EVOLUTION OF QUARTZ CRYSTAL CLOCK 



577 



important thing being to obtain a rotation speed of 1 rps. This is a very 

 low speed for a phonic wheel motor but has the obvious advantage of great 

 simplicity since it permits of controlling seconds devices without the use of 

 gearing. Only one shaft is involved and the bearing problem is reduced 

 to the simplest possible terms. A hardened steel cam, integrally mounted 

 with the phonic wheel rotor, is used to operate a single electrical contact, so 

 connected into the circuits controlled by it that the instant oj break is the sole 

 time-determining operation. A break signal is preferable to a make signal 

 chiefly because it is easier to avoid irregular effects, such as result from 

 contact chatter, when a circuit is being opened than when it is being closed. 

 If a pallet of sapphire or ruby is used for the mechanical contact on the cam^ 



EED Back t^^ (ido poles) 



PICK-UP 

 STATOR 



^0 



K TIME *■ 



Fig. 34 — Suggestion of elements for a quartz clock for long time continuous operation. 



and if small currents are used through the contacts, made preferably of 

 platinum-iridium or similar alloy, it would be reasonable to expect trouble- 

 free performance through several hundred milHon operations. 



Ordinarily, the "hunting" of a phonic wheel motor operating on a fre- 

 quency as low as 100 cycles would cause time errors too large to neglect in a 

 device such as just described. However, by the use of feedback in the 

 motor amplifier circuit, such as indicated schematically in Fig. 34, the 

 effective hunting can be reduced to the point where the time errors caused 

 by it would become negligible for most purposes. 



Various circuits could be suggested for making use of the break signal for 

 timing purposes, the one shown in Fig. 34 being typical and suitable for 

 various methods of precise measurement and control. It is capable of 

 providing an electrical impulse with a steep wave front and of adjustable 

 duration. The grid of the vacuum tube is normally biased to cutoff by the 



