December 8, 192 1] 



NATURE 



461 



Electrotechnical Theory. 



(i) The Theory of the Itidiiction Coil. By Prof. 

 E. Taylor-Jones. Pp. xi + 217. (London: Sir 

 1. Pitman and Sons, Ltd., 192 1.) 12s. 6d. net. 



(2) The Theory of Direct-current Dynamos and 

 Motors. By John Case. Pp. xiii-ri96. (Cam- 

 bridge : W. Heffer and Sons, Ltd., 1921.) 

 155. net. 



(3) Engineering Electricity. By Prof. R. G. 

 Hudson. Pp. viii + 190. (New York : John Wiley 

 and Sons, Inc. ; London : Chapman and Haii, 

 Ltd., 1920.) 135. 6d. net. 



(i) \ LTHOUGH it is more than ninety years 

 J~\. ago since Faraday made the first in- 

 duction coil, yet it is only quite recently that a 

 satisfactory working theory of it has been evolved. 

 The problem is one of considerable commercial 

 importance, for induction coils and magnetos are 

 widely used for ignition purposes in gas and 

 petrol engines as well as for X-ray work and 

 radio-communication. Prof. Taylor-Jones has 

 made a special study of the problem, and the 

 theoretical results deduced from his oscillation 

 transformer theory are in good agreement with 

 experiment. 



The principal hypothesis he makes is that after 

 the interruption of the primary current the system 

 acts like an oscillation transformer. The currents 

 in both the primary and secondary circuits act like 

 systems with two degrees of freedom. The wave 

 of current in each circuit has two oscillatory com- 

 ponents which have in general different fre- 

 quencies and different damping factors. They 

 sometimes also have different initial amplitudes. 



The author's method of treatment is similar 

 to that adopted by Drude. As the value of the 

 current in the secondary at any instant varies 

 along the length of the wire, and as it is prac- 

 tically impossible to discuss distributed mutual 

 inductance, the author supposes that there is a 

 " mutual coefficient " between the primary and the 

 secondary, and a different "mutual coefficient" 

 between the secondary and the primary. He is 

 also forced to suppose that the distributed capa- 

 city of the coils can be imitated by localised con- 

 densers placed across their terminals. Making 

 these assumptions he has to find the roots of an 

 algebraical equation of the fourth degree in order 

 to solve his differential equations. It is shown 

 that in certain cases the solution can be simpli- 

 fied considerably. 



This theory shows how to calculate approxi- 

 mately the size of the primary condenser of an 

 induction coil in order to get the greatest differ- 

 ence of potential between the secondary terminals, 

 while the experiments show that the simplified 

 NO. 2719, VOL. 108] 



formulae obtained by neglecting the resistances 

 of the coils are a help in practical work. Very 

 interesting photographs of the wave form of the 

 secondary potential are shown, and useful dis- 

 cussions of the problems of the Tesla coil and the 

 high-tension magneto are given. We can recom- 

 mend this book to research workers. 



(2) This book will prove very useful to students 

 who are reading for a degree . in engineering 

 science. The author develops in a very intelligible 

 way the theory underlying the design, operation, 

 and testing of direct-current machinery. He dis- 

 cusses the dynamo as a machine for converting 

 mechanical into electrical energy, and the motor 

 as a machine for doing the reverse operation. 

 The use of direct-current motors in traction 

 systems is also discussed, and there is a chapter 

 on boosters and multiple-wire systems. A special 

 feature of the book is the large number of engin- 

 eering problems given, many of which have been 

 taken from recent university examination papers. 

 It would have been an improvement if the answers 

 to all of them had been given. 



When discussing the heating and cooling of 

 dynamos the author regards the heat dissipated ; 

 being mainly due to radiation. As a matter of 

 fact, the great bulk of the heat dissipated is due 

 to thermal convection. This is the reason why 

 it is customary to make ventilating ducts so as 

 to secure a good circulation of air, the convection 

 increasing as the square root of the velocity of 

 the air current. We notice that the author adopts 

 the induction-factor method of discussing the 

 motor problem — a method first given by Carus 

 Wilson many years ago. It seems particularly 

 helpful when discussing numerical examples. A 

 very neat graphical method of obtaining the losses 

 in a machine by letting it slow down and getting 

 an angular velocity-time curve by means of a 

 tachometer is also given, and the geometrical 

 data found from it give the required constants. 



We have found little to criticise in the book. 

 In the statement of Faraday's law given on p. i 

 it should be stated that the induced E.M.F. is 

 due to the rate at which the flux of induction 

 (not the number of lines of force) linked with the 

 circuit is altering: the \:omplete formula (11), 

 p. 10, for the self-inductance of a pair of parallel 

 wires is not correct, for the second term should be 

 I, and not 2. There is a mistake also in the 

 numerical working out of the example at the top 

 of p. II. 



(3) Junior students in technical colleges can be 

 recommended to study Prof. Hudson's text-book. 

 It can also be commended to the notice of 

 engineers who desire to revise their knowledge 

 of the principles on which electrical engineering 



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