EVOLUTION OF QUARTZ CRYSTAL CLOCK 523 



tradition, electrical maintenance and control has been applied in the most 

 accurate pendulums in the world. 



The free-pendulum clock makes use of the idea, first proposed by Rudd, 

 of allowing a master pendulum to swing free of all sustaining or other 

 mechanism for a considerable number of periods and of imparting to it, 

 after each group of free swings, a single impulse large enough to maintain 

 the next equal number. The advantage is that no friction effects of driving 

 mechanism are coupled to the pendulum except during that minimum time 

 required to impart energy to it. Actually, in theory, the phase error 

 introduced by one large impulse after n free swings is exactly the same as 

 the sum of the phase errors for n small impulses. That can be deduced 

 from the phase diagram of Fig. 5. But experience has shown that a pen- 

 dulum is actually more stable when the sustaining mechanism is detached 

 from it the greater part of the time. 



The Synchronome free-pendulum clock includes also the basic idea of 

 the gravity remontoir first applied by Lord Grimthorp (then Sir Edmund 

 Beckett Denison) in the design of the mechanism of Big Ben, London, 

 constructed in 1854 — and still in continuous operation. The ingenious 

 application of these principles and the electrical means devised by F. Hope- 

 Jones and W. H. Shortt for its accomplishment have resulted in the con- 

 struction of the most accurate pendulum clocks in the world by the Syn- 

 chronome Clock Company of London. The history and development of 

 the free-pendulum clock is elegantly described by F. Hope- Jones in his book 

 on Electric Clocks^. 



The predominant characteristics of a pendulum resonator, as used in a 

 clock, have just been discussed' in order to show the parallel between them 

 and the properties of other resonant systems. It will be shown how some 

 of the factors that have been troublesome in the development of pendulums 

 have been rather easily taken account of in other types of control devices 

 and in particular in the quartz crystal clock. 



The Evolution of Electric Oscillator Clocks 



It almost never happens that a result of any considerable value is obtained 

 at a single stroke or comes through the efforts of a single person. More 

 often even the most important advances come as the climax of a long series 

 of ideas which have accumulated over a period of years until the next step 

 becomes almost self-evident and is accompHshed either through the necessity 

 for a new result or as a logical next step. 



This was preeminently the case in the crystal clock development and 

 involved the putting together of a considerable number of ideas that had 

 been accumulating through a century or more of related activity. The 



