is 
 possible to cause almost any solution to be 
 transferred through it into the subjacent 
 tissues, by placing an electrode thoroughly 
 moistened with the solution over the por- 
 tion of the skin selected, and connecting it 
 with the positive terminal of the source of 
 continuous E. M. F. employed, while the 
 
864 ELECTRICITY. 
 
 negative electrode is placed in contact with 
 some other portion of the body. Methods 
 of treatment based upon this action, 
 whereby drugs or medicaments are directly 
 introducted into the parts to be acted on, 
 are called cataphoric medication. 
 
 The combination of electrolysis with 
 cataphoric medication is sometimes called 
 metallic electrolysis. Thus, if a moistened 
 copper electrode be placed over a surface 
 of skin, or mucous membrane, and be con- 
 nected with the positive pole of a source of 
 continuous E. M. R, while the negative 
 pole is placed in connection with some 
 other part of the body, a salt of the metal 
 will be formed at the surface of the metal 
 by electrolysis, and this salt, entering into 
 solution at the surface of the skin, will be 
 carried through the skin or membrane by 
 cataphoric action. 
 
CHAPTER XIV. 
 
 DANGERS IN THE THERAPEUTIC USE OF 
 ELECTRICITY. 
 
 An uninsulated electric conductor carry- 
 ing a current, though held in the hand, is 
 not dangerous from the passage of the cur- 
 rent, unless the wire is so overheated that 
 the wire becomes dangerously hot. If, 
 however, a high E. M. F. be connected 
 with the wire, then holding such wire in the 
 hand may become dangerous, if a circuit be 
 established through the body for the pas- 
 sage of an electric current from the E. M. 
 F. That is to say, the body may receive 
 a dangerously powerful electric current. 
 A man standing on a dry, wooden floor, 
 
 365 
 
366 ELECTRICITY IN 
 
 may safely touch a wire connected with a 
 high pressure. For example, he may hold 
 in his hand a bare wire leading from a 
 dynamo supplying a number of arc lamps 
 in series, and, therefore, having a differ- 
 ence of electric potential, relatively to the 
 ground, of several thousand volts. The 
 man will, probably, be absolutely uncon- 
 scious of any effect produced by the current 
 passing through the wire. But, should 
 the man, while touching this wire, come 
 into contact with some other electric con- 
 ductor, such, for example, as an iron beam 
 connected with the ground, or a grounded 
 wire, he may receive a dangerously power- 
 ful, and even fatal, electric current through 
 his body; for, if a ground exists any- 
 where in the circuit of the wire he holds, 
 he will thus permit an electric circuit to 
 be closed through his body, having in it a 
 considerable E. M. F. 
 
ELECTRO-THERAPEUTICS. 367 
 
 In other words, merely touching at one 
 point a circuit through which a powerful 
 current is passing, is not sufficient to cause 
 a current to pass through the body. Not 
 only a point of entrance, but also a point 
 of exit and a complete circuit must be 
 provided through the body, before a cur- 
 rent can be received. It is for this reason, 
 that the rule is frequently adopted in elec- 
 tric lighting stations, where conductors 
 carrying high pressure currents are em- 
 ployed, always to keep one hand in the 
 pocket when touching a conductor. By 
 this means, if the floor is insulated, it will 
 be very difficult to establish a circuit 
 through the body. 
 
 The current strength which will be re- 
 ceived by the body under any given condi- 
 tions in which a circuit is established will, 
 of course, depend upon the E. M. F. and 
 
368 ELECTRICITY IN 
 
 x>n the resistance of the entire circuit in 
 which the body is introduced, according to 
 Ohm's law. The resistance of the human 
 body may vary enormously, as already 
 pointed out, so that it is almost impossible 
 to say what the current strength will be in 
 any particular case, but, generally speak- 
 ing, the greater the surface area of skin 
 coming into contact with the electrodes, 
 and the moister the skin, the greater will 
 be the danger of receiving a fatal shock 
 from a powerful E. M. F. 
 
 Generally speaking, a continuous E. M. 
 F. of 20 volts, applied anywhere to the 
 human body through the unbroken sur- 
 face of the skin, may be regarded as harm- 
 less, since the current strength that can be 
 made to pass through any portion of the 
 body by means of such an E. M. F. is very 
 feeble. Alternating E. M. Fs., at frequen- 
 
ELECTRO-THERAPEUTICS. 
 
 cies commercially employed, may be pain- 
 ful under certain circumstances at pres- 
 sures as low as even 5 volts; as, for 
 example, when the hands are immersed in 
 a jar of saline solution, and these jars are 
 connected with an alternating pressure of 
 5 volts effective. As the pressure is in- 
 creased above 5 volts of alternating E. 
 M. F., or 20 volts of continuous E. M. F., 
 the physiological effects become more 
 painful, and the continuance of such a cur- 
 rent may produce serious effects. Fifty 
 volts of alternating E. M. F. is capable of 
 killing a dog, in two or three seconds, when 
 suitably applied through large wet elec- 
 trodes, in such a manner as to meet with 
 a comparatively reduced resistance in the 
 body of the animal. 
 
