EVOLUTION OF QUARTZ CRYSTAL CLOCK 539 



temperature of operation, which was controlled at a value in the neighbor- 

 hood of 40 degrees C. 



The method for frequency subdivision used in this tirst quartz crystal 

 clock is illustrated in Fig. 11. The inductance element of an electric circuit 

 oscillator, designed to operate at the desired low frequency, has a core of 

 variable permeability so that the frequency can be adjusted over a narrow 

 range through the control of direct current in an auxiliary winding. A 

 harmonic of this low frequency, generated in the tube following the oscil- 

 lator, is compared with the incoming high frequency in the vacuum tube 

 modulator. The harmonic chosen has nominally the same frequency as 

 that of the control, or crystal oscillator, so that one output of the modulator 

 is a direct current whose magnitude and sign vary with the phase relation 

 between the inputs to the modulator. The use of this method to regulate 

 the low-frequency oscillator insures that the low frequency is some exact 

 simple fraction of the high frequency. If, therefore, a synchronous motor 

 is operated from the low frequency thus produced, its rate represents ac- 

 curately that of the high-frequency source as though it had been possible to 

 use that source directly. 



Several other electrical circuits were proposed around 1927 for the sub- 

 division of high frequencies. The method in most general use at present is 

 an adaptation of the "multivibrator" first used by Henri Abraham and 

 Eugene Block in 1919 for the measurement of high frequencies^^ They 

 used their circuit to produce a wave rich in harmonics and having a funda- 

 mental that could be compared directly with that of a tuning fork standard. 

 By various means now well known the high frequency could be compared 

 with one of the harmonics of this special oscillator. 



This procedure was reversed by Hull and Clapp'^, who discovered that 

 the fundamental frequency could be controlled by coupling the high-fre- 

 quency source directly into the circuit of the multivibrator. This, in fact, 

 is a general property of any oscillator in which the operating cycle involves 

 a non-linear current-voltage characteristic, being most pronounced in those 

 of the relaxation type. Van der Pol and \'an der Mark in 1927 reported on 

 some experiments on ''frequency demultiplication" using gas tube relaxation 

 oscillators^^. The multivibrator is, in effect, a relatively stable relaxation 

 oscillator^-^, and with slight modification has been used extensively as the 

 frequency-reducing element in quartz-controlled time and frequency stand- 

 ards throughout the w^orld. 



One serious difficulty with the multivibrator type of submultiple generator 

 has been that, if the input fails or falls below a critical level, it will continue 

 to deliver an output which, of course, will not then have the expected fre- 

 quency. Certain variables in the circuit, such as tube aging, may cause a 



