EVOLUTION OF QUARTZ CRYSTAL CLOCK 



553 



corded photographically with great accuracy. The difference thus observed, 

 after allowing as well as possible for known systematic errors, is the best 

 known single check on the time indication of a clock. A series of such 

 observations constitutes the best known measure of the rate of a clock. 

 The great value of the method is that the comparisons are made directly 

 without the need of any intermediate mechanism thus eliminating a large 

 part of the "personal error" of observation. The probable error of observa- 

 tion as derived from a number of such measurements on a good night may be 

 as small as one or two milliseconds^^. The average rate of a clock thus 

 determined depends on the number of days over which the rate is computed 

 and in a two-week period may be compared with the rate of the earth, that 

 is, with astronomical time, with an accuracy of one part in one hundred 



CRYSTAL 

 CONTROLLED 

 OSCILLATOR I 



ADJUSTING 



PHASE 



SHIFTER 



illll I I I I,. II 



CONTINUOUS 



CORRECTING 



PHASE SHIFTER 



MEAN SOLAR 

 CLOCK 



Fig. 23 — The use of an electrical phase shifter to adjust the timing of a signal. (From 

 'The Crystal Clock", 1930) 



million or about a third of a second a year. All this, of course, is contingent 

 on the stability of the quartz clock, which, except for long-time effects, may 

 be demonstrated independently. 



A rotating mechanism controlled directly from a crystal clock is admirably 

 adaptable to the transmission of precise time signals. Rhythmic signals of 

 any desired structure can be produced readily by means of cams, special 

 generators, or interrupted light beams, and the timing of those signals can 

 be adjusted as precisely as the clock time is known by simply advancing or 

 retarding the signal generators. Such adjustment is attained readily by 

 means of differential gearing in the mechanical system, or by means of con- 

 tinuous phase shifters in the electrical driving circuit. The use of electrical 

 phase shifters for this purpose was first proposed in 'The Crystal Clock" 

 paper^^ previously mentioned. Figure 23, taken from that paper, illus- 

 trates the manner of using the phase shifter with one type of time signal 



