528 BELL SYSTEM TECHNICAL JOURNAL 



the velocity of that element. In the vacuum tube oscillator, it is a relatively 

 simple matter to design the feedback circuits to meet this condition very 

 accurately. 



In 1921 and 1922 Eckhardt, Karcher and Keiser^^-^o described the 

 development of a precise fork and vacuum tube driving means, pointing 

 out the following uses: '^As a sound source; as a small scale time standard; 

 as a current interrupter; as a synchronizer." The chief emphasis seems to 

 have been on the second item because in the same year Eckhardt described 

 a high-speed oscillograph camera using the same fork as a precise 'timing 

 device. The study and improvement of the tuning fork oscillator were 

 carried on continuously and soon such oscillators were used in several 

 national physical laboratories and commercial research institutions as stand- 

 ards of frequency and time interval. 



The next two reports of progress appeared in 1923, one by D. W. Dye 

 of the National Physical Laboratory in Teddington, and the other by J. W. 

 Horton, N. H. Ricker and W. A. Harrison of Bell Telephone Laboratories, 

 New York City. Both of these papers disclosed work done over a period 

 of two or three years and described apparatus that had been in operation 

 for a considerable period. Di*. Dye employed a 1000-cycle steel tuning 

 fork and a phonic wheel motor operating synchronously from it with a 

 gear reduction and cam to produce periodic electrical signals which he 

 compared with a clock by means of a chronograph^^ Horton, Ricker, and 

 Marrison used a 100-cycle steel fork, a synchronous motor with a gear 

 reduction to produce electrical impulses at one-second intervals, and a 

 clock mechanism operating directly from these signals^^. This appears to 

 be the first time that a vacuum tube-controlled oscillator was ever used to 

 operate a complete clock mechanism. Shortly thereafter, a clock was built 

 in which the 100-cycle motor was geared directly to the clock mechanism 

 instead of operating through a stepping device. A contacting device was 

 retained, however, for the purpose of making precise time measurements. 



For precise measurements of rate over long time intervals, means were 

 provided to compare the seconds pulses controlled by the synchronous 

 motor directly with time signals received by radio from the Naval Observa- 

 tory. To facilitate these comparisons, a two-pen siphon recorder was built 

 by means of which the time marks were laid down side by side on a moving 

 strip of pap)er in such a way that accurate subdivisions of a second could be 

 made on any part of the record. 



This same two-pen recorder and lOO-cycle fork time standard was used 

 during the total solar eclipse of January 24, 1925 to time the progress of 

 the moon's shadow as observed at a number of stations in the path which 

 were all connected by a round-robin telegraph circuit, through the Hell 



