﻿2(j Mr. C. V. Raman on Motion in a 



It is obvious that the maintaining forces in it should be 

 excessively feeble compared with the first or the second. 

 Perhaps experiments with interrupter-forks of higher fre- 

 quencies and independent driving of the motor may succeed 

 in showing the existence of controlled rotation-speeds at 

 these ratios. 



Combinational Rotation- speeds under Double Excitation. 



When the electromagnet of the synchronous motor is 

 excited simultaneously by the intermittent currents from two 

 separate interrupter-forks having different frequencies, main- 

 tenance of uniform rotation is possible not only at the various- 

 speeds related to the synchronous speeds due to either of the 

 intermittent currents acting by itself, but also at speeds 

 related jointly to the frequencies of the two currents. 



The preliminary experiments on this point were made 

 without the assistance of any independent driving of the 

 motor, and it was found at once that differential rotation of 

 the motor was easily maintained, the number of teeth passing 

 per second being equal to the difference of the frequencies 

 of the two interrupter-forks. 



When the "differentially"' revolving wheel was examined 

 by reflexion in mirrors attached to the prongs of the two 

 interrupter-forks, it was found that the patterns seen in 

 neither of them were stationary. They were found to be 

 moving steadily forward or backward with a definite speed, 

 with occasional slight to and fro oscillations superposed 

 thereon. This continuous rotation of the patterns seen was 

 obviously due to the fact that the frequencies of the forks 

 and their difference did not bear any simple arithmetical 

 ratios to each other, and it enabled a rotation-speed main- 

 tained by joint action to be distinguished by mere inspection 

 from one maintained by either of the two currents separately. 



Using this optical method, and assisting the rotation of 

 the motor with independent driving by a water-motor, various 

 other combinational speeds were found to be maintained. 

 Of these, the most powerfully and steadily maintained was 

 the simple summational rotation. The summationals and 

 differentials of the second series, i. e. those in which the 

 half-frequencies of the fork enter, were also noticed. The 

 rotation-speeds were determined by actual counting and a 

 stop-watch. 



The mathematical theory of these combinational speeds is 

 very similar to that given for the case of excitation by one 

 periodic current. For the field strength in this case is al>o 



