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." 153
PART III.
CORRECTIONS TO THE SIMPLE THEORY OF THE EXPERIMENT.
(i) THE ELECTROSTATIC CAPACITY OF THE COIL.
The chief cause of difficulty in comparing the experimental results with theory
arises from the fact that the coil has considerable capacity, and further that this is
not distributed uniformly along the length of the wire.
The coil consists of two similar portions almost identical.
Each half is wound with about 60 layers of gutta percha covered wire containing
about 30 turns to a layer. The interior diameter is 27'5 cm. and the exterior
is 48'7 cm., while the axial depth of the coil is about 5'2 cm. The number of turns
of the coil were not counted exactly when it was wound.
After the experiments the case was opened and the coil measured as far as practicable. It was found that the number of layers in a radial direction as estimated from those
which could be seen and counted was 64, and they occupied 10'35 cm. Thus the average
distance between the centres of consecutive layers is 10'35/64 or *164cm. The inner
layer contained 28 turns, and of these 25 lie in a space of 3'9 cm.; thus the distance
between consecutive turns is '156 cm. The thickness of the uncovered wire was found
to be '049 cm.; thus the thickness of two coverings is '107 cm.
The two halves are separated by a sheet of glass with a circular hole in its centre;
the sheet is about '27 cm. in thickness.
The whole coil is enclosed in a wooden box, the ends of the wires being brought
to terminals which are well insulated from the wood.
Now if we consider any turn of the one coil lying near the glass, it is faced on
the opposite side of the glass by a similar turn, which during the experiments will be
at a very different potential. Charges will thus accumulate on these turns and their
capacity must be considered in the theory. If we consider a turn in the centre of
either coil it is surrounded by other turns at nearly the same potential as itself, and
does not therefore become much charged.
The outer layers of the coil will have some capacity, but if the wood case be
treated as an insulator this will be small, and thus we may consider that the chief
capacity of the coil lies in the faces in contact with the glass.
We may thus represent the two coils in the following diagrammatic manner:
Consider a number (n-1) of equal condensers, each of capacity S'; each plate of a
condenser represents two adjacent turns of the wire, which lie on the same side of
the glass, and face two corresponding turns, representing the second plate of the condenser,
on the opposite side of the glass.
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