494 • Mr. R, Appleyard on 



for settling down, are brought into fair contact with the 

 dielectric, the contact is not so perfect as when mercury is 

 employed. It is curious to note here, as in all these ex- 

 periments, that where tinfoil electrodes are used the resistance 

 appears to decrease with the increase of voltage, whereas with 

 mercury contact this effect practically vanishes. I have 

 shown (Proc. Physical Soc. vol. xiii. p. 157, 1894) that with 

 celluloid sheets between unyielding metal plates, the change 

 of apparent resistance with voltage is remarkably great. It 

 is a matter of considerable importance in the study of dielec- 

 trics, and I have purposely described the experiments in 

 detail so as to endeavour to establish the fact that what is 

 often called change of dielectric resistance with voltage is, as 

 a rule, merely a surface effect, and that it can be greatly 

 reduced, if not entirely eliminated, by ensuring perfect con- 

 tact with the electrodes. Moreover, the effect is generally 

 found associated with so-called "negative electrification/' 

 the second-minute reading being greater than the first-minute 

 reading. 



In order to see how little the true resistance is affected by 

 abrupt changes of voltage, an examination may be made of 

 Tables III., IV., and VII. , where only one minute was allowed 

 between each change for the dielectric to lose its charge. The 

 residual effect can be estimated by switching off' the current at 

 the end of a given minute, and immediately connecting the 

 insulated electrodes through the galvanometer to earth. With 

 a view to avoiding too lengthy a table illustrating this, I have 

 calculated the values of [(deflexions) x (multiplying power of 

 shunt)] in the following case, omitting shunts, and omitting 

 megohms, from the tabulated figures. It may be stated, 

 however, that the megohms, worked out from the 1st, 11th, 

 21st, 31st, 41st, and 51st mins., are respectively 43*2, 42*6, 

 42*3, 42 2, 420, and 41*7. The procedure was to apply the 

 battery for 5 mins., and at the end of the 5th minute to switch 

 the battery out of circuit and connect the insulated electrode 

 to earth through the galvanometer, taking earth-readings for 

 five minutes. At the tenth minute the battery was reversed, 

 and the same process was continued in a cycle of reversals 

 and earthings up to the 53rd minute. In the following table 

 the reversals and earthings are sufficiently indicated by the 

 signs of the deflexions, and by the smallness of the earth- 

 readings in comparison with the battery readings. The fall 

 after each change of connexions was perfectly steady, and in 

 no instance was there anything approaching a rise after the 

 deflexion corresponding to a given change had been recorded. 

 On another day, after allowing the sheet to rest for 12 hours. 



