EVOLUTION OF QUARTZ CRYSTAL CLOCK 557 



subject published in 1946.^°^ The early scaling circuits operated on the 

 binary system, but recently various circuits have been developed that give 

 the count in scale-of-ten notation with certain advantages, chiefly that of 

 convenience, associated with the common decimal system of notation. 

 A discussion of some modern binary and decade electronic counters^"^ ^^3,5 

 published by I. E. Grosdoff in September, 1946. 



Methods of measurement such as this, and the stable properties of the 

 quartz clock which make them desirable, are of importance in the precise 

 measurement of time because the nature of variations in rate, so small that, 

 if continued unchanged they would accumulate to only one second in a 

 thousand years, may be studied under controlled conditions in the labora- 

 tory, and with such facility that a comparison with this precision can be 

 made every ten seconds. 



In a simpler manner, the short period of one oscillation of the quartz 

 oscillator is of direct interest to the astronomer in connection with means for 

 the intercomparison of his clocks in time. This reduces simply to counting 

 the number of cycles gained or lost by one oscillator, referred to another, 

 and may be accomplished in a great number of ways, yielding, on the basis 

 of whole numbers of cycles, an absolute accuracy of time comparison of 

 0.00001 second. 



An elegant method for accomplishing this^^^, which also indicates auto- 

 matically which clock is fast or slow, employs a special vacuum tube circuit 

 to produce a polyphase current having the frequency however small of the 

 difference between any two oscillators nominally the same. This polyphase 

 current is used to operate a special synchronous motor whose angular posi- 

 tion corresponds at all times to the phase angle of the vector representing the 

 polyphase current. This relation holds all the way to zero frequency differ- 

 ence, in which condition the angular position of the motor, now at rest, 

 indicates the phase relation between the two high frequencies. If the beat 

 frequency goes through zero, the motor reverses. By this means, it is 

 possible with very simple equipment to set up dial indicators showing con- 

 tinuously the time comparisons between any group of quartz clocks, taken 

 in pairs, with an absolute accuracy of 0.00001 second. Of course, to operate 

 other indicators, contacts, etc. from this device is a simple mechanical 

 problem. 



The principle of operation of the polyphase modulator is illustrated in 

 Fig. 25, which shows one of the many possible forms of this device. Other 

 modulator elements than vacuum tubes are used in some applications. In 

 the form shown here it is necessary only to assume that the vacuum tubes 

 produce second-order modulation, the lowest-frequency component of which 

 is employed. If inputs at the two frequencies /i and /2, which are nearly 

 the same, are delivered into the two balanced modulators A and B in such a 



