﻿22 Mr. C. V. Raman on Motion in a 



field of force are very well suited for lecture demonstration , 

 as the Lissajous figures obtained by the method described 

 above can be projected on the screen on a large scale, and 

 form a most convincing demonstration of the fact that the 

 frequency of the maintained motion is an exact sub- multiple 

 of the frequency of the exciting current. 



On Synchronous Rotation under Simple Excitation. 



It is well known that with an intermittent current passing 

 through its electromagnet, the synchronous motor can main- 

 tain itself in " uniform " rotation, when for every period of 

 the current, one tooth in the armature-wheel passes each 

 pole of the electromagnet. In other words, the number of 

 teeth passing per second is the same as the frequency of the 

 intermittent current. From a dynamical point of view it is 

 of interest, therefore, to investigate whether the motor could 

 run itself successfully at any speeds other than the " syn- 

 chronous" speed. Some preliminary trials with the motor 

 unassisted by any independent driving proved very en- 

 couraging. The phonic wheel I have is mounted on ball- 

 bearings, and runs very lightly when the large stroboscopic 

 disk usually kept fixed upon it is taken off, and there is no 

 current passing through the motor. When a continuous or 

 intermittent current is flowing through the motor, the latter 

 does not, however, run very lightly, being subject to very 

 large electromagnetic damping, apparently due to Foucault 

 currents in the iron. In the preliminary trials, however, I 

 found that, using the intermittent unidirectional current 

 from an interrupter- fork of frequency 60, the motor could 

 run successfully of itself at half the synchronous speed, i. e. 

 with 30 teeth passing per second. It of course ran very 

 well at the usual synchronous speed, i. e. with 60 teeth 

 massing per second. By increasing the speed, it was found 

 that the motor could also run well of itself at douhle the 

 synchronous speed, i. e. with 120 teeth passing per second. 

 Using an interrupter-fork of low frequency (23'5 per second) 

 the motor, it was found, could also run of itself at triple the 

 synchronous speed. No certain indication was, however, 

 obtained of the intermediate speeds, i. e. 1-h and 2J times 

 respectively the synchronous speed. 



To test these points, therefore, independent driving was 

 provided. This was very satisfactorily obtained by fixing a 

 small vertical water-wheel to the end of the axis of the 

 motor and directing a jet of water against it. The water- 

 wheel was boxed in to prevent any splashing of water on the 

 observer. By regulating the tap leading np to the jet, the 



