354 
LORD RAYLEIGH OH THE CONSTANT OF MAGNETIC 
It will be seen that the cycle consisted of eight readings, four with positive and 
four with negative rotation of the plane of polarisation, and that this cycle is 
repeated three times. 
The three readings under any one head vary in consequence of the diminution of 
the current as well as from errors of observation. The value of p was 
at the beginning. p = 7018 
at the end. p=7 016 
Mean. p=7017 
Thus in the first observation at 6 h 3^ ra , when the cycle reading was 261° 44', the 
difference of potentials at the extremities of the [1-^] was (1 x Clark I., the 
temperature of Clark I. and of the [1-g-] being 17°‘6. 
For the mean double rotations in the four positions of the double image prism 
we have 
269 177—261 45 : 3 = 7 32-4 
359 23-0-351 56‘3 = 7 267 
89 19-3- 81 49-7 = 7 29'6 
179 20-0-171 52-3 = 7 277 
Mean . . . 7 29 "1 
Since all the effects are proportional to the current, it is sufficient to compare the 
mean rotation with the mean value of r, viz., 1413 ; so that the double rotation 
7° 29'"1, or 449'-1, corresponds to a difference of potentials equal to 
( 1 + 7bl7 / XClark I * = 7oi7 xC LARK I. 
The double rotation that would have been found if the current had been just 
strong enough to balance Clark I. (at the actual temperature) is 
7017 
8430 
X449 , -l = 373'-8. 
19. This result is a function of the temperatures of the cell and of It as well as of 
the CSo; and it is rather unfortunate that all three temperature corrections tell in 
the same direction. A rise of the thermometer involves a rise in It and a fall in the 
force of the standard cell, so that on both accounts the current giving the balance is 
diminished. At the same time the smaller current acts less advantageously in 
