Memoirs presented to the Cambridge philosophical society on the occasion of the jubilee of Sir George Gabriel Stokes, bart., Hon. LL. D., Hon. SC. D., Lucasian professor.

144 MESSRS GLAZEBROOK AND LODGE, EXPERIMENTS ON THE OSCILLATORY
The coil whose coefficient of self-induction L is required forms one of the arms of a
Wheatstone bridge, Fig. 2. Let P be the resistance of the arm. Two of the other arms
R and S are two resistances whose ratio-preferably one of equality-is known, and a
balance is obtained by adjusting the fourth arm Q. When this balance is found we
have the relation P/Q -R/S.
If the connections in the battery circuit be now reversed, a current due to selfinduction in the arm P passes through the galvanometer. Let a the first throw of
the galvanometer be observed.
Now alter the resistance Q by an amount îQ. In consequence there will be a deflection of the galvanometer needle; let 0 be this deflection; let x, x' be the currents in
the arm P before and after the alteration of Q, X the logarithmic decrement, and
let T be the time of a complete oscillation. Then, remembering that P and Q are
equal, we have (Rayleigh, "On the Value of the British Association Unit in Absolute
Measure," Phil. Trans. Part II., 1882)
x T ( 2sin 2
L=GQ- -(1X) 2
x 477 tan '
THE RESISTANCE BOXES.
In our experiments the coil P, already partly described, was wound in two sections
each with a resistance of about 100 ohms, so that when both sections were in use
P was approximately 200 ohms. The other resistances were taken from two boxes of coils
of platinum silver wire by Messrs Elliott Bros., correct in "Legal Ohms" at 17~ C. The
boxes had been calibrated in previous experiments, and the coils agreed closely with
each other. R and S were two coils of 100 ohms from one of these boxes; for the arm
Q an arrangement of two resistances in multiple arc was used. One of these was
205 ohms, the other was a large resistance of about 8000 ohms, and by varying this a fine
adjustment could be easily obtained.
DESCRIPTION OF THE GALVANOMETER USED.
The galvanometer employed was a ballistic instrument of about 64 ohms resistance.
It has two channels of rectangular section. Each channel contains 20 layers of thin
copper wire and 16 layers of thick, making about 465 and 202 double turns respectively,
so that there are 667 double turns in each channel, and about 2668 single turns on the
galvanometer.
The two thicknesses of wire were employed in order to fill the channels, and at
the same time permit the resistance of the galvanometer to be varied as required. The
ends of the wires are connected to binding screws on the bobbin marked A, B, &c., a, b, &c.