﻿Periodic Field of Force. 23 



velocity of the latter conld be adjusted. The speed of the 

 phonic wheel was ascertained by an optical method, i. e. by 

 observing the rim of the wheel as seen reflected in a mirror 

 attached to the prong of the interrupter-fork. When the 

 motor "bites," the pattern seen becomes stationary and 

 remains so for long intervals of time or even indefinitely, 

 and the speed of the wheel can be inferred at once from the 

 nature of the pattern seen. 



It was found in these trials that the motor could " bite w 

 and run at the following speeds. (Frequency of interrupter 

 GO per sec.) 



(a) j the synchronous speed : stationary pattern of rim of 

 moving wheel seen as a single sine-curve : wave- 

 length \ the interval between teeth. Number of 

 teeth passing electromagnet per second = 30. 



(!>) Synchronous speed : stationary pattern of rim of 

 moving wheel seen as a sine-curve, wave-length = 

 interval between teeth. Number of teeth passing 

 electromagnet per second = 60. 



(r) 1-g- times the synchronous speed : stationary pattern 

 of rim of wheel seen as three interlacing waves. 

 Number of teeth passing electromagnet per second 

 = 90. 



id) 2 times the synchronous speed : stationary pattern 

 seen as two interlacing curves. Number of teeth 

 passing per second = 120. 



(e) 2^ times the synchronous speed : this was only ob- 

 tained with difficulty. Number of teeth passing per 

 second = 150. 



(/) 3 times the synchronous speed : stationary pattern 

 seen as three interlacing curves. Very satisfactory 

 running. Number of teeth per second = 180. 



(<j) 4 times the synchronous speed : stationary pattern 

 seen as 4 interlacing curves. Number of teeth per 

 second = 240. 



(h) 5 times the synchronous speed : stationary pattern 

 seen as 5 interlacing curves. Number of teeth per 

 second = 300. 



The outstanding fact of observation is that while speeds 

 which are equal to the "synchronous " speed or any integral 

 multiple of it are readily maintained, only the first two or 

 three members of the other series (/. e. having ratios -V, l.V, &c. 

 to the synchronous speed) can be obtained, and the "grip '' 

 of the wheel by the periodic magnetic forces, i. e. the 

 stability of the motion, is hardly so great as in the integral 



