EVOLUTION OF QUARTZ CRYSTAL CLOCK 575 



This is no longer a serious problem for there are various electronic means 

 such as described by C. F. Booth and E. J. C. Dixon^^^ for continuous tem- 

 perature control, by means of which the variations may be kept very small, 

 and very effective thermal lagging methods^^s g^j-g ^^^ known. 



The bridge method for temperature control has been applied in many 

 forms. One of the simplest and most effective procedures has been to 

 utilize a bridge-stabilized oscillator of the type developed by L. A. Meacham 

 ioT frequency control, and to use it instead for temperature control. For this 

 purpose, all four arms of the bridge are noninductive resistances wound as 

 heaters on the oven to be controlled. In the feedback circuit of the oscillator, 

 a rough frequency control is included simply for the purpose of setting up an 

 oscillation in the circuit which includes the bridge. The conjugate pairs of 

 bridge arms are made of resistance wire with different temperature coefficients 

 and so proportioned that the bridge balances at the desired temperature. 

 The amplitude at which this bridge oscillator oscillates depends upon the 

 temperature departure from the balance value. Since the alternating cur- 

 rent output of the oscillator flows in the bridge arms, the amount of heating 

 is proportional to the temperature error, and hence the control is automatic. 



Continuity of Operation 



An astronomical clock, in addition to having as nearly constant a rate as 

 can be attained, should also be able to operate over long periods of time with- 

 out change or interruption. The reason for this is that many of the phe- 

 nomena that are of interest in time measurement occur in continuous succes- 

 sion and the greatest amount of information can be obtained only by the use 

 of clocks with which measurements can be ma^e in unbroken sequence. 

 Quartz clocks that have been used for astronomical purposes to date have 

 not had a very commendable record in this respect and already a good deal 

 has been said in the clock literature about this aspect — as though it were an 

 inherent property of the quartz clock. 



However, it is only a matter of simple engineering, making use of tech- 

 niques and apparatus already well known and available, to design a quartz 

 clock which should operate continuously for many years. A chain is only 

 as strong as its weakest link — and the clock comprises a chain of apparatus 

 parts every link of which must function perfectly and continuously. This 

 chain consists of (1) the crystal-controlled oscillator, (2) a frequency de- 

 multiplier to obtain a low frequency to operate a motor, (3) a power amplifier 

 to obtain sufficient current to drive the motor and (4) the motor itself, associ- 

 ated with any of a wide assortment of time signal-producing or measuring 

 equipment. In addition to the links in this chain, a power supply must be 

 maintained, and the temperature of the crystal must be controlled, both 

 continuously. 



