xif SUGGESTIONS TO TEACHERS 



(4) The book contains comparatively few sketches; this gives 

 the student an opportunity to illustrate the important relations 

 by sketches of his own. Making sketches and drawings of electric 

 machines to scale, with their mechanical features, should be one 

 of the important features of an advanced course, even though it 

 m&y not be popular with some analytically-inclined students. 

 Mechanical drawing develops precision of judgment, and gives 

 the student a knowledge of machinery and apparatus that is 

 tangible and concrete. 



(5) The author has avoided giving definite numerical data, 

 coefficients and standards, except in problems, where they are 

 indispensable and where no general significance is ascribed to 

 such data. His reasons are: (a) Numerical coefficients obscure 

 the general exposition. (6) Sufficient numerical coefficients and 

 design data will be found in good electrical hand-books and pocket- 

 books, one of which ought to be used in conjunction with this 

 text, (c) The student is liable to ascribe too much authority to 

 a numerical value given in a text-book, while in reality many 

 coefficients vary within wide limits, according to the conditions 

 of a practical problem and with the progress of the art. (d) 

 Most numerical coefficients are obtained in practice by assuming 

 that the phenomenon in question occurs according to a definite law, 

 and by substituting the available experimental data into the corre- 

 sponding formula. This point of view is emphasized throughout 

 the book, and gives the student the comforting feeling that he 

 will be able to obtain the necessary numerical constants when 

 confronted by a definite practical situation. 



(6) The treatment of the magnetic circuit is made as much 

 as possible analogous to that of the electrodyamic and electro- 

 static circuits treated in the companion book. The teacher will 

 find it advisable to make his students perfectly fluent in the use 

 of Ohm's law for ordinary electric circuits before starting on the 

 magnetic circuit. The student should solve several numerical 

 examples involving voltages and voltage gradients, currents and 

 current densities, resistances, resistivities, conductances, and 

 conductivities. He will then find very little difficulty in master- 

 ing the electrostatic circuit, and with these two the transition 

 to the magnetic circuit is very simple indeed. The following 

 table shows the analogous quantities in the three kinds of cir- 

 cuits. 