 At ordinary commercial frequencies, it 
 would appear, from experiments con- 
 
370 ELECTRICITY IN 
 
 ducted upon dogs, horses aud cows, that 
 the danger of a given alternating-current 
 pressure is two to three times as great as 
 that of the same amount of continuous-cur- 
 rent pressure, and, moreover, under the 
 action of a powerful alternating current, 
 the animal is deprived of volitional control 
 of its muscles, which, are thrown into 
 tetanic rigidity, a much greater strength of 
 the. continuous current being necessary to 
 produce a similar effect, even in a partial 
 degree. At extremely high frequencies, 
 however, far above those at present com- 
 mercially employed, we have seen that the 
 physiological effect of alternating currents 
 is considerably less than that of continu- 
 ous currents of the same strength. 
 
 Under ordinary circumstances, a man re- 
 ceives a shock from a wire through his 
 hands and feet. A pressure of 100 volts 
 
ELECTRO-T] 
 
 Co 
 
 e than appre- 
 
 contmuous is not rau 
 ciable when the hands a 
 same may be said of 50 volts 01 alterna- 
 ting current. A pressure of 500 volts is 
 capable of giving a very severe shock, 
 especially when a man standing on the 
 wet ground, touches a conductor in connec- 
 tion with a trolley wire, at about 500 
 volts pressure. Rare instances are said to 
 have occurred in which this continuous 
 current pressure has been fatal to man. 
 Such a pressure is very readily capable of 
 killing a horse, partly owing to the fact 
 that its skin is almost entirely unpro- 
 tected. It would also appear from such' 
 experimental knowledge as we possess that 
 animals are more readily killed by electric 
 pressures than human beings. 
 
 The current strength which it is danger- 
 ous to employ depends both upon its point 
 
 c 
 
372 ELECTRICITY. 
 
 of application and upon its duration. A 
 current of 250 milli amperes is, in some cases, 
 harmless when conducted through por- 
 tions of the human body for a short inter- 
 val of time, while, in other cases, this 
 strength of current, passed through vital 
 organs, might produce fatal results. In 
 continuous-current strength, however, any- 
 excess of 25 milliamperes is usually at- 
 tended with pain under normal conditions, 
 and is, therefore, regarded as a strength of 
 current that should only be administered 
 with due precautions. Even this strength 
 of current through delicate organs, such as 
 the eye, might produce serious results. 
 
INDEX. 
 
 Action of Electrified Sphere, Mechanical Model 
 of, 149, 150. 
 
 Active Conductor, Magnetic Flux Paths of, 192, 
 193. 
 
 ■ Loop, Influence of, on Magnetic Needle, 
 
 193, 194. 
 
 Activity, Definition, 125. 
 
 , Electric, Unit of, 126. 
 
 , Mechanical, Unit of, 125, 126. 
 
 Adapter Connections for Continuous-Current Cir- 
 cuits, 316, 317. 
 
 for Continuous-Current Circuits, 314, 315. 
 
 Aero-Ferric Magnetic Circuit, 196. 
 
 Alternating, Current, 121. 
 
 Current Dynamo, 119. 
 
 Current Magneto-Electric Generator, 246, 
 
 247. 
 
 373 
 
374 INDEX. 
 
 Alternating-Current Transformer, 309, 310. 
 
 Current Transformer for Cautery, 312, 313. 
 
 E. M. F., 113, 114. 
 
 Alternation, Definition of, 117. 
 
 Alternator, Electro-Therapeutic, 307, 308. 
 
 Alternators, 199, 299. 
 
 Amalgam for Frictional Electric Machines, 141. 
 
 Ammeter, Definition of, 90. 
 
 Ampere, 81. 
 
 , Definition of, 90. 
 
 Ampere-Turn, Definition of, 207. 
 
 Animal Electricity, Conclusions in Regard to, 
 9, 10. 
 
 Anode, 357. 
 
 Apparatus for High-Frequency Alternating Cur- 
 rents, 352. 
 
 Armature of Electromagnet, 204. 
 
 B 
 
 Battery, Chloride Storage, 57. 
 
 of Silver Chloride Cells, 39. 
 
 , Voltaic, Definition of, 50. 
 
 , Voltaic Plunge, 52. 
 
 Begohm, Definition of, 66. 
 Bichromate Voltaic Cell, 41, 42. 
 Bluestone or Gravity Voltaic Cell, 29, 30. 
 Body, Human, Electric Resistance of, 76, 77, 78. 
 
INDEX. 375 
 
 Body, Human, Electrolytic Decomposition Pro- 
 duced in, 349. 
 
 , Human, Heat Produced in by Different 
 
 Current Strengths, 135, 136, 137. 
 
 Bonetti Electrostatic Machine, 180. 
 
 Breeze, Electric, 331. 
 
 , Static, 331. 
 
 C 
 
 C. E. M. F., Produced by Chemical Decom- 
 position, 133. 
 
 , Produced by Magnetic Activity, 133. 
 
 , Produced by Resistance of Circuit, 132. 
 
 Calculation of Resistance, 69, 10. 
 
 Calorie, 19. 
 
 , Lesser, 135. 
 
 Calorimeter, 133, 134. 
 
 Carbon Pressure Rheostat, 326. 
 
 Rheostat, 323, 324. 
 
 Cataphoresis, 361. 
 
 and Electrolysis, 356 to 364. 
 
 Cataphoretic Medication, 364. 
 
 Cautery, Alternating-Current Transformer for, 
 312, 313. 
 
 , Electric, Knives for, 319, 320, 321. 
 
 , Platinum Snare, 320, 321. 
 
 Cell, Charged, 54. 
 
 , Exhausted or Run Down, 54. 
 
