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.

EXPERIMENTS ON THE OSCILLATORY DISCHARGE, ETC. 137
that the following research was undertaken, and much care was accordingly bestowed
upon it.
It may be here noted that Lord Kelvin himself, in one of his popular lectures *,
suggests this method of electric oscillation as just conceivably one of the methods by
which v could be practically determined; and he puts the matter in a geometrical way,
which it may be interesting freely to paraphrase thus:
Take a wheel of radius equal to the geometric mean of the following two lengths,
the electrostatic measure of the capacity of a condenser, and the electromagnetic measure
of the self-induction of its discharge circuit; make this wheel rotate in the time of
one complete electric oscillation of the said condenser (as if it were being driven by
an electrically oscillating piston and crank), then it will roll itself along a railway -with
the velocity v.
And indeed (as Maxwell discovered) ethereal waves excited by the discharge are
actually transmitted through space at this very speed.
GENERAL REQUIREMENTS OF THE METHOD.
The first essential is a condenser of capacity directly measurable from its dimensions. Its dielectric must accordingly be air, its plates must be a reasonable distance
apart, and they should be either spherical or have a guard-ring. The necessary smallness of capacity of a condenser satisfying these requirements is a difficulty, especially
when a quantity so large as the velocity of light is the subject of measurement. A
difficulty of the same sort is, however, common to all methods, and is what makes "v"
a quantity so much more difficult to determine than for instance "the ohm."
To compensate for the smallness of practicable electrostatic capacity a discharge
circuit of very great inductance must be employed, or else the time-determination will
be difficult from its excessive minuteness.
The inductance must be secured in combination with as much conductance as
possible, or the discharge will fail in being oscillatory. To this end Messrs W. T. Glover
and Co. were requested to supply a regularly wound hank or coil of No. 22 (s. w. G.)
high conductivity copper, very thinly india-rubber covered, of shape such as to give
maximum self-induction, and of size estimated to give between 5 and 6 secohms, i.e., in
magnetic measure, a length of 5 or 6 earth quadrants.
This would be afforded by a coil of 4 inches cross-sectional area and mean diameter
15 inches, with three or four thousand turns of wire. But to guard against the danger
of sparking or leaking between layers it was decided to reduce the dangerous tension to
one-quarter by having the coil in two halves. Accordingly it was made as follows (to
quote Messrs Glover's statement):
* Sir W. Thomson's Lectures alnd Addresses, Vol. i. p. 119. Lecture on Electrical Units to the Inst. C. E.
VOL. XVIII. 18