April 29, 1922] 



NA TURE 



545 



just at this point. Einstein has met the greatest of 

 all objections both to Newtonian mechanics and to 

 Euclidean geometry. He has satisfied the logicians, 

 and it so appears that, beginning with this sole end in 

 view, he has found the explanation of the outstanding 

 discrepancy with observation. 



After the historic setting the most important 

 element in an exposition of this theory is an 

 analysis of the nature of measurement and of exact 

 observation. The new element in the general 

 theory of relativity is directly concerned with this. 

 Einstein insists on the fact that the use of co- 

 ordinates to distinguish between events is a piece of 

 mathematical machinery; that the physical facts 

 are there, and are the same, whatever descriptive 

 method we may employ. On the other hand, measure- 

 ment is simply a particular part of physical observation. 

 On this point much more exact thinking is needed. 

 The expositions of relativity, on the other hand, almost 

 without exception encourage more than usually loose 

 thinking. The strictest logical analysis cannot be 

 avoided. 



But after all the test of a popular exposition is 

 whether it is really illuminating to the amateur reader. 

 The reviewer is not entitled to pass hasty judgment. 

 Nor is he entitled to compare these books on the score 

 of their strict accuracy in detail. For the success of 

 the author's attempt is relative to the previous know- 

 ledge and habit of thought of the reader. 



One or two words of reference to the par icular 

 features of these publications may, however, be made. 



Dr. Schmidt's account is colloquial and entertaining, 

 and shows that the author feels the story of physical 

 science to be part of the wonder book of the universe. 

 Dr. Thirring is more severe and academic ; but at the 

 same time is lucid and free from exaggerations and mis- 

 leading illustrations. Mr. Bolton's essay is interesting 

 as the expansion of the 3000-word essay which won 

 the prize offered by the Scientific American. Mr. 

 Bolton remarks in his preface that the general drift of 

 the theory was a greater obstacle to an understanding 

 of the subject than the details of the advanced mathe- 

 matical work, and he has written the book with a 

 lively recollection of his own troubles. The fourth of 

 Messrs. Methuen's publications is the most interesting. 

 It is a collection of the best portions of the essays sent 

 in for the Scientific American prize. This book will 

 give hours of interesting reading from a multitude of 

 points of view. 



Dr. Dalton's Lectures in Johannesburg have been 

 reproduced attractively and are very readable, while 

 Prof. Gustav Mie gives us the point of view of one who 

 has himself contributed a good deal to the discussion 

 of fundamental physical theories. 

 NO. 2739, VOL. 109] 



The Induction Motor. 



The Induction Motor and other Alternating Current 

 Motors. By B. A. Behrend. Second edition, 

 revised and enlaiged. Pp. xxiiiH-272. (New York 

 and London : McGraw-Hill Book Company, Inc., 

 1921.) 24s. net. 



A FULL discussion is given in this book of the 

 practical theory of the induction motor and 

 of several of the main types of alternating current 

 motors. The author also gives a historical account 

 of the invention of the induction motor and of the 

 development of its theory. He attributes the inven- 

 tion to Nikola Tesla in 1888. In England it is generally 

 attributed to Ferraris, who certainly made an induction 

 motor, the rotating part of which was a solid copper 

 cylinder, in the autumn of 1885. In this connection 

 also, Baily has some claim to be called the inventor, 

 as he showed a disc revolving in a rotating magnetic 

 field to the Physical Society of London in June 1879. 

 Tesla and the Westinghouse Co., however, were the 

 first to make a motor similar to those used to-day. 

 They had great difficulties to contend with, as the 

 standard frequency of alternating current supply in 

 America in 1888 was 135. 



In 1895 the author first developed his theory of 

 the induction motor. He showed that in an ideal 

 motor the locus of the extremity of a vector repre- 

 senting a phase current is a ciicle, and that from this 

 circle the engineer can foretell the working of the 

 machine. This circle diagram has proved of the 

 greatest value to the designer and is in world-wide 

 use, although it is known that in consequence of certain 

 assumptions made in the course of the proof it is only 

 an approximation. An immense amount of ingenuity 

 has been expended in trying to make it more accurate, 

 but we are very doubtful of the value of these corrected 

 diagrams. Very often the authors unwittingly in- 

 troduce new assumptions — for instance, that all 

 the vectors lie in one plane — which may introduce 

 appreciable errors into their results. 



There are two parts in an induction motor, the 

 stator or fixed part containing the windings carryiag 

 the polyphase currents which produce the rotating 

 magnetic field, and the rotor, which is rotated by this 

 field and from which mechanical power is taken from 

 a pulley on its shaft. The induced alternating currents 

 in the rotor are quite distinct, and have a different 

 frequency from the alternating currents in the stator. 

 The mutual inductance coefficients between the stator 

 and rotor windings are not constants, and the induct- 

 ances of the windings are only approximately constant. 

 The problem as therefore difficult, and great credit is 

 due to the author for discovering that the speed. 