376 INDEX. 
 
 Cell, Primary, Definition of, 54. 
 
 , Secondary, Definition of, 54. 
 
 , Storage, Chloride, 56, 57. 
 
 , Storage, Definition of, 54. 
 
 , Voltaic, Bluestone or Gravity, 29, 30. 
 
 , Voltaic Dry, 48, 49. 
 
 , Voltaic, Exciting Liquid of, 30. 
 
 , Voltaic, Leclanche, 27, 28. 
 
 , Voltaic, Partz Gravity, 46, 47. 
 
 , Voltaic, Silver Chloride Form of, 36, 37, 38. 
 
 Cells, Voltaic, Double-Fluid, 30. 
 
 , Voltaic, Single-Fluid, 30. 
 
 — ' , Voltaic, Various Couplings of, 88, 89. 
 
 Charged Cell, 54. 
 
 Charging Current, 55. 
 
 Chemical Decomposition or Electrolysis Produced 
 
 in Human Body, 349. 
 Chloride Storage Battery, 57. 
 
 Storage Cell, 56, 57. 
 
 Circuit, Aero-Ferric, 196. 
 
 , Closed, Definition of, 34. 
 
 , Electric, 333. 
 
 , Electrostatic, 156. 
 
 , Ferric Magnetic, 196. 
 
 , Magnetic, 191. 
 
 , Magnetic, Character and Dimensions, 
 
 Effect of Reluctance of, 209, 210, 
 
INDEX. 377 
 
 Circuit, Magnetic Methods of Varying M. M. F. 
 
 of, 212. 
 
 , Non-Ferric Magnetic, 196. 
 
 of Alternating-Current Transformer, 199. 
 
 Classification of Electric Sources, 26. 
 Closed Circuit, Definition of, 34. 
 Coil, Faradic, 248. 
 
 , Inducing, 233. 
 
 , Induction, Simple Form of, 249. 
 
 , Primary, 234. 
 
 , Secondary, 234. 
 
 Coils, Faradic, Adjustable Vibrator for, 274. 
 Comb of Points of Frictional Electric Machine, 
 
 141. 
 Commutator, Two-Part, Diagram of, 244, 245. 
 Condenser, Definition of, 175, 176. 
 Connections for Adapter, 316, 317. 
 
 of Medical Induction Coil, 290, 291. 
 
 Contact Theory, Volta's, 6, 7. 
 Continuous Current, 121. 
 
 Circuits, Adapter for, 314, 315. 
 
 — Dynamo, 107. 
 
 Generators, 303. 
 
 Continuous E. M. F., 107. 
 Convective Discharge, 330, 331, 332. 
 
 Discharge, Rotation Produced by, 321. 
 
 Convention as to Direction of Magnetic Flux, 191. 
 
378 INDEX. 
 
 Core of Medical Induction Coil, 267. 
 Coulomb, 80. 
 
 , Micro, 147. 
 
 per second, 81. 
 
 Counter E. M. F., 130. 
 Couple, Voltaic, Definition of, 30. 
 Couplings, Various, of Voltaic Cells, 88, 89. 
 Current, Alternating, 121. 
 
 , Charging, 55. 
 
 , Continuous, 121. 
 
 , Direct, 122. 
 
 , Electric, 80 to 106. 
 
 ' — , Electric, Definition of, 80. 
 
 , Electrostatic, 156. 
 
 , Endosmotic, 360. 
 
 , Exosraotic, 360. 
 
 , Pulsating, 121. 
 
 Strength, Effective, 288. 
 
 Strength Employed in Electrocutions, 82. 
 
 Currents, Static-Induced, 344. 
 Cycle, Definition of, 117. 
 
 D 
 
 D'Arsonval Galvanometer, 105, 106. 
 Dangers in the Use of Electricity, 365 to 372. 
 
INDEX. 379 
 
 Decomposition, Chemical, or Electrolysis Pro- 
 duced in Human Body, 349. 
 
 , Electrolytic, 83, 34-9. 
 
 Dielectric Medium, 15V. 
 
 Resistance, 167, 168. 
 
 Direct Current, 122. 
 
 Direction of Induced E. M. F., Rule for, 225. 
 
 Discharge, Conductive, 331, 332. 
 
 , Convective, 330, 331. 
 
 , Convective, Rotation Produced by, 321. 
 
 , Disruptive, 333. 
 
 , Impulsive, 344, 345. 
 
 — , Oscillatory, 333. 
 
 , Silent, 331, 332. 
 
 Discharges, High -Frequency, 329 to 355. 
 
 of Medical Induction Coils, Characteristics 
 
 of, 298. 
 
 Displacement, Electric, 148. » 
 
 Lines, 157. 
 
 Disruptive Discharge, 333. 
 
 Dissociation, Molecular, 356. 
 
 Dissymmetrical Alternating E. M. F., 119. 
 
 E. M. F., 118. 
 
 Double-Fluid Voltaic Cells, 30. 
 
 Dr. Ohm, 64. 
 
 Dry Voltaic Cell, 48, 49. 
 
 Voltaic Cell, E. M. F. of, 49. 
 
380 INDEX. 
 
 Dubois-Raymond Type of Medical Induction Coil, 
 
 261, 262. 
 Dynamo, Continuous-Current, 107. 
 Dynamo-Electric Generator, 299. 
 Dynamos, 299. 
 
 , Alternating-Current, 119. 
 
 and Alternators, Fundamental Principle 
 
 Involved in Production of E. M. F. by, 
 
 300, 301. 
 
