February 8, 1894] 



NATURE 



J/ 



DYNAMOS AND TRANSFORMERS. 

 Dynamos, Alternators, atid Transformers. By Gisbert 

 Kapp, M.Inst.C.E., M.Inst.E.E. (London: Biggs 

 and Co.) 



THE author of this work is well known as a successful 

 designer of dynamos and transformers. In his 

 preface he states his object to be " to place before the 

 reader an exposition of the general principles underlying 

 the construction of dynamo-electric apparatus, and to do 

 this without the use of high mathematics and complicated 

 methods of investigation." He further says, on p. 26 : 

 " In attempting to establish a working theory of dynamo- 

 electric machinery, or rather in setting forth the rules and 

 formute now used by the designers of such machines, we 

 shall therefore not follow the lead of the pioneers in 

 science so much as that of their more popular expounders, 

 and that of practical experience. The treatment will 

 thus necessarily lack that mathematical elegance of which 

 the scholastic mind is so fond, but, on the other hand, it 

 will be more easily grasped and adopted by the practical 

 engineer who works as much by the aid of his mechanical 

 instinct as by that of science." 



This is the promise, and we may say at once that in 

 our opinion there is plenty of room for mathematical 

 elegance on the lines laid down. If the results of difficult 

 investigations be assumed, and correct deductions be 

 made and set forth in exact language and in appropriate 

 notation, as required for the particular practical applica- 

 tion, the exposition will be both elegant and scientific. 

 Practical experience also is On the same footing as any 

 experimental result in physics, and deductions made 

 therefrom may be scientific in the highest degree. From 

 the extracts above quoted, one would not expect to find 

 one-third of the book occupied with a theoretical ex- 

 position of electro-magnetism on lines similar to those 

 that may be found in a dozen or more existing works. 

 But such is the case, and we regret to say that there is 

 much in the exposition of the author that is open to 

 severe criticism on the score of the inaccurate and 

 frequently incorrect use of scientific and practical ex- 

 pressions, whose meanings are thoroughly well-estab- 

 lished and generally understood. The word " energy " 

 is employed in a sense with which, we imagine, theoretical 

 and practical men will be alike unfamiliar. " Work " 

 and " rate of doing work " are throughout the book em- 

 ployed as interchangeable expressions, the word 

 "energy" having double duty thrust upon it. It is 

 scarcely necessary to observe that " energy '' is expressed 

 in units of work, and that it is improper to use it in the 

 sense of "rate of doing work " or " power." This is all 

 the more extraordinary for the reason that the author 

 defines " power " or " activity," and in one part of the 

 book freely uses these terms to denote what he calls 

 " energy " in another part. 



More serious is the circumstance that the author 

 thinks it proper to express "rate of doing work" in 

 units of " work." On p. 42 we find the statement : " This 

 is called the Watt, and is equivalent to 10,000,000 ergs." 

 It is as if the distance between London and Brighton 

 were described as being fifty miles per hour. 



In the same context the C.G.S. unit of power is termed 

 the erg-second. We have heard of the Watt-second, the 

 NO. 1267, VOL. 49] 



volt-ampere, the ampere-second, &c., but never before of 

 the erg-second, which has no meaning whatever as a 

 hyphened expression built up in the manner customary 

 with electricians. 



On p. 141 electromotive force is under discussion by 

 means of the well-known rail and slider ; and on p. 142 

 occurs the following passage, which may be said to fairly 

 beat the record: — 



" It was shown in chapter iv. that the mechanical force, 

 P, acting upon a conductor in a field of 13 lines per square 

 centimetre is (C.G.S. measure) 



P = /^33 



where / is the length of conductor and c the current. If 

 we move the slider with a velocity of v centimetres per 

 second the energy required will be 



Vv^lc J3 V ergs. 



The energy represented by the current is ec. if by e we 

 denote the electromotive force expressed in a suitable 

 measure. We have therefore the equation 



ec=lc'>^v (28) 

 or, 



e = l^ V 



from which we find that the induced electromotive force 

 is given by the product of length of conductor, strength 

 of field and velocity, all in C.G.S. measure. 



Formula (28) gives the energy in ergs. 



To obtain it in Watts, we divide by 10,000,000, and 

 have 



Watts = /6-B V 10-'." 



Herein we find almost every conceivable blunder. 

 Power is termed " energy," and expressed in ergs ; 

 formula (28) is said to give the energy in ergs, whereas, 

 of course, it gives the power in ergs per second ; and to 

 wind up the comedy of errors, an expression said to 

 represent ergs is divided by the number 10", and, magi- 

 cally, it appears as Watts. 



Was confusion ever worse confounded ? 



The practical man — whose intellect the author con- 

 siders robust enough for the expression, '' line integral of 

 magnetic force,'' and for the comprehension of the (freely 

 employed) integral calculus — is very easily pleased if he 

 finds this sort of information improving. Surely, above 

 all things, he demands accurate statements, and resents 

 having symbols thrown at his head in this contemptuous 

 manner. 



On p. 27 the north pole of a bar magnet is described 

 as " the end which, if the bar were freely suspended 

 would point to the geographical north." 



Why the qualifying adjective " geographical " ? If the 

 author desired to evade a definition of the magnetic 

 meridian, he surely might have preferred a " suppressio 

 veri " to a " suggestio falsi." 



Many examples might be given of the looseness of 

 the language employed. We already possess a number 

 of excellent books on electro-magnetism written by 

 thoroughly practical men in precise and accurate lan- 

 guage ; and we must enter, for the reasons above given, 

 a strong protest against the theoretical portion of this 

 book. 



It is a pleasure to turn to the really practical part of 

 the work, where the reader will find valuable informa- 

 tion concerning dynamo design. In the case of large 

 machines, the author favours the multipolar type. He 



