81 



short-circuited by the brushes, are in the 

 same condition as the secondary of a 

 transformer short-circuited upon itself. 

 They will therefore be liable to burn up 

 if the armature cannot start at once. 



3. The third method of producing 

 motive power from alternating currents 

 has hitherto received most attention, as 

 being the most promising. It is due to 

 a discovery made about a year and a half 

 ago by Professor Galileo Ferraris, of 

 Turin. This scientist found that a cop- 

 per cylinder, suspended between two 

 coils, was set into rotation if alternating 

 currents of the same period, but of differ- 

 ent phase, were sent through these coils 

 which were placed at right angles to each 

 other. The explanation, given by Pro- 

 fessor Ferraris, was that the resultant 

 field of the two currents revolved round 

 the common center-line of the coils, and, 

 by means of eddy currents created in the 

 copper cylinder, dragged the latter after 

 it. This principle has been practically 

 developed by Mr. Nicola Tesla and others, 

 and motors have been actually built in 