 , Motors and Transformers, 299 to 328. 
 
 , Self -Exciting, 302. 
 
 , Separately-Excited, 302. 
 
 E 
 
 E. M. F., 25. 
 
 , Alternating, 113, 114. 
 
 and not Electricity Produced by Electric 
 
 Sources, 24', 25. 
 
 , Continuous, 107, 121. 
 
 , Continuous-Current Dynamo, 110. 
 
 , Continuous, Graphic Representation of, 
 
 107. 
 
 , Counter, 130. 
 
 , Dissymmetrical, 118. 
 
 , Dissymmetrical Alternating, 119. 
 
 , Effective, 288. 
 
 , Effective Thermal, 288. 
 
INDEX. 381 
 
 E. M. F. in Dynamos and Alternators, Funda- 
 mental Principle Involved in Production 
 of, 300, 301. 
 
 , Induction of, by Magnetic Flux, 221. 
 
 , Intermittent, 112. 
 
 , Methods of Discharge of, 329, 330. 
 
 , Negative, Graphic Representation of, 109. 
 
 of Continuous-Current Dynamo, Graphic 
 
 Representation of, 110. 
 
 of Dry Cell, 49. 
 
 of Edison-Lalande Cell, 44. 
 
 of Induction Coil, Methods of Varying 
 
 Value of, 250, 251. 
 
 of Partz Gravity Cell, 47. 
 
 of Self-induction, Direction of, on Break- 
 ing Circuit, 229. 
 
 of Self-induction, Direction of, on Com- 
 pleting Circuit, 229. 
 
 of Silver-Chloride Cell, 38. 
 
 of Zinc-Carbon Cell, 41. 
 
 , Positive, Graphic Representation of, 107. 
 
 Produced by Friction, 138, 139. 
 
 Produced by Friction, High Value of, 139, 
 
 140. 
 
 , Pulsatory, 110. 
 
 , Sinusoidal, 121. 
 
 , Symmetrical, 118. 
 
382 INDEX. 
 
 E. M. F., Symmetrical, Wave of, 119. 
 
 , Unit of, 28. 
 
 E. M. Fs., Franklinic, 144. 
 Edison-Lalande Cell, E. M. F. of, 44. 
 
 Voltaic Cell, 43, 44, 45. 
 
 Effect, Skin, 350. 
 
 Effective Current Strength, 288. 
 
 Thermal E. M. F., 288. 
 
 Electric Activity, Source of, 128, 12§. 
 
 Breeze, 331. 
 
 Calorimeter, 133, 134. 
 
 Circuit, 333. 
 
 Current, 80 to 106. 
 
 Current, Definition of, 80. 
 
 Displacement, 148. 
 
 Osmose, 361. 
 
 Resistance, 63 to 79. 
 
 Resistance of Flesh, 75. 
 
 Resistance of Human Body, 76, 77, 78. 
 
 Sources, Classification of, 26. 
 
 Unit of Work, 125. 
 
 Electricity and Magnetism, Relation Between, 
 
 184, 185. 
 and Magnetism, Transmission of, Through 
 
 Vacua, 20, 21, 22. 
 
 , Animal, Conclusions in Regard to, 9, 10. 
 
 , Decomposition by, 83. 
 
INDEX. 383 
 
 Electricity, Nature of, 13, 14. 
 
 , Unit of Quantity of, 80. 
 
 Electrocutions, Current Strengths Employed in, 
 
 82. 
 Electrode, Negative, 35 7. 
 
 , Positive, 357. 
 
 Electrodes, 357. 
 
 Electrolysis and Cataphoresis, 356 to 364. 
 
 , Definition of, 83. 
 
 , Metallic, 364. 
 
 Electrolyte, Definition of, 30. 
 Electrolytic Decomposition, 83, 349. 
 Electromagnet, 200, 201. 
 
 , Aero-Ferric Circuit of, 200, 201. 
 
 , Horse-Shoe, 202, 203. 
 
 , Yoke of, 204. 
 
 Electromagnetic Induction, 237, 238. 
 
 Inrush, 338, 339. 
 
 Motors, 304, 307. 
 
 Electromotive Force, 13 to 62. 
 
 Force, Abbreviation of, 25. 
 
 Force, Nature of, 24, 25. 
 
 Force, Varieties of, 107 to 123. 
 
 Electro-Negative Ions, 357. 
 Electropliorus, Description of, 166. 
 
 , Operation of, 167 to 171. 
 
 Electropoion Fluid, 41. 
 
384 INDEX. 
 
 Electro-Positive Ions, 35 7. 
 
 Electrostatic Attraction and Repulsion, General 
 
 Laws of, 164, 165, 166. 
 
 Circuit, 156. 
 
 Circuit, Application of Ohm's Law to, 156. 
 
 Circuits of Toepler-Holtz Machine, 173. 
 
 Current, 156. 
 
 Flux, 148. 
 
 Flux, Line or Curves of, 148. 
 
 Flux Paths, Representation of, 160. 
 
 Induction, 144, 145, 146, 159. 
 
 Law, General, of Attraction and Repulsion, 
 
 164, 165, 166. 
 
 Resistance, 156. 
 
 Electro-Therapeutic Alternator, 307, 308. 
 Electro-Therapeutics, Galvani's Contribution to, 9. 
 Elements, Voltaic, 31. 
 
