820 



SCIENCE 



[N. S. Vol. XXXIII. No. 856 



SCIENTIFIC BOOKS 

 Electric Motors. By H. M. Hobart. Lon- 

 don, Whitaker & Co. 1910. 



The name of Hobart is so well known to 

 designing engineers in the electrical engineer- 

 ing field that any work from his pen needs no 

 introduction to members of that profession. 

 The volume under consideration being the 

 second edition of his " Electric Motors " of 

 1904 is not altogether new to the electrical 

 engineering fraternity, but there has been a 

 considerable revision and modernization of 

 the subject and an increase in the size of the 

 book. The large amount of data included 

 has also been revised to correspond to recent 

 practise. 



The scope of the book includes the electrical 

 design, predetermination of characteristics, 

 testing, operation and methods of special ap- 

 plication of practically all types of motors 

 now in commercial use, including the inter- 

 pole D.C.- motor and the single-phase series 

 and repulsion A.C. motors. 



Mr. Hobart's method of treatment is largely 

 empirical and practical and each procedure is 

 premised by copious data taken from tests. 



It assumes a knowledge of the fundamental 

 principles and familiarity with design. Thus 

 it will serve better as a reference book for 

 engineers and instructors than as a text-book 

 for the student in technical schools. 



No general method is given for the prelim- 

 inary selection of dimensions for a design, 

 but the reader is left to consult the data given. 

 This is a common custom among designing 

 engineers who, from their long experience and 

 memory, can guess at the right proportions 

 the first time, whereas the general practitioner 

 or student needs some criterion for his selec- 

 tion. Few books on design lead up to the 

 preliminary choice of design and dimensions 

 by a discussion of those fundamental condi- 

 tions which bear upon these proportions; such 

 as allowable peripheral speed, allowable ampere 

 conductors per inch or ampere stream on the 

 armature and reasonable magnetic densities. 



The book contains a very valuable compila- 

 tion of data on various designs, the product 

 of several different European manufacturers. 



This will make it a useful reference book for 

 the American engineer, as it gives him in- 

 formation on the methods which others, hav- 

 ing a different view-point, have used to solve 

 those problems which he has met and probably 

 solved in his own way. But it will be re- 

 gretted by the general practitioner and the 

 novice in designing, that American practise 

 is not described. It is needless to say that 

 designs which are very successful abroad 

 could not be marketed here owing to the dif- 

 ferent conditions, particularly with reference 

 to the cost of labor. 



The treatment of the interpole motor, while 

 very good, is not as extensive as a good many 

 engineers would desire. There is need of a 

 carefully systematized work of authority on 

 this subject in English, something comparable 

 to Arnold's work in German to which most 

 designing engineers in direct-current as well 

 as alternating-current work are obliged to 

 refer more or less frequently. 



The chapters on the Design of Induction 

 Motors are very complete, and by means of the 

 data included would enable one already fa- 

 miliar with the principles to produce very 

 satisfactory designs; but there are certain 

 features lacking and certain methods of treat- 

 ment which would make it difiicult for a per- 

 son not in the habit of practising design to 

 understand the subject. Thus the subject of 

 the leakage reactance or inductance is treated 

 almost altogether as a function of the " circle 

 diagram " and no statement or diagram is 

 given showing the paths of the leakage flux 

 and the effect on the value of this leakage flux 

 of the number and shape of the teeth. 



While it' is possible to take into account 

 this leakage reactance in the calculation of 

 the characteristics of the machine by means 

 of the circle diagram by referring to data 

 given, it is nevertheless very valuable to thor- 

 oughly appreciate during the actual process 

 of design, exactly how much each particular 

 feature of the design contributes to this quan- 

 tity. This can only be determined by actu- 

 ally deriving the leakage path from the shape 

 of the punchings. Incidentally, the effect of 

 fractional pitch on the leakage reactance is 



