EVOLUTION OF QUARTZ CRYSTAL CLOCK 515 



This sort of operation is known sometimes as relaxation oscillatiort and 

 appears in many forms. In the clock, the rate-controlling feature depends 

 upon the length of time it takes a member having a given moment of inertia 

 to move from one angular position to another under a given applied torque. 

 Thus, the rate depends to first order on the applied torque. 



Although De Vick's clock was one of the most famous in all history, it was 

 not because of its good record of timekeeping. In its original form, it is 

 said that it often varied as much as two hours a day from true time. Out- 

 wardly, this clock on the Palais de Justice appears about the same as it 

 did originally, but the "works" have been modernized and it keeps much 

 better time now. 



The history of timekeeping during the next three hundred years consisted 

 mainly in improvements and in a great variety of applications of the prin- 

 ciples contained in De Vick's clock. During this period great numbers of 

 clocks of all sizes, from tower clocks to portable table clocks were made, 

 controlled by various forms of the crown wheel, verge and foliot balance. 

 All of these timekeepers belong to the class that we have just called aperiodic. 

 Their accuracy, in general, was still poor and the indicator on their dials 

 consisted of but one hand — the hour hand. It was not until the invention 

 and application of the pendulum that the next major improvement was 

 born in timekeeping. 



The Epoch or Resonajnt Control 



All that has been said so far is a prelude to the shortest but by far the most 

 productive epoch in timekeeping, that of resonant control. The heart of 

 every precision clock is an oscillatory device which depends upon resonance 

 for its constancy of rate. The history of precision clock development con- 

 sists largely of the choice and design of stable resonant elements and of 

 devising means for using them so that as far as possible their inherent 

 properties alone control their rates of oscillation. Once in stable oscilla- 

 tion, it is only necessary to control the indicating of dials and other suitable 

 mechanisms in order to constitute a complete clock.* Presumably this 

 can always be done, but in some cases it is more convenient to do than in 

 others, as will appear. 



The resonant element may be any of a wide variety of forms, mechanical 

 or electrical, all characterized by the single property that, if deformed from 

 a rest condition and released, the stored energy is transformed back and 

 forth from potential to kinetic at a rate depending chiefly on the effective 

 mass and the effective stiffness, or other like properties, a small proportion 



* Encycl. Brit. 14th Ed. "A clock consists of a train of wheels, actuated by a spring or 

 weight or other means, and provided with an oscillating governing device which so regulates 

 the speed as to render it uniform." 



