﻿On Motion in a Periodic Field of Force, 15 



The conditions for maximum potential are the same when 

 a rapid interruptor is employed as for the slow break used in 

 the above experiments. They also hold whether the primary 

 current is supplied by a small storage-battery, a 100-volt 

 battery, or the 200-volt mains. Plate I. fig. 8 shows the 

 course of the secondary potential at two successive " breaks " 

 effected by a motor mercury interruptor. The effect of the 

 small potential at the " make w is also noticeable. On 

 this occasion the 100-volt battery was used to supply the 

 primary current, the total (steady-current) resistance in the 

 primary circuit being 11 ohms. The greatest spark-length 

 was 18*4 cm., and the mean primary current, as indicated 

 by an amperemeter in the primary circuit, was *3 ampere. 

 The current immediately before interruption would, however, 

 be over 2 amperes. 



Bangor, November 1914. 



II. On Motion in a Periodic Field of Force. 

 By C. V. Raman, M.A.* 



[Plate IL] 

 Vibrations maintained by a Periodic Field of Force, 

 rj^HE experimental study of the motion of a dynamical 

 -L system in a periodic field of force leads to results of 

 quite exceptional interest. One aspect of the problem, i. e. 

 the oscillatory motion of the system about a position of equi- 

 librium in the field, has some affinities to the case of vibra- 

 tions maintained by a variable spring which I have dealt 

 with in my previously published work, but the two classes 

 of investigations lead to results which differ from one another, 

 yet are related in a most remarkable way. By experimenting 

 on stretched strings subjected to a variable tension, I showed 

 that a normal variation of spring will enable the oscillations 

 of the system to be maintained, whan the frequency of these 

 oscillations is sufficiently nearly equal to % of, or -J times, or 

 | times, or f times, &c. the frequency of variation of the 

 spring, these ratios forming an ascending series f. By ex- 

 periments on the vibrations of a body about a position of 

 equilibrium in a periodic field of force (to be described 

 below), I have shown that the frequency of the oscillations 

 maintained may be equal to, or half of, or one- third, or one- 

 fourth, &c. of the frequency of the field; in other words, ir 



* Communicated by the Author. 



t See Phil. Mag. Oct. 1912, "The Maintenance of Forced Oscillations 

 of a New Type." 



