EVOLUTION OF QUARTZ CRYSTAL CLOCK 559 



way that there is a 90-degree phase shift between the two input voltages for 

 one of the frequencies, the lowest-frequency component appears as a sinusoidal 

 current in the output circuits 1, 2, 3 and 4 separated in phase by 90 electrical 

 degrees in cyclic rotation. The principal output, therefore, is a 4-phase 

 current having the frequency of the difference between the two inputs. If 

 the magnetic circuits are arranged geometrically as shown, the resulting 



Fig. 27 — Spark chronograph — close view of mechanism. 



magnetic vector will rotate clockwise or counterclockwise depending on 

 which frequency is high, or will remain stationary, indicating the phase 

 relation, if the two frequencies are exactly equal. 



Motors have been designed and are commercially available suitable for 

 operating synchronously from such polyphase modulators, and form an 

 excellent basis for the intercomparison of quartz oscillators and clocks with 

 ultra-high precision. 



For making records of time comparisons the spark chronograph^ shown 

 in Figs. 26 and 27 has served a very useful purpose, combining in a single 