 , Voltaic, Varieties of, 33. 
 
 Endosmotic Current, 360. 
 Ether, Luminiferous, 19, 20. 
 
 , Transmission of Heat by, 16, 17, 18. 
 
 , Universal, 14. 
 
 Exciting Liquid of Voltaic Cell, 30. 
 
 Exhausted or Run Down Cell, 54. 
 
 Exosmotic Current, 360. 
 
 External Damping Tube for Induction Coil, 281, 
 
 282. 
 
INDEX. 385 
 
 F 
 
 Faradic Coil, 248. 
 
 Coil, Adjustable Vibrator for, 274, 2*75. 
 
 Ferric Magnetic Circuit, 196. 
 
 Flesh, Electric Resistance of, 75. 
 
 Flow, Electric, Unit of Rate of, 81. 
 
 Fluid, Electropoion, 41. 
 
 Flux Density, 213. 
 
 , Electrostatic, 148. 
 
 , Magnetic, 190. 
 
 , Magnetic, Apparent Failure to Produce 
 
 Physiological Effects on Human Body, 
 214, to 218. 
 
 , Magnetic, Induction of E. M. F. by, 221 
 
 to 247. 
 
 , Passage of through Human Body, 218, 
 
 219, 220. 
 
 Paths, Effect of Shape of Body on Direc- 
 tions of, 151 to 155. 
 
 Paths, Electrostatic Representation of, 160. 
 
 -, Remanent, 202. 
 -, Residual, 201. 
 
 Foot-pound, Definition of, 124. 
 
 per second, Definition of, 125. 
 
 Force, Electromotive, 13 to 62. 
 
 , Electromotive, Abbreviation of, 25. 
 
 , Electromotive, Nature of, 24, 25. 
 
386 INDEX. 
 
 Force, Electromotive, Varieties of, 107 to 123. 
 
 , Magneto-motive, 206. 
 
 , Magneto-motive, Unit of, 207. 
 
 Franklin, 144. 
 Franklinic E. M. Fs., 144. 
 Frequency, Definition of, 118. 
 
 of Oscillation, 337, 338. 
 
 Friction, Development of E. M. F. by, 138, 139. 
 Frictional and Influence Machines, 138 to 184. 
 
 Electric Machine, Comb of Points of, 141. 
 
 Electric Machine, Plate Form of, 141, 142. 
 
 Electric Machines, 140, 141. 
 
 - Electric Machines, Amalgam for, 141. 
 
 Electric Machines, Rubber of, 141. 
 
 Frog, Galvanoscopic, 2. 
 
 G 
 
 Galvani, Discovery of, 1 to 5. 
 Galvanometer, D'Arsonval, 105, 106. 
 
 , Mirror, 99, 100. 
 
 , Mirror, Sensitive, 103, 104, 105. 
 
 Galvanoscopic Frog, 2. 
 
 Gauss, Definition of, 213. 
 
 Generator, Alternating Magneto-Electric, 246, 247. 
 
 , Dynamo-Electric, 299. 
 
 , Magneto-Electric, 239. 
 
 Generators, 299. 
 
INDEX. 387 
 
 Generators, Continuous- Current, 303. 
 
 Gilbert, Definition of, 207. 
 
 Graphic Representation of Continuous E. M. F., 
 
 107. 
 Representation of Oscillatory Discharge, 
 
 334. 
 Gravity or Bluestone Voltaic Cell, 29, 30. 
 Grenet's Voltaic Cell, 41, 42. 
 Grid of Storage Cell, 56. 
 
 H 
 
 Heat, Transmission of, by Ether, 16, 17, 18. 
 
 High-Frequency Alternating-Currents, Apparatus 
 for, 352. 
 
 Discharges, 329 to 355. 
 
 Discharges, Physiological Effects of, 347, 
 
 348. 
 
 Electric Oscillations, Conditions Requisite 
 
 for, 340, 341. 
 
 Holtz Influence Machine, Form of, 179. 
 
 Horse-Shoe Electromagnet, 202, 203. 
 
 Human Body, Electric Resistance of, 76, 77, 78. 
 
 Body, Electrolytic Decomposition Pro- 
 duced in, 349. 
 
 Body, Heat Produced in, by Different 
 
 Current Strengths, 135, 136, 137. 
 
 Body, Passage of Flux through, 218, 219, 
 
 220. 
 
388 INDEX. 
 
 I 
 
 Impulsive Discharge, 344, 345. 
 Impurities, Effect of, on Resistivity, 71, 72. 
 Incandescent Lamps for Exploratory Purposes, 
 
 317. 
 Induced E. M. F., Direction of, 225. 
 Inducing Coil, 233. 
 Inductance, 332. 
 
 of Secondary of Induction Coil, 264. 
 
 Induction Coil, External Damping Tube for, 281, 
 
 282. 
 
 — Coil, Medical, 248 to 298. 
 
 Coil, Rapid Interrupter for, 278, 279. 
 
 Coil, Ribbon Vibrator for, 276, 277. 
 
 Coil, Simple Form of, 249. 
 
 Coil, Internal Damping Tube For, 281, 
 
 282. 
 Coils, Medical, Relative Effectiveness of, 
 
 285, 286. 
 
 , Electromagnetic, 237, 238. 
 
 , Electrostatic, 144, 145, 146, 159. 
 
 , Magneto-Electric, 238, 239. 
 
