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.

DISCHARGE OF AN AIR CONDENSER, WITH A DETERMINATION OF "v." 145
A to a is one wire, B to b another. In our experiments the coils were connected up in
series, the total resistance being about 64 ohms at a temperature of 13~'2 C.
The needle of the galvanometer was suspended from the Weber suspension by three
single cocoon fibres of 60 centims. in length.
The magnet was a small bar of hardened steel 15 centim. long, '6 centim. broad,
and '12 centim. thick; its weight was '708 grm. The magnet was attached by two
small screws to a brass stirrup to which the mirror was fixed. A piece of brass wire
6 6 centims. long, with a screw thread cut on it, was fixed to this stirrup at right angles
to the plane of the mirror, projecting equally on either side of the mirror. Two small
brass cylinders could be screwed along this brass wire, and by means of them the moment
of inertia and time of swing of the needle could be adjusted as required. The stirrup
and mirror weighed 6'6 grms.
The galvanometer has a solid wooden base of about 18 centims. diameter, and
this base was supported on three levelling screws. A graduated circle is fixed to the
base, and the coils can be turned about a vertical axis, and their position read by
means of a vernier. This was found useful in adjusting the coils parallel to the magnetic
meridian.
The galvanometer rested on a stone bracket built up from the ground. A scale
placed approximately north and south at a distance of about 347 centims. from the
magnet was reflected in the mirror and viewed through a telescope.
The scale rested on a solid stone support on the floor of the room. The mirror,
about 1'5 centim. square, was a specially good one, selected by a fortunate chance from
among a number in the laboratory. The divisions of the scale were in millimetres, and
after practice these could be subdivided by the eye with great accuracy to tenths. The
scale itself was of paper; though this material is unsuitable for many purposes because of
the changes produced in it by the weather, in our experiments these changes are of
small consequence, for we require only the ratio of the throw produced by the induction
current to the steady deflection produced by the permanent current; and the time which
elapsed between the measurements was only a few minutes. Any shrinking or alteration of the scale will go on very approximately uniformly throughout its length and
not alter the ratio of two lengths, which were never very unequal, as measured by the
scale. The scale had been carefully compared with the standard metre and the necessary
correction applied to the readings.
The distance between the mirror and the scale only enters our result in the small
correction necessary to reduce the scale readings so as to give the ratio of the sine of
half the throw to the tangent of the deflection. It was unnecessary, therefore, to
measure it with any great accuracy or to take steps to ensure its remaining the same
from day to day; so long as it did not change during the half-hour occupied by each
experiment, all the conditions required by us were satisfied.
The scale was carefully set so that the line joining its griddle point to the centre
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