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XIV. A Method for Determining the Rate of Tuning -forks. 

 By Albert A. Michelson*. 



THE tuning-fork is already employed to a great extent as 

 a measurer of small intervals of time ; and this use 

 promises to become more extended as its advantages become 

 more fully appreciated. Any method which will facilitate 

 the operation of finding the rate, or increase its accuracy, 

 should therefore merit consideration. 



The following plan appears to be open to fewer objections 

 than any now in use; and its accuracy is limited only by the 

 accuracy of the pendulum. 



The fork to be rated, for example an Ut 2 making about 128 

 double vibrations per second, is compared with an Ut 2 kept 

 in vibration by electro-magnets and which shall be designated 

 EUt 2 ; and this is compared directly with the seconds pendu- 

 lum ; and then the two forks are once more compared, to make 

 certain that EUt 2 has not changed in the interval. Thus Ut 2 

 is determined. The whole operation may be performed in 

 less than ten minutes. 



To determine the rate of EUt 2 we will suppose that this is 

 within a small fraction of the whole number of vibrations per 

 second, and that this whole number is known, say 128 v. s. 

 A mirror is attached to one prong of EUt 2 ; and in front of 

 this, and at a distance of two or three feet, is placed a Greissler 

 tube. This last is illuminated once every second. The 

 tube itself is continuously illuminated, and its image in 

 the mirror presents the appearance of a broad band with well- 

 defined edges. Against this the narrow flash is projected. 



Evidently if EUt 2 makes an exact whole number of vibra- 

 tions per second, then this flash will always find the fork in 

 the same phase of vibration ; and consequently its image will 

 always appear at the same part of the band. If, however, the 

 fork makes, say, 128*1 v. s., then it will occupy successively 

 the positions 



0123456789 10 



That is, there would be ten flashes between the positions (0) 

 and (10). If, therefore, ten flashes occur in one " period," 

 then the whole number 128 is to be increased (or diminished) 



* From Silliman's American Journal for January 1883 



