678 
LORD RAYLEIGH OH THE VALUE OF THE BRITISH 
current is reversed, we may write 2Lx for Lx, understanding by x the same as before. 
As this method was the one actually adopted, we will write the result in the 
appropriate form 
L.x T sin \ol 
SP.a/ 7r tan 6' 
In the formula as originally given by Maxwell, and as stated in the former paper, 
the distinction between x and x (the currents before and after the resistance balance 
is upset) was neglected. This step is legitimate if SP be taken small enough, to which 
course however there are experimental objections. In order that tan 6 might be of 
suitable magnitude, it was found necessary to make the ratio of SP : P equal to about 
•3 a fraction too large to be neglected. 
In carrying out the experiment it was found more convenient to insert the additional 
resistance in the branch Q, leaving P unaltered. By the symmetry of the arrangement 
it is evident that this alteration is immaterial, and that we may take the formula in 
the form 
L_L_ SQ.af T sin 
Q P Q.x 7 r tan 0 ’ 
x being the current in the branch Q when the resistance balance is perfect, x' the 
diminished current when the additional resistance SQ is inserted. 
The principal difficulties in carrying out the experiment arose from variation in the 
battery and in the resistance balance. From these causes the results of two days’ 
experiments were rejected, as unlikely to repay the trouble of reduction. On the 
last day (December 3, 1881) the first difficulty was overcome by using three large 
Daniell cells (charged with zinc sulphate) in multiple arc. As precautions against 
rapid change of temperature the copper coils were wrapped thickly round with strips 
of blanket and deposited in a closed box. The delicacy of our arrangements was such 
that about Tcrno a degree centigrade would manifest itself, so that it was hopeless 
to try to maintain the resistance balance absolutely undisturbed. The mode of 
applying a suitable correction will presently be explained. On December 3, partly by 
good luck, the necessary correction remained small throughout. In order to avoid a 
direct action of the current upon the galvanometer needle, the coil was placed at a 
considerable distance, at the same level, and with its plane horizontal. Any out¬ 
standing effect of the kind would, however, be eliminated from the final result by the 
reversals practised. 
The induction throws were always taken by reversal of the battery current. A 
reversal has two advantages over a simple make or break. In the first place the effect 
is doubled and is therefore more easily measured; and in the second the battery is more 
likely to work in a uniform manner, the circuit being always closed except for a 
fraction of a second at the moment of reversal. The key was of the usual rocker and 
mercury cup pattern. 
The galvanometer was one belonging to the laboratory of about 80 ohms resistance. 
