A /7v'(l/. /./;/) rUXIXG FORK 



149 



This niflhod o{ coiitri)! is slow yil sonsitive to wry slikjlu rli. lilies 

 in frwiuency. 



The method vf conirollinj; the l"re(|iienry of llie fork by a (lamping 

 wimlinR was found to tK> the most simple and salisfactor>- nu-lliod. 

 rhe amoimt of variation of frequency wiiich this winding will produce 

 under extreme ixjnditions should be slightly greater than the maxinuim 

 variation to which the fork is subjectcnJ in operation when uncon- 

 trolliHl. This has been found to be about .05%, when temperature 



CUOC»^ 



r=«|ih-l-H|ih-i(-i-ih--iH 



-,|,|,|_l_,^.._,p_J 



Fig. 2^Clock Control of Fork Frequency. (.Schematic Diagram) 



v'ariations are held to within a few degrees. The only requirement 

 for the coil is that the current available from the vacuum tube must 

 be sufficient to produce the necessary control. This requirement is 

 easily met. It has been found possible with the equipment described 

 to obtain an effect at least 10 times greater than neccssar\-. 



The change in frequency of the fork due to the current in the damp- 

 ing winding is a combination of several effects. The current will 

 increase the decrement of the fork, due to the losses induced in the 

 metal of the fork while vibrating in a magnetic field. This will cause 

 a decrease in frec|uency. The magnetic force acting on the tines of 

 the fork, even though it be assumetl to cause no losses in the fork, is 

 unsymmetrical, having a greater effect at the ends of the swing of the 

 fork. This unsymmetrical force may also be shown to cause a de- 

 crease in frequency. Again there is a change in frequency due to the 

 change in amplitude alone. For the type of fork here used this 

 change may Ije an increase or decrease, depending on the range over 

 which the change occurs. 



It is obvious that when the control is operating, the voltage of the 

 condenser and hence the space current of the vacuum tube, will 

 fluctuate each half second. Since it is only the mean value of space 



