THE DIELECTRIC 119 



tube. As long as the taps are turned off, the trough represents a 

 perfect insulator, for the energy is undissipated. Release of the 

 trough corresponds to discharge, and we have oscillations corre- 

 sponding to the electric oscillations in the discharge of a condenser. 

 If the taps are all turned on equally the trough represents a leaky 

 dielectric of the same conductivity throughout, and holding it in 

 the displaced position for a time and then releasing it, it returns 

 to and remains, after the oscillations have ceased, in a position 

 short of the original position. But if the taps are turned on 

 unequally if, sav, the two end taps are turned off and the two 

 middle ones are turned on it is easily seen that the phenomena 

 of residual discharge are exactly imitated. For on turning the 

 trough through a given angle and holding it there, the energy in 

 the middle compartments decreases, and on release the trough only 

 moves part way back, going to the point at which the mean level 

 is the same on the two sides. There is now a negative difference of 

 level in the middle compartments if the original difference is 

 called p<itivr. If the trough is held in its new position for a 

 short time the negative difference is reduced, and on release the 

 trough returns by another amount towards its original position, 

 and this may be repeated several times until finally the original 

 position is practically regained. 



