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.

186 MESSRS GLAZEBROOK AND LODGE, EXPERIMENTS ON THE OSCILLATORY
PART VI.
FINAL EXPERIMENTS.
With the air condenser described in Part V., a number of spark photographs on
Mr Swan's 4-inch square plates were taken with the plate revolving 64 times a second.
From seven to nine circles were attempted on these plates with three or four sparks on
each circle.
Tin plates which are lettered from A to Z were afterwards read with great care
by Mr J. W. Capstick who writes: "The measurements will be found to be within a
very few minutes of the correct reading. In one case I accidentally went over a spark
twice, and though I was then at the end of six hours' almost continuous work at
them, and the spark was an exceptionally indefinite one, the greatest divergence in the
readings was only 3 or 4 minutes.
"The plates are very much better than any I had done previously, and the setting
of the microscope was generally a simple matter. The sparks were in general so
definite and regular that I did not think it necessary to make drawings of them."
[This had been done with some of the earlier plates.]
Mr Capstick remarks-as will be seen from the Tables-that there is some irregularity in the sparks, and that, unless it is desired to study this, greater accuracy of
reading is hardly necessary.
The analysis of this long series of plates has been a work of time; we give below
the results of a study of all the plates from G to U. In the earlier plates, marked
A to F, the work was in some respects of a preliminary character; there was no plate
marked Q. In the spin for plate P the coils were in multiple arc, and the coefficient
of self-induction for this arrangement was not determined.
We give as an example the actual record for two of the circles on plate U*.
This illustrates the method of dealing with the results.
SPARK RECORD ON PLATE U.
Coil B only used.
Outer circle.
Actual readings. Differences. Averages.
Spark (1) 194~ 0'
186 49 140 36'
179 24 (14 45)
(172 4) 14 25 1 4
164 59 (14 24)
157 40
* The record for this plate happened to cone first in one of the note-books in which results were recorded.