608 
ME. J. HOPKINSON 0^ THE 
quadrant is considerable and of the same sign, it cannot be assumed that the deflection 
for a given difference is the same as if the charges were small, or of equal and opposite 
sign; in fact, if the potentials of the quadrant and the jar of the electrometer are of 
the same sign, the sensibility of the instrument will be diminished ( vide Maxwell’s 
4 Electricity and Magnetism,’ vol. i. p. 273). On this account the results for 4>(t) given 
below should be increased by about -jV part of their value. The experiment consisted 
in insulating the flask from the battery, and observing the difference of potential between 
the flask and the battery after a suitable interval. 
The flask was charged and instantly insulated at 8.25 p.m. The image traversed 
164 divisions in 10 seconds. The flask was again connected with the battery, and insu- 
lated from time to time. 
I. gives the middle of the period of insulation, measured from 8.25 ; II. the division 
traversed ; III. the duration of insulation ; IV. the value of ip(t). 
I. 
II. 
ill. 
IY. 
5 seconds. 
164 
10 seconds. 
0-28 
1 minute. 
26 
20 „ 
0-022 
2 minutes. 
14 
20 „ 
0-012 
3 
11 
20 „ 
0-0094 
5‘ „ 
8 
20 „ 
0-0069 
10 „ 
34 
2 minutes. 
0-0049 
15 „ 
28 
2 
91 
0-0040 
20 „ 
22 
o 
0-0031 
30 „ 
36i 
4 „ 
0-0026 
60 „ 
25 
4 „ 
0-0018 
15 hours. 
11 
6 „ 
0-0005 
Glass No. 5. — Optical soft crown. Density 2*55. Composed of silica and potash, 
with lead and lime in small quantity. 
Experiments a. and /3. — 68 divisions of the electrometer-scale equal one Daniell’s 
element. 
The flask was charged for 5 seconds, insulated, and the loss in the subsequent 
10 seconds observed. The result may be regarded as giving an approximation to 
The mean of two experiments gives a fall from 471 to 452^, or 4/(1) =0*23. 
Charging for 45 seconds, and observing the loss during 30 seconds, gave •4/(l) = 0 , 06- 
The flask was connected with the battery continuously, and only insulated at intervals, 
and connected with the electrometer for a short time to determine the rate of loss. The 
following values are thence deduced : — 
t 5 . 10 . 30 . 60 . 120 . 180 . 300 . 
4(t) . . . 0-025 0-017 0-012 0-009 0-007+ 0-007- 0-006 
co ) probably does not differ much from 0-005 or 0-004. 
