612 Mr. J. Viriamu Jones [May 24, 



suppose that the fork permits the light to pass through it 128 times 

 per second, and that the row on the disc which looks stationary has 

 fourteen teeth on it ; then one tooth takes the place of the next in 

 T ^ a second, but this corresponds to ^ of a revolution, so that one 

 revolution is performed in y 1 /^ of a second, and the number of turns 



A ' 128 Ql 



per second is -yj- ~ "t ' 



And generally 

 2P 



n = 1T 



where P = the pitch of the fork, 



n = the number of turns of the disc per second, 

 Q = the number of teeth in the stationary row. 



We may then make the stationariness of a tooth, as seen by an 

 observer through the slits, a test of the uniformity of the rate of 

 rotation. If a tooth begins to move past a fixed reference wire placed 

 immediately in front of the rotating disc or drum, the observer can at 

 once bring it back by altering the current in the motor.* 



If the tuning fork were itself a reliable time indicator to the 

 degree of accuracy required, we might calculate from its pitch 

 the speed corresponding to stationariness of a given row of teeth. 

 But my experience is that the vibration period of a fork maintained 

 in vibration electrically is not sufficiently constant. If stopped and 

 set going again it may start with a period different from that of its 

 last performance by several parts in 10,000. No previous deter- 

 mination of the pitch of the fork can therefore be relied on to give us 

 the rate of rotation to a hundredth per cent., though once started the 

 fork goes sufficiently uniformly to give us the means of control. 



The period of a bowed fork does not vary in this fashion, but it 

 is more troublesome to use for the purpose. The constant bowing 

 required takes too much of the attention of the observer. 



Accordingly, it is necessary to measure the rate of rotation during 

 each run while the galvanometer observations are being made. This 

 may be done telegraphically with great accuracy. The rotating disc 

 is by means of an eccentric attached to its axle caused at each revolu- 

 tion to make and break an electric circuit passing through a Bain's 

 electro-chemical telegraph instrument. The standard clock telegraphs 

 seconds to the same instrument. The two records lie side by side on 

 the same tape, and simple counting with careful fractional estimate 

 gives with great precision the mean rate of rotation during the few 

 minutes occupied in taking a set of galvonometer readings. 



With this means of measuring the rate of rotation we may, 

 establishing synchronism with a fork in the way I have indicated, 

 find the pitch of the fork. 



* So far as I know this method of measuring a rate of rotation was first used 

 by Lord Kayleigh. The method used by Macleod and Clarke for measuring the 

 pitch of a tuning fork bears much likeness to it. 



