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.

176 MESSRS GLAZEBROOK AND LODGE, EXPERIMENTS ON THE OSCILLATORY
PLATE No. I.
On its first or outer circle several sparks were taken, with cylinder and disc condenser joined by wood w'.
Second circle, several more with the same.
Third circle, with the cylinder condenser alone, the dise condenser being disconnected.
Fourth circle, both condensers in parallel, joined as in Figure 8, but by wire, not wood.
The following is the micrometric analysis of this plate, the numbers being given
rather fully as a specimen. It was the first plate carefully read. We do not quote the
actual circle readings but the successive differences or lengths of the constituent halfoscillations; the last one was usually faint, and some were better marked than others.
It will be seen that there were very few oscillations in each spark, because of the
smallness of the condenser and the resistance of the circuit. It would not indeed have
been surprising if the damping had affected the period perceptibly; but the only obvious
effect is the lengthening out of the last swing by the high resistance of the decaying
spark.
Successive alternation intervals on fourth or inner circle of Plate No. 1 for different
sparks:6~ 48', 6~ 18', 6~ 5.5', 7~ 6'
6~ 36', 6~ 15', 6~ 40', 6~ 54'
6~ 29', 6~ 41', 6~ 32', 7~ 6' (lower power object glass),
6~ 32', 6~ 35', 6~ 25', 7~ 11' (plate recentred),
6~ 32', 6~ 35', 6~ 26', 7~ 7' (repetition),
6~ 23', 6~ 33', 6~ 16', 7~ 2' (plate recentred),
6~ 32', 6~ 38', 6~ 34', 70 28' (apparatus reset).
It is clear that the last or decaying half oscillation is unduly lengthened by reason
of the high resistance of the dying spark, so, omitting it, we have as the average of a
half oscillation for this circle 6~ 31'.
Similarly omitting the last reading, which in nearly all cases is longer than the others,
the average length of a half oscillation on the third circle is 5~ 19'; on the second circle
5~ 18'; and on the first or outer circle 5~ 19'.
Since the plate was making 64 revolutions per second, this gives as the observed
frequency:
For the cylinder condenser connected by wood to the dise condenser 2170 per second.
For the cylinder condenser alone...2170
For the cylinder and dise condenser properly connected... 1770
These numbers shew that the wooden connectors separating the condensers act as
expected, at least in preventing combined discharges, and thus act effectively in isolating
the machine terminals from the capacity discharged.