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.

138 MESSRS GLAZEBROOK AND LODGE, EXPERIMENTS ON THE OSCILLATORY
" 4,330 yards of No. 22 tinned copper wire covered with 2 coats of pure indiarubber to the diameter of '035 inch. This was the only covering. In two parallel
coils, internal diameter 103 inches, 4 inches deep, and 2 inches wide." See Figure 1.
This pair of coils were then packed carefully and permanently in a round walnut
box or drum, with a thin sheet of glass between them, and the
2^ terminals of each coil were led to the outside and finished off on four
-j i ",, ebonite pillars.
They could, therefore, be connected up in series, or parallel, or
used separately; but in practice they were usually joined in simple
104 series. With this coil many preliminary experiments were made at
Liverpool.
yI_ _ 1The self-induction of the double coil was estimated as about
- - -2 5 secohms or "quadrants," but no attempt was made to ineasure it with
any care at this time, because it was better to do it when all the
FIG. 1. apparatus was in position in the basement room set aside for the
experiments described in Part II.
The chief part of the whole business consisted in taking clear images of a spark
on a moving sensitive plate, getting every detail of the oscillation clearly recorded on
the negatives, so that they could be subsequently analysed under a microscope and the
time of an oscillation accordingly determined.
The sparks used were extremely feeble, and each was drawn out by motion into
a band, so that in order to get every detail clear the plates had to be super-sensitive.
For such plates we were indebted to the kindness of Mr J. W. Swan, who sent on
several occasions a special packet of Messrs Mawson and Swan's most highly sensitized
plates, which answered admirably.
The next principal part consisted in the microinetric reading of the records on the
photographic plates. The reading is rather a tedious process as a great many numbers
have to be recorded for each plate, and care is necessary to disentangle the several
sparks, which to economise time and labour at the experimental end were usually taken
during a single spin.
The details of the method of obtaining the record will now be described.
TIME OF ONE OSCILLATION.
The long-established method of observing spark oscillation by means of a revolving
mirror was at first used; but this plan, though easy for observation, does not readily
lend itself to precise measurement. It is desirable to obtain a photographic record
which can be studied at leisure, and it seemed therefore best to form an image of
the spark on a plate moving so rapidly that its constituent oscillations were clearly
visible.