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 c"v." 139
For metrical purposes there are many advantages in thus moving only the sensitive
plate, though for mere display Mr Boys's more recent plan of spinning a succession of
lenses is able to give more striking results.
Accordingly an old packing case was made light-tight, and used as the camera. In
it were contained: first the spark-gap, a pair of adjustable brass knobs about half-aninch in diameter, clamped to a glass pillar, one vertically over the other and with a
clear space, on the average about 2 millimetres, between them; next the lens, an
ordinary camera lens on a special stand; and lastly the sensitive plate in its conjugate
focus, arranged so that the image was not very much smaller than the object. The
photographic plate is supported firmly in a revolving wooden carrier or frame fixed to
the horizontal axle of a whirling machine (one of Weinhold's) which was firmly clamped
to a stone pillar outside the camera and was driven by a long carefully spliced whipcord belt by means of one of Bailey's "Thirlmere" turbines standing on a distant
sink, and having a large grooved pulley to give the necessary "gearing up." One end
of the whirling machine axle passed through into the box in a light-tight manner, and
it was supplied with a self-oiling syphon wick. The ordinary speed at which it was
driven was 64 revolutions per second; occasionally it rose as high as 85, but the
water pressure was not often enough for this.
The turbine could have been fed from a cistern in the roof, but greater pressure
was attainable in the mains, and though liable to fluctuation this was found at certain
times in the day or evening regular enough for good observation.
MODE OF CONTROLLING AND DETERMINING THE SPEED.
Uniformity of rotation was essential, and to secure it the method employed by
Lord Rayleigh in his determination of the ohm was imitated. A small cardboard
stroboscopic disk was painted with several circles of radial markings, or "teeth," the
ones chiefly used being 3, 4, 5, 6, 8 teeth respectively in a circumference, especially
the pattern 4.
This disk was watched through a pair of slits carried by the prongs of a large
electroinagnetically maintained Koenig fork, whose loads were adjusted to give 128 vibrations per second precisely. The slits permitted vision at the middle of each swing,
consequently 256 glimpses a second. Hence whenever the 4 pattern on the stroboscopic
disk was distinct and stationary as seen through the slits, it meant that the sensitive
plate on the same axle was spinning 64 times in a second.
Photographs of sparks were taken only when the pattern was stationary and the
speed thus known to be regular.
To determine the speed absolutely it was necessary to calibrate or specially observe
the period of the fork. To this end two methods were employed: one the ordinary
method devised by Lord Rayleigh, for comparing an electromagnetically maintained fork
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