746 Prof, Townsend and Mr. Hurst on the 
the insulators, although there was perfect insulation for 
potentials of about 300 volts. The accompanying rough 
sketch shows the form of the apparatus, and will serve to 
explain the method which was adopted to get rid of the 
irregular deflexions of the electrometer. 
lo Electrometer 
rm 
500 
The metal plates B and © were held together by four 
ebonite pillars HK, and the conductivity takes place between 
the plates A and B. The distance between the plates A and 
B was adjustable by means of the micrometer-screw 8S. The 
upper plate was in connexion with the insulated quadrants 
of an electrometer, and the lower plate B was in connexion 
with a terminal of a battery of cells and was thus maintained 
at any required potential V. 
A little consideration will show that the insulation may be 
very irregular when the potential V is raised to 500 volts 
although there is good insulation for a potential of 340 volts. 
It would be almost impossible to ensure that the insulation 
of the supports E should be equally good along the whole 
length from B to C, so that a charge would creep gradually 
over the surface of the insulator and the electric force parallel 
to the surface would no longer be uniform. — For potentials 
less than 340 volts this would produce a small uniform drift 
of the electrometer, but for higher potentials it would be 
quite possible for the surface charges to concentrate the 
electric force between two points on the surface of the 
insulators so as to give rise to a minute discharge in 
the gas. 
The irregular disturbance of the insulation was attributed 
to small discharges arising from effects of this kind, and in 
order to insulate the plate B rings of tinfoil T were fixed 
round the supports EK and maintained at a potential less than 
V, for example at 200 volts when the lower plate was 500 volts. 
The potential-difterence between any two adjacent points on 
the surface of the insulator could not then attain a sufficient 
value to produce a discharge in the gas near the ebonite 
illar. 
i It was found that the electrometer insulated perfectly when 
the lower ,plate was at 500 and the rings at 200 volts, but 