 , Mutual, 232 to 235. 
 
 of E. M. F. by Magnetic Flux, 221 to 247. 
 
 of E. M. F. by Magnetic Flux, Varieties 
 
 of, 221. 
 
ind: 
 
 
 7* 
 
 Induction of E. M. F., Me 
 
 226,227, 228. 
 
 Influence Machine, 144 
 
 Machine, a form of Electrophorus, 169, 170. 
 
 Machine, Oscillatory-Current Circuit of, 
 
 345, 346. 
 
 Inrush, Electromagnetic, 338, 339. 
 
 Insulators, 68. 
 
 Intensity, Magnetic, Unit of, 213. 
 
 Interconnection of Primary and Secondary Wind- 
 ings of Medical Induction Coil, 293, 294. 
 
 Intermittent E. M. F., 112. 
 
 Internal Damping Tube for Induction Coil, 280. 
 
 Ions or Radicals, 357. 
 
 j 
 
 Jar, Leyden, 176, 177. 
 
 Joint Resistance, 73. 
 
 Joule, Definition of, 125, 126. 
 
 per second, Definition of, 126. 
 
 Julien Storage Battery, 59. 
 
 K 
 
 Kathode, 357. 
 
 Knives for Electric Cautery, 319, 320, 321. 
 
390 INDEX. 
 
 L 
 
 Lamps, Incandescent, for Exploratory Purposes, 
 
 317. 
 Law, General, of Electrostatic Attraction and 
 
 Repulsion, 164, 165. 
 
 , Ohm's, 84 to 90. 
 
 Leclanche Cell, E. M. F. of, 28. 
 
 Voltaic Cell, 27, 28. 
 
 Lesser Calorie, 135. 
 
 Ley den Jar, 176, 177. 
 
 Jar Discharge, Oscillatory Character of, 
 
 341. 
 Light, Nature of, 23, 24. 
 , Transmission of, by Luminiferous Ether. 
 
 19, 20. 
 Lines, Displacement, 157. 
 
 or Curves of Electrostatic Flux, 148. 
 
 Luminiferous Ether, 19, 20. 
 
 M 
 
 M. M. F., 206. 
 
 of Circuit, Methods of Varying Value of, 
 
 212. 
 Machine, Frictional Electric, 140, 141. 
 Machines, Frictional and Influence, 138, 183. 
 Magnet, North Pole of, 191. 
 , South Pole of, 192. 
 
INDEX. 391 
 
 Magnetic Circuit, 191, 192. 
 
 Circuit, Ohm's Law applied to, 207. 
 
 Circuit, Varieties of, 196. 
 
 Field, Rapidly Oscillating, Apparatus for 
 
 Producing, 354, 355. 
 
 Flux, 190. 
 
 Flux, Apparent Failure to Produce 
 
 Physiological Effects on Human Body, 
 
 214 to 218. 
 
 Flux, Convention as to Direction of, 191. 
 
 Flux, Induction of E. M. F. by, 221 to 
 
 247. 
 
 — Flux Paths of Active Conductor, 192, 193. 
 
 Flux, Unit of, 211. 
 
 Intensity, Unit of, 213. 
 
 Needle, Influence of Active Loop on, 193, 
 
 194. 
 
 Reluctance, 206. 
 
 Resistance, 206. 
 
 Magnetism, 184 to 220. 
 
 and Electricity, Relation Between, 184, 
 
 185. 
 
 and Electricity, Transmission of, through 
 
 Vacua, 20, 21, 22. 
 
 , Definition of, 184. 
 
 Method of Producing, 188, 189. 
 
 , Permanent, 201. 
 
392 INDEX. 
 
 Magnetism, Residual, 202. 
 
 Magneto-Electric Generator, 239. 
 
 Generator changes in Magnetic Circuit of, 
 
 241, 242. 
 
 Induction, 238, 239. 
 
 Magneto-Motive Force, 206. 
 
 Force, Unit of, 207. 
 
 Mechanical Analogue of Induction of E. M. F., 
 226, 227, 228. 
 
 Analogue of Relation Between Electricity 
 
 and Magnetism, 185 to 188. 
 
 Model of Action of Electrified Sphere, 
 
 149, 150. 
 
 Vibrator, 335, 336. 
 
 Medical Induction Coil, 248, 298. 
 
 Induction Coil, Characteristics of Dis- 
 charge Produced by, 298. 
 
 Induction Coil, Connection of Vibrator in, 
 
 269 to 275. 
 
 Induction Coil, Connections of, 290, 291. 
 
 Induction Coil, Core of, 267. 
 
 Induction Coil, Diagram of Primary In- 
 duced E. M. Fs., 259. 
 
 Induction Coil Discharges, Characteristics 
 
 of, 298. 
 
 Induction Coil, Dubois-Raymond Type, 
 
 261, 262. 
 
INDEX. 393 
 
 Medical Induction Coil, Effect of Increasing Fre- 
 quency of Vibration, 
 
 Induction Coil, Interconnection of Primary 
 
 and Secondary Windings of, 293, 294. 
 
 Induction Coil, Methods of Increasing 
 
 Frequency of Flux Oscillations Pro- 
 duced by, 252. 
 
 Induction Coil, Methods of Increasing 
 
 Magnetic Flux of, 252. 
 
 Induction Coil, Operation of, 254, 258. 
 
 Induction Coil, Primary Connections of, 
 
 253, 254. 
 
 — Induction Coils, Relative Effectiveness of, 
 
 285, 286. 
 
