4i8 



NATURE 



[June 3, 1920 



Applications of Electricity. 



(i) Telephonic Transmission: Theoretical and 

 Applied. By J. G. Hill. (Manuals of Tele- 

 graph and Telephone Engineering.) Pp. xvi + 

 398. (London : Longmans, Green, and Co., 

 1920.) Price 215. net. 

 (2) The Principles of Electrical Engineering and 

 their Application. By G. Kapp. Vol. ii., Appli- 

 cation. Pp. viii + 388. (London: Edward 

 Arnold, 1919.) Price i8s. net. 

 (i)'~pHE applications of electricity in the tele- 

 1 graph and telephone services are now so 

 numerous and so highly specialised that no one can 

 claim to have an expert knowledge of every branch. 

 It has been decided, therefore, to produce a series 

 of handbooks which will cover the whole of the 

 ground involved. The editor of the series is Sir 

 William Slingo, late engineer-in-chief of the Post 

 Office, and most of the authors are on the staff 

 of the Engineering Department. Judging from 

 the present volume and from the names of the 

 authors preparing the other volumes of the series, 

 we shall soon have a very complete and thorough 

 account of English tejegraphic and telephonic 

 practice. 



This book is written for experts engaged in the 

 practical applications of telephony, and must be 

 judged from this point of view. It is now ancient 

 history how the early telephonists did their best 

 to diminish the capacity and resistance of their 

 lines with the object of securing good communi- 

 cation. In 1887 Oliver Heaviside pointed out 

 that this rule was quite fallacious. The two 

 qualities of the line which it is necessary to study 

 are the attenuation of the signals- and the velocity 

 with which they are propagated. Heaviside 

 stated this clearly and showed that his " dis- 

 tortionless " circuit gave the complete solution of 

 the problem. In 1900 Prof. Pupin showed 

 how a distortionless circuit might be secured very 

 approximately by putting inductance coils at cer- 

 tain intervals in the line. When the distance 

 between the coils is small there will be little re- 

 flection of the waves by them, and in this case 

 the practical working will be satisfactory. 



There are many engineers employed in telephone 

 work who have great difficulty in following the 

 advanced mathematical reasoning of Heaviside 

 and Pupin, and yet they have to evaluate their 

 complicated formulae in everyday work. For 

 their benefit the author introduces additional 

 chapters describing the transmission of direct 

 currents along a leaky line and getting the equi- 

 valent circuits. This should give those engineers 

 confidence to attack the complete mathematical 

 problem which is given in appendices. 

 NO. 2640, VOL. 105] 



The symbols and general arrangement of the 

 formulae are mostly those used by Kennelly and 

 Fleming, whose work is much appreciated by the 

 British Post Office. To the general man of science 

 most of the book will appear to be endless varia- 

 tions of complicated formulae, involving complex 

 variables, deduced from comparatively simple 

 differential equations. But a study of the book will 

 show him how laborious it is to get numerical solu- 

 tions, and how ingenious are some of the methods 

 employed to get the constants of the line. The 

 chapter on " the human voice in telephony " is 

 illustrated by excellent oscillograms of the alter- 

 nating currents produced by certain words. There 

 is also a chapter on the thermionic valve as a 

 telephonic relay which is of great interest. Very 

 instructive characteristic curves of the valve are 

 shown. A curve is also given which proves the 

 enormous variation of magnification with input. 

 The use of these telephonic relays is most promis- 

 ing, and great developments may shortly take 

 place. 



The book will be of great value to the telephonic 

 engineers for whom it is written. We were much 

 interested in the electrical constants of many of 

 the cables used in practice which have been cal- 

 culated by the author. They prove conclusively 

 the great value of advanced mathematical theory 

 in telegraphy and telephony. 



(2) As a pioneer of the applications of electricity 

 Prof. Kapp has had the pleasure of seeing many 

 of his theorems become incorporated in the 

 routine teaching of technical colleges and many 

 of his methods widely adopted in everyday prac- 

 tice. The technical student, therefore, will find 

 much that is familiar in this volume ; but he will 

 also find that the proofs given in many cases 

 have been appreciably simplified. The author in- 

 variably keeps practical considerations in the fore- 

 ground and rarely, if ever, digresses on points of 

 abstruse theory. In discussing the running of 

 machines he makes little endeavour to elaborate 

 the theory, but gives, in most cases, a clear 

 first approximate solution. The book, therefore, 

 will be welcomed by the engineer and the student. 

 The former will gain a clearer view of the prin- 

 ciples on which his machines work, and the latter 

 will find that many long mathematical solutions 

 can be much shortened by elementary graphical 

 methods. 



In the earlier chapters direct-current machinery 

 is described. The treatment of the critical speed 

 of turbo-dynamos Is very neat, and the results 

 agree with experiment. Little space is given to 

 losses which are relatively unimportant— bearing 

 friction, for instance. The methods of coupling 

 dynamos for parallel running are fully described. 



