A HIGH PRECISION STANDARD OF FREQUENCY 



505 



rotor below the flywheel is the rotor of the unipolar 10-cycle generator. 

 The disc above the motor rotor is the armature of the 100-cycle 

 generator. The squirrel cage armature of an induction motor for 

 starting is immediately above the 100-cycle generator rotor. 



A single constant frequency generator is no longer sufficiently 

 reliable as a standard of frequency of high precision and, as has been 

 the practice where accurate time standards are maintained, three 



Fig. 15 — 1,000-cycle synchronous motor, generators, and clock. 



similar units have been installed. Means are provided for inter- 

 checking them continuously and automatically with the highest 

 precision justified. The use of three such generators with means for 

 interchecking them makes it possible to determine very quickly if 

 and when one generator fails to operate properly. 



Only one submultiple generator, clock, and multiple output amplifier 

 is provided, but a special 3-way switch is used by means of which 

 any one of the three primary oscillators may be selected and used as 

 the controlling unit. The oscillators may be interchanged in any 

 order without interrupting the circuits controlled by them. 