 Medication, Cataphoretic, 361. 
 
 Medium, Dielectric, 157. 
 
 Megohm, Definition of, 66. 
 
 Metallic Electrolysis, 364. 
 
 Milliameter, Construction of, 94 to 98. 
 
 , Definition of, 90. 
 
 , Varieties of, 91 to 94. 
 
 Milliampere, Definition of, 82. 
 
 Mirror, Galvanometer, 99 to 102. 
 
 , Galvanometer, Sensitive, 103, 104, 105. 
 
 Molecular Dissociation, 356. 
 
 Motors, Dynamos and Transformers, 299 to 
 328. 
 
394 i:n t dex„ 
 
 Motors, Electromagnetic, 304 to 307. 
 Mutual Induction, 232 to 235. 
 
 N 
 
 Nature of Electricity, 13, 14. 
 
 Negative E. M. F., Graphic Representation of, 109. 
 
 Electrode, 357. 
 
 Plate of Voltaic Cell, 31. 
 
 Pole of Voltaic Cell, 34. 
 
 Non -Ferric Magnetic Circuit, 196. 
 Non-Polarizable Voltaic Cells, 32. 
 North Pole of Magnet, 191. 
 
 o 
 
 i 
 Oersted, Definition of, 211. 
 
 Ohm, Definition of, 65, 66. 
 
 Ohm, Dr., 64. 
 
 Ohm's Law, 84 to 90. 
 
 Law, Application of to Electrostatic Cir- 
 cuit, 156. 
 
 Law, Application of to Magnetic Circuit, 
 
 207. 
 
 Oscillations, Frequency of, 337, 338. 
 
 , High-Frequency, Electric Conditions 
 
 Requisite for, 340, 341. 
 
 Oscillatory Character of Leyden Jar Discharge, 341. 
 
INDEX. 395 
 
 Oscillatory Current Circuit of Influence Machine, 
 345, 346. 
 
 Discharge, 333. 
 
 Discharge, Graphic Representation of, 334. 
 
 Osmose, 360. 
 , Electric, 361. 
 
 Pair, Voltaic, Definition of, 30. 
 Parallel-Connected Resistances, 73. 
 Partz Gravity Voltaic Cell, 46, 47. 
 Period, Definition of, 117. 
 Permanent Magnetism, 201. 
 
 Physiological Effects of High-Frequency Dis- 
 charges, 347, 348. 
 Plate Form of Frictional Electric Machine, 141. 
 Platinum Snare Cautery, 320, 321. 
 Plunge Battery, Voltaic, 52. 
 Polarization of Voltaic Cell, 32. 
 Pole, Negative, of Voltaic Cell, 34. 
 
 , Positive, of Voltaic Cell, 34. 
 
 Portable Silver-Chloride Battery, 53. 
 
 Positive E. M. F., Graphic Representation of, 109. 
 
 Electrode, 357. 
 
 Plate of Voltaic Cell, 31. 
 
 Primary Cell, Definition of, 54. 
 
396 INDEX. 
 
 Pulsating Current, 121. 
 Pulsatory E. M. F., 110. 
 
 R 
 
 Radicals, Electro-Negative, 357. 
 
 , Electro-Positive, 357. 
 
 or Ions, 357. 
 
 Rapid Interrupter for Induction Coil, 278, 279. 
 Rapidly Oscillating Magnetic Field, Apparatus 
 
 for Producing, 354, 355. 
 Ratio of Transformation, 311. 
 Reluctance, Effect of Character and Dimensions of 
 • Circuit on, 209, 210. 
 
 , Magnetic, 206. 
 
 of Human Body, 218, 219, 220. 
 
 , Unit of, 211. 
 
 Reluctivity, 208. 
 Remanent Flux, 202. 
 Residual Magnetism, 201, 202. 
 Resistance, Calculation of, 69, 70. 
 
 , Dielectric, 167, 168. 
 
 , Electric, 63 to 79. 
 
 , Electric, Definition of, 63, 64. 
 
 , Electric, of Flesh, 75. 
 
 , Electric, of Human Body, 76, 77, 78. 
 
 , Electrostatic, 156. 
 
 , Joint, 73. 
 
INDEX. 397 
 
 Resistance, Magnetic, 206. 
 
 , Specific, 67. 
 
 , Unit of, Electric, 64. 
 
 Resistances, Parallel-Connected, 73. 
 
 , Series-Connected, 72. 
 
 Resistivities, Effect of Temperature on, 71. 
 
 , Table of, 68. 
 
 Resistivity, Definition of, 67. 
 
 , Effect of Impurity on, 71, 72. 
 
 of Water, 71. 
 
 Rheostat, Carbon, 323, 324, 325. 
 
 , Carbon Pressure, 326. 
 
 , Water, 327, 328. 
 
 Rheostats, 321 to 328. 
 
 Ribbon Vibrator for Induction Coil, 276, 277. 
 Rubber of Frictional Electric Machines, 141. 
 Rule for Direction of Induced E. M. F., 225. 
 
 S 
 Scale, Mirror, Galvanometer, 102, 103.' 
 Secondary Coil, 234. 
 
 Induced E. M. F. of Medical Induction 
 
 Coil at High Frequency under Load, 273. 
 
 of Induction Coil, Inductance of, 264. 
 
 or Storage Cell, Forms of, 55 to 62. 
 
