EVOLUTION Of QUART/ CRYSTAL CLOCK 525 



tions per second, with delicate mechanical means, similar to a clock escape- 

 ment, for sustaining the fork in vibration and for counting the number of 

 vibrations over any desired interval of time. For this purpose, the escape- 

 ment mechanism was geared to the hands of a clock, so that when the fork 

 had its nominal frequency the clock would keep correct time. Dr. Konig 

 credits the invention of the fork-clock to N. Niaudet''"' in these words: 



"Cette disposition avail ete realisee pour la premiere fois dans I'horloge a diapason 

 que N. Niaudet fit presenter a I'Academie des Sciences le 10 decembre 1866, et que a figure 

 aux expositions universelles de Paris 1867 et de Vienne 1873."* 



Thus, as early as 1866, the essential elements had been developed sepa- 

 rately from which a clock of the electric oscillator type could have been 

 constructed. But it was not until more than half a century later, when 

 there was more apparent need for such a clock, that it was actually realized. 

 It was chiefly for the purpose of studying temperature coefficients and like 

 properties of tuning forks that Konig constructed and used his famous 

 mechanical fork-clock. There is no evidence that there was at that time 

 any idea of using a fork-clock as a timekeeper. 



It was for the purpose of making still more precise studies of the properties 

 of tuning forks that H. M. Dadourian^^ in 1949 made use of the phonic 

 wheel motor for the first time for counting the number of cycles executed by 

 a fork over an extended period of time to measure its rate. By means of a 

 chronograph the time interval corresponding to the total of a very large 

 number of periods could be measured precisely in terms of a standard clock, 

 thus providing a direct '^absolute" measure of fork rate. For this he found 

 already invented for him all of the essential component parts, including 

 the fork with electromagnetic drive, and the phonic wheel motor. 



The phonic wheel motor, which in some modified form is an essential 

 part of nearly all oscillator clocks, was invented by two investigators, 

 apparently quite independently and for entirely different purposes. The 

 first published reports of each appeared in 1878. 



The first of these is an American patent that was granted on May 7, 

 1878 to Poul La Cour^^, a Danish telegraph engineer. The application was 

 filed in Washington on April 9 of the same year, and described a fork- 

 controlled impulse motor similar to those still used in many modern syn- 

 chronous clocks. The other publication was a report in Nature for May 23 

 of the March 30 Physical Society Meeting. In this, Lord Rayleigh de- 

 scribed a motor which he developed to measure the frequency of sound by a 

 stroboscopic method. ^^ Both of these original disclosures indicated a 



* ''This apparatus was realized for the first time in the fork-clock which N. Niaudet 

 described at the Academy of Sciences on December 10, 1866, and which was shown at the 

 expositions of the University of Paris in 1867 and the University of Vienna in 1873." 



