﻿402 Mr. Rollo Appleyard on Dielectrics. 



" Residual Charge" 



The residual charge becomes a very small quantity a few 

 minutes after breaking circuit ; consecutive voltages can 

 therefore be applied without having to wait very long for the 

 practical disappearance of the last charge. In the above tests 

 it was made a rule never to send a current into a sheet until the 

 previous charge had vanished to within J per cent. Although, 

 under these conditions, the charge is quickly got rid of, yet 

 during the first few seconds the amount left in may be very 

 considerable. Conversely, the celluloid takes up its change 

 comparatively tardily. If, for instance, an attempt is m~de 

 to determine the capacity of the sheet, when in the mercury 

 apparatus, by the sinn^e discharge method, an eye which is 

 accustomed to watching the " throw " of the galvanometer will 

 at once observe that the needle is under the action of an ex- 

 tended discharge and not of a sudden impulse. When first 

 this phenomenoi was observed I carried out the following ex- 

 periment : — The celluloid vv as charged from 20 cells for 15 

 seconds. It was then discharged, not through the galvano- 

 meter, but throagh the short-circuit key. After two seconds 

 the short-circuit key was opened, and there was a discharge 

 of a few degrees to the right — that is, in the " discharge " 

 direction ; this fell in a few seconds across the zero to 5 

 divs. on the left, where it remained perfectly steady. This 

 curious reversal of sign was at first rather puzzling ; if the 

 spot had merely fallen to zero, it would have been easy to 

 refer it to complete loss of charge ; if it had fallen a little to 

 the right of zero, it could have been attributed to incomplete 

 loss of charge ; but falling, as it did, on the negative side of 

 zero, a novel case was presented. 



After several repetitions, charging with various potentials 

 up to as high as 900 volts, this reversal still manifested itself, 

 although the time required to attain it was of course greater 

 as the testing-voltage increased. Days and nights of short- 

 circuiting would not wipe it out ; whatever was - tried, it 

 remained always the same negative deflexion. It is, in fact, 

 an initial permanent E.M.F. of about '0006 of a volt with 

 which nothing seems to interfere. It is apparently due to 

 the contact of the mercury with the celluloid ; it did not 

 appear when metallic plates were used. This point having 

 been so far settled, the experiments on slow discharge were 

 repeated with 100, 200, 400, and 600 cells ; the corresponding 

 discharge-readings — after short-circuiting the sheet for two 

 seconds in each case before discharge— were respectively 18, 

 36, 57, and 80 divs., falling to zero, or very nearly to zero in 

 a few minutes, and ultimately crossing the zero to —5. The 

 discharge from a standard microfarad charged to 30 volts is 



