491 
of Edinburgh , Session , 1881-82. 
In the cases where R alone was charged we have 
V- Y 0 = — XJ. 
U P 
When both R and S were charged to the same potential. 
V - V 0 = 2— U . 
U p 
These formulae reproduce all the results quoted above ; they 
further indicate that the differences are proportional to the potential 
of the disturbing body, as was found to be the case in some rough 
experiments which were tried to test this conclusion. 
The fact that the presence of an oppositely charged body facili- 
tates discharge from a conductor, suggests the curious conclusion 
that a spark may under certain circumstances be caused to pass 
between two bodies by merely bringing up near them a body 
oppositely charged to that on which the point of maximum surface 
density is to be found. This experiment was repeatedly tried with 
perfect success. P was gradually charged up until the electrometer 
indicated that it was near the potential of rupture. Then the 
machine was stopped and R suddenly charged oppositely to P by 
connecting it with a large Ledyen jar standing ready for the purpose. 
The result was to. cause disruptive discharge between P and Q. A 
similar phenomenon no doubt sometimes occurs in nature. Suppose 
there were a positively charged thunder cloud at some distance from 
the earth, near, but not quite at, the point of discharging ; the advent 
of another cloud negatively charged, but at a smaller height, might 
cause the lightning to pass down from the higher cloud to the earth. 
Among the various circumstances that might affect the strength 
of a dielectric I thought, for reasons needless to relate, that it would 
be interesting to examine whether its magnetic state be material. 
With this view I arranged two spark terminals between the poles of 
a powerful electromagnet in different positions relatively to the lines 
of magnetic force and measured the difference of potential required 
to produce a spark when the magnet was excited and when it was 
not. The result was entirely negative. Although the magnetic 
force* employed went as high as 6000 absolute C. G. S. units, I 
* For the rough measurement of the strength of the magnetic field I am 
indebted to Mr. Albert Campbell. 
VOL. XI. 3 E 
