﻿292 Lord Rayleigh on the Minimum Current 



and 



dM/dt=Gm p . 2 fi/l. cos pt. ... (6) 



The formula corresponding to (5) is thus derived from it by 

 simple introduction of the factor 2/3/Z. 



The forks were excited by bowing, and the observation of 

 amplitude was effected by comparison with a finely divided 

 scale under a magnifying- glass. It was convenient to ob- 

 serve the extreme end of a prong where the motion is greatest, 

 but the double amplitude thus measured must be distinguished 

 from 2/3. In order to allow for the distance between the re- 

 sultant poles and the extremities of the prongs, the measured 

 amplitude was reduced in the ratio of 2 to 3. The observa- 

 tion of the magnetic moment at the magnetometer is not 

 embarrassed by the diffusion of the free polarity. 



In order to explain the determination more completely, I 

 will give full details of an observation with a fork c' of 

 frequency 256. The distance I between the middles of the 

 prongs was *875 inch, and the double amplitude of the 

 vibration at the end of one of the prongs was *09 inch. Thus 

 2/3 is reckoned as '06 inch. The inductor-coil was the 

 ebonite coil already described, and the sound was judged to 

 be of the standard distinctness when, for example, B = 15 

 inches, or C = 15'5 inches, and the added resistance was 

 1000 ohms, so that R = 1100 x 10 9 . The quantities required 

 for the computation of (5) as extended are 



n= 242, 



P = 2ttx 256, 



H = *18, 



A= 10-25, 



?>=15, 



d=410, 



C= 15-5, 



R=llxl0 n , 



D = 1370, 



2/3=*06, l='S75 ; 



and they give for the current corresponding to the standard 

 sound 9*8 x 10~ 8 c.G.s., or 9*8 X 10 -7 amperes. 



A summary of the results obtained with forks of pitch 

 c, c', <?', g 1 , c", <?", g" is annexed. As the pitch rose, the 

 difficulties of observation increased, both on account of the 

 less duration of the sound and of the smaller amplitudes 

 available for measurement. In one observation with tele- 

 phone T 2 at pitch c", the resistance, estimated at 11 ohms, was 

 that of the coil, telephone, and leads only. No trustworthy 

 result was to be expected under such conditions, but the 

 number is included in order to show how small was the 

 influence of self-induction, even where it had every opportunity 



