489 
of Edinburgh, Session 1881-82. 
on disruptive discharge. R and S are two parallel metal discs faced 
with plates of glass aa and bb , to prevent discharge between them 
and P or Q. R can be raised to any required potential by charging 
the Leyden jar L. Q and S, the outer coating of L, and the other 
electrode of the Holtz machine, are all connected with the earth. 
For a given distance PQ of 6 mm. or so, provided the distance, 
RS is not too small, the mere presence of R and S does not sensibly 
increase or diminish the difference of potential required to produce 
a spark between P and Q. But^the case is very different if R be 
raised to a considerable potential, as will be seen by the following 
tables. In the columns headed P and R, the sign of the charge of 
these conductors is given ; under V, the difference of potential (in 
an arbitrary unit) required to produce the spark ‘ in the last column 
the difference between the two values of Y. 
p 
R 
Y 
Difference. 
+ 
■T7 
129 
- 20 
■> 
+ 
0 
149 
... 
PQ = 6 mm. 
+ 
+ 
160 
+ 27 
RS = 25 mm. 
+ 
0 
133 
J 
+ 
+ 
142 
+ 22 
'■ 
+ 
0 
120 
•• 
- 
PQ= 3 mm. 
+ 
- 
107 
-25 
RS = 25 mm. 
+ 
0 
132 
... 
J 
+ 
140 
-30 
- 
0 
170 
... 
h 
PQ = 5‘5 mm. 
- 
-- 
190 
+ 20 
RS= 25 mm. 
- 
0 
170 
... 
J 
A modification of the experiment was made by insulating S, and 
charging and discharging it along with R, with the following result : — 
