138 Some Features of the Electric Induction Motor. [April 28, 



placing the planes of the coils A and C at right angles to one another, 

 and B at right angles to A, as in the figure. 



If a second intermediate coil D were placed across coil B, there 

 would be currents induced in it by A, and a force exerted on it by 

 the field due to C, resulting in a series of impulses similar to those 

 acting on B, but differing from them by a quarter of a period. The 

 sum of the two sets of impulses on the intermediate conducting 

 system was a uniform twisting moment. 



If the two coils B and D were replaced by a continuous con- 

 ductor — such, for instance as a copper drum — this action could go on 

 while the conducting system rotated. In the experiment shown, a 

 copper drum was suspended in front of two coils placed at right 

 angles, and excited by currents differing by one quarter of a period. 

 When both coils were excited the drum rotated at a uniform speed. 



A striking feature in the Induction Motor was the rotating mag- 

 netic field caused by the variations of magnetism in the different 

 coils. This was illustrated by an experiment in which a small 

 permanent magnet carrying a mirror was centred upon a vibrating 

 rod placed between two pairs of electromagnets. When two oppo- 

 site magnets were excited the mirror vibrated between them and 

 reflected a spot of light on to the screen, tracing out a vertical line. 

 When the second pair of opposite magnets was excited the spot traced 

 out a horizontal line of equal amplitude. Both sets were then excited 

 together by currents in step with each other, with the result that the 

 spot vibrated in a line making an angle 45° with the former lines of 

 vibration. When the exciting currents were made to differ by one 

 quarter of a period, the spot of light reflected from the mirror 

 described a circle, showing that the magnetic field produced by two 

 magnets set at right angles and excited by two currents differing by 

 a quarter of a period is of uniform intensity and rotates at a uniform 

 rate. 



The action of the Induction Motor was based on principles which 

 governed the action of the better known transformer, and for pur- 

 poses of calculation it was convenient to regard the Induction Motor 

 as a transformer. One form only of the Induction Motor had been 

 alluded to ; of other possible forms that in which three magnetising 

 coils were used, with currents differing from one another by one-third 

 of a period was the one most commonly employed in practice. 



