THE FREQUENCY METER 43 



regulating switch of the motor an ordinary 

 variable resistance in series with the field magnets 

 the motor can be speeded up until, say, the spring 

 labelled 99 vibrates freely. The motor is then 

 revolving % 9 times a second, or just under 3,000 



FIG. 20. 



times per minute. This is the condition for the 

 motor of the sending apparatus. The face of the 

 meter is shown in Fig. 20. 



Similarly at the receiving station the operator 

 runs his motor at just over 3,000 revolutions per 

 minute. Each motor is geared doWh with a worm 

 drive in oil, which has to work very sweetly in order 



