526 BELL SYSTEM TECHNICAL JOURNAL 



considerable amount of previous study, even including the fluid-filled 

 flywheel to reduce hunting. It may be impossible at this time to know who 

 actually put in motion the first phonic wheel motor. 



DifiSculties inherent to contact-controlled devices prevented the develop- 

 ment of highly accurate fork standards of this type, and there is no evidence 

 so far that any thought had been given to the use of a tuning fork as a 

 timekeeper. 



The method of usuig a microphone instead of a contact was proposed by 

 A. and V. Guillet ^^, in 1900 and has been used considerably in frequency 

 standards of moderate accuracy, but that too had limitations which made 

 it impossible to utilize fully the inherent stability of a good tuning fork. 



The Use of Vacutmt Tubes 



The first opportunity for really precise control of the frequency of a 

 mechanical vibrating system, and the next step in the oscillator clock 

 evolution, came with the invention of the thermionic vacuum tube at the 

 turn of the century. The development of the vacuum tube has been a more 

 or less continuous process^^ starting with the studies of electrical conduction 

 in the neighborhood of hot bodies by Elster and Geitel, Edison, and Fleming, 

 and later developed into the first practical devices by Fleming^* and 

 DeForest^*^ in England and America respectively. The first patent for 

 such a device, a two-element tube, was issued to J. A. Fleming in 1904.^^ 

 The first patent on a tube containing three elements and suitable for use as 

 an amplifier was issued to Lee DeForest in 1907.^^ 



The vacuum tube as an amplifier found almost immediate and widespread 

 application in telephony and, next to the basic telephone elements, was the 

 most important single factor contributing to long distance communication. 

 For this purpose large amounts of amplification were required. Very 

 often in the operation of early amplifiers, enough signal from the output 

 would somehow get coupled into the input circuit to make the entire circuit 

 break into oscillation on its own account at some frequency for which the 

 amplifier and feedback circuit were particularly efficient. 



Although this was very annoying in an amplifier, it led naturally in 1912 

 to the invention of the vacuum tube oscillator, consisting essentially of an 

 amplifier with coupling between the output and the input and some definite 

 means for regulating the frequency of oscillation. The first to seek patent 

 protection in vacuum tube oscillators were Siegmund Strauss^* in Austria, 

 Marconi Company in England^'*, A. Meissner in Germany, and Irving 

 Langmuir, E. H. Armstrong and Lee DeForest^^ in America. Many specific 

 forms have since been invented and widely used, some of the more familiar 

 types being associated with the names of Colpitts, Hartley and Meissner. 



