
DISRUPTIVE DISCHARGE OF ELECTRICITY. 635 
charge. After the discharge took place the image fell back and oscillated 
about zero or a point near it. 
The force resisting the deflection of the mirror was the action of two external 
magnets upon several small magnets fixed to the back of the mirror. The two 
external magnets were at the beginning of the experiments properly placed on the 
top of the case, and kept by melted paraffin in the same positions throughout. 
The distance between the electrodes, where the spark passed, was measured 
by resting a finely divided glass millimetre scale on the flat surface of the collar 
of the receiver, and adjusting the rod until a thin wire soldered to a ring form- 
ing the upper part of the rod coincided with a scratch on the scale. For some 
time we had no other means of inferring the pressure of the insulator inside 
the receiver than the indications of a gauge, the greatest range of which was 
200 mms.; but finally we had a barometer tube in connection. 
When the second system of conductors is charged to a positive potential, 
the potential of the third system is also positive, but necessarily less than that 
of the first. By increasing the distance between the balls A and B, the 
potential of the latter system may be reduced to any fraction of the potential 
of the former. Were this lessening not made, the potentials which formed the 
subject of the research would be too great to be measured by any electrometer. 
To ascertain minutely the law according to which the potential of the third 
system diminished when the distance of the ball B from the ball A was increased; 
I took the series of observations recorded in Tables I. and IL., and plotted on 
figs. 7 and 8. The insulated balls used in the series of observations of 29th 
May were of unequal diameters—2°5 inches and 2 inches respectively ; and they 
were supported by slightly dissimilar stems. They were replaced on the 12th 
June by the balls and stand represented in the sketch, and which were specially 
constructed for the purpose. The new balls are each of 2°25 inches diameter. 
The slide on which the ball B moves is graduated to half millimetres, so that 
its position during any series of observations can be accurately recorded. It 
was generally 42°75 centimetres, sometimes 32°75 centimetres. 
In addition to the difference in the balls, the constants were very different, 
being in the one case the difference of potential required to pass a spark 
between two ball electrodes of one inch diameter through 6 centimetres of air 
at a pressure of 160 mm.; and in the other between two discs 4 inches 
diameter through ‘5 centimetre of air at 740 mm. pressure. Yet the equations 
expressing the dependence of the potential of the third system upon the dis- 
tance between the centres of the insulated balls, are very similar. In the case 
of the first curve, the equation V =4522:67"-'—20:24, where V denotes the 
induced potential on the ball B, and 7 the distance of its centre from the centre 
of the ball A, satisfies all the observed values of 7 beyond 22°5 centimetres ; 
while the equation 
V = 2903'57 —* + 58°073 — 318077, 
