1881.] Determination of the Ohm in Absolute Measure. 137 



the following numbers for the positions of rest during the experi- 

 ments : — 





766-28 



765-59 



765-33 



765-53 



Deflected reading 



367'60 



lloo*40 



obo IS 



iioo uy 





-398-61 



+ 400-81 



-399 10 



+ 400-56 





+ 2-08 



- 2-93 



+ 2-08 



- 2-94 



Reduction of temperature 











to Time = 9 h 37 m 



+ 0-05 



+ 0-05 



- 0-21 



+ 0-35 



Corrected deflection. . 



-396-55 



+ 397-93 



-397-23 



+ 397-97 



Mean deflection 397'42. 



C^S +0-0248. 



When all the spinnings had been reduced in this way, the final cal- 

 culations for the actual resistance were made. The determination of 

 all quantities involved has been explained, with the exception of the 

 measurement of the absolute pitch of the tuning-fork. 



Bate of Vibration of Tuning-fork. — As has already been explained, 

 the tuning-fork which was used to regulate the speed was on every 

 night compared with a standard fork, and our determinations, there- 

 fore, all depend on the absolute pitch of this standard fork. The 

 method used to determine that pitch has been described by Lord 

 Rayleigh.* 



. A fork, vibrating about 32 times a second, maintained by means of 

 an electric current, and driving a second fork of fourfold frequency, 

 was compared directly with the clock. The vibrations of the driven 

 fork were simultaneously compared with the standard by counting the 

 number of beats in a given time. A' few experiments have to be 

 made in order to see whether the fork gains on the clock, or vice versa, 

 and also whether the standard vibrates quicker or slower than the 

 driven fork< This can be done by gradually shifting weights on the 

 driver. The difference in the time of vibration of the clock and 

 driving fork was generally such as to give one cycle in between 20 or 

 30 seconds. The driven fork gave at the same time from 5 to 11 beats 

 per minute. 



The experiments agreed well with each other, and both the rate of 

 vibration and the temperature variation are in close agreement with 

 the determinations made by Professor McLeod and Mr. G. S. Clarkef 

 of other tuning-forks which, like ours, were made by Konig. 



The following series of determinations was made at a temperature 

 of about 13° C. :— 



* " Nature," xvii, p. 12, 1877. 

 f " Phil. Trans." vol. 171, p. 1, 1880. 