 Self-exciting Dynamos, 302. 
 Self-induction, 222, 229, 230, 332. 
 
398 INDEX. 
 
 Self-induction, Counter Electromotive Force of, 
 
 229, 230. 
 Sensitive Mirror Galvanometer, 102, 103, 104. 
 Separately-Excited Dynamos, 302. 
 Series-Connected Resistances, 72. 
 Series Connection of Voltaic Cells, 50. 
 Short Circuit, Definition of, 87. 
 Silent Discharge, 331, 332. 
 Silver-Chloride Cell, E. M. F. of, 38. 
 
 Cells, Battery of, 39. 
 
 Portable Battery, 53. 
 
 ■ Voltaic Cell, 36, 37, 38. 
 
 Single-Fluid Voltaic Cells, 30. 
 Sinusoidal E. M. F., 121. 
 
 Wave, 120, 121. 
 
 Sources, Electric, Classification of, 26. 
 
 Skin Effect, 350. 
 
 Snare, Platinum, Cautery, 320, 321. 
 
 South Pole of Magnet, 191. 
 
 Sparking Distance Through Air-Gap, 140. 
 
 Specific Resistance, 67. 
 
 Static Breeze, 331. 
 
 — Induced Currents, 344. 
 
 Step-Down Transformer, 309. 
 Storage Cell, Chloride. 56, 57. 
 
 Cell, Definition of, 54. 
 
 Cell, Grid of, 56. 
 
INDEX. 399 
 
 Storage Cell, Julien, 59. 
 
 — - — or Secondary Cells, Forms of, 55 to 62. 
 
 Symmetrical E. M. F., 118. 
 
 Wave of E. M. F., 119. 
 
 T 
 
 Table of Resistivities, 68. 
 
 Temperature, Effect of on Resistivities, 71. 
 
 Therapeutic Uses of Electricity, Dangers in, 365 
 
 to 372. 
 Toepler-Holtz Influence Machine, Construction of, 
 
 172. 
 
 — Machine, Operation of, 173, 174, 175. 
 
 Transformation, Ratio of, 311. 
 Transformer, Alternating-Current, 309, 310. 
 
 , Step-Down, 309. 
 
 Transformers, Motors and Dynamos, 299 to 328. 
 
 Tregohm, Definition of, 66. 
 
 Two-Part Commutator, Diagram of, 244, 245. 
 
 u 
 
 Unit of E. M. F., 28. 
 
 of Electric Activity, 126. 
 
 of Electric Resistance, 64. 
 
 of Electric Work, 126. 
 
 of Magnetic Flux, 211. 
 
400 INDEX. 
 
 Unit of Magnetic Intensity, 213. 
 
 of Magneto-Motive Force, 207. 
 
 of Mechanical Activity, 125. 
 
 of Quantity of Electricity, 80. 
 
 of Rate of Electric Flow, 81. 
 
 of Reluctance, 211. 
 
 of Work, 124. 
 
 Universal Ether, 14. 
 
 v 
 
 Varieties of Electromotive Force, 107 to 123. 
 
 of Magnetic Circuit, 196. 
 
 Vibrator, Adjustable for Farad ic Coil, 274. 
 
 — ■ , Mechanical, 335, 336. 
 
 Volt, Definition of, 28. 
 
 Volta, 6. 
 
 Volta's Contact Theory, 6, 7. 
 
 Voltaic Battery, Definition of, 50. 
 
 Voltaic Cell, Bi-Chromate, 41, 42. 
 
 Cell, Edison-Lalande, 43, 44, 45. 
 
 Cell, Elements of, 31. 
 
 Cell, Exciting Liquid of, 30. 
 
 Cell, Grenet, 41, 42. 
 
 Cell, Leclanche, 27, 28. 
 
 Cell, Negative Plate of, 31. 
 
 Cell, Partz Gravity, 46, 47. 
 
 Cell, Polarization of, 32. 
 
 Cell, Positive Plate of, 31. 
 
INDEX. 401 
 
 Voltaic Cell, Silver Chloride Form of, 36, 37, 38. 
 
 Cells, Connection of, in Series, 50. 
 
 Cells, Double-Fluid, 30. 
 
 Cells, Non-Polarizable, 32. 
 
 ; — Cells, Single-Fluid, 30. 
 
 Cells, Zinc-Carbon, 40, 41. 
 
 Couple, Definition of, 30. 
 
 Dry Cell, 48, 49. 
 
 Elements, 31. 
 
 Elements, Varieties of, 33. 
 
 Pair, Definition of, 30. 
 
 Volt-Coulomb, Definition of, 126. 
 Voltmeter, Definition of, 129. 
 , Description of, 131. 
 
 w 
 
 Water-Gramme-Degree-Centigrade, 135. 
 
 , Resistivity of, 71. 
 
 Rheostat, 327, 328. 
 
 Watt, Definition of, 126. 
 
 Waves, Sinusoidal, 120, 121. 
 
 Weber, Definition of, 211. 
 
 Wimshurst Electrical Machine, 182, 183. 
 
 Work and Activity, Electric, 124 to 137. 
 
 , Electric, Unit of, 125. 
 
 Rate of Doing, 125. 
 
 , Unit of Electrical, 126. 
 
402 INDEX. 
 
 Y 
 
 Yoke of Electromagnet, 203. 
 
 z 
 
 Zinc-Carbon Cell, E. M. F. of, 41. 
 
 Voltaic Cells, 40, 41. 
 
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