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 0F AN AIR CONDENSER, WITH A DETERMINATION OF "v." 143
READING OF THE RECORD.
The negative when thoroughly finished was subjected to careful rnicrometric examination.
To this end the plate was fixed on a horizontal circular graduated plate, part of
a spectrometer, reading with verniers at opposite ends of a diameter, and capable of
rotation with a slow motion tangent screw. Above the plate was clamped a microscope
of moderate power, with crosswires in its eye-piece; and below the plate a scrap of
mirror was arranged inclined at 45~ to throw the light up.
The centre of the plate was made to coincide with the centre of rotation, and the
microscope was placed over one of the spark rings. The plate was turned until the
beginning of a spark-trace appeared. Some definite feature of it was then brought
under the crosswire, and the verniers were read. Then another feature was sighted,
and the verniers read again, and so on, all along the trace of that spark; and similarly
with every spark round that circle. Then the microscope was shifted till over another
circle, and the process repeated.
By far the most distinct features, and the most useful for precise setting, were
the sharp spits or radial lines already referred to and visible in the positives or rough
copies of some of the preliminary plates.
All the readings were done on the negatives, and the best or final series of plates
have had no positive copies taken from them as yet.
PART II.
THE MEASUREMENT OF THE SELF-INDUCTION OF THE COIL.
Theory of the Method.
The method adopted for the measurement of the self-induction is that devised by
Maxwell, in his papers on "A Dynamical Theory of the Electromagnetic Field," Collected
Pctpers, Vol. I. p. 549.
B
F=iG. 2. D
FIG. 2.