488 Mr. R. Appleyard on 



Change of Resistance ivith Volts. — Mr. Rayner observed 

 considerable change of resistance according to the volts 

 applied. For example, using successively 200, 1000, and 

 200 volts he obtained respectively 22,000," 9600, and 18,500 

 megohms with a sample o£ press-spahn. These changes 

 appear to be excessive, and it is necessary to consider whether 

 they are in any way to be associated with the method of 

 applying the test. 



Absorption. — Another question of great importance is the 

 change of resistance with time, when the volts are constant. 

 Mr. Rayner, following ordinary usage, calculated the resist- 

 ance from the deflexion observed after " electrification " had 

 proceeded for 1 minute, and he states that the deflexion after 

 2 minutes did not differ by more than about 5 per cent, from 

 the deflexion obtained after 1 minute. In view of the 

 valuable information to be derived from observations of the 

 rate and direction of the change of deflexion, the actual 

 readings at the 1 minute and 2 minutes, respectively, should 

 be recorded. In what follows, this will be done. 



Surface Leakage. — In the apparatus employed for the 

 following tests, every part of the system was provided, where 

 necessary, with a Price's guard-wire. A preliminary test was 

 made to examine whether there was surface leakage between 

 the electrodes. For this purpose the edge of a sheet of 

 press-spahn was covered with tinfoil, and this was connected 

 to the guard-wire. It was found, however, that this did not 

 affect the deflexion. The surface* leakage was therefore 

 negligible. 



Temperature. — The temperature of the room during all 

 the following tests varied from 15° C. to 16° C. 



Humidity. — Press-spahn, as is proved by Mr. Rayner's 

 results, is very hygroscopic ; consequently the resistance 

 cannot be regarded as a fixed quantity. Each set of tests 

 must therefore be considered as distinct from those which 

 precede or follow it. It is assumed that the conditions in 

 regard to moisture remain constant during a short test. 



Method of Testing. — 'The direct-deflexion method was em- 

 ployed throughout, the readings being recorded at successive 

 minutes. In order to indicate the rate of change of deflexion, 

 the actual readings, or their equivalent, are in each case 

 tabulated. The dielectric-resistance is worked out from the 

 1 minute reading, and is expressed in megohms. 



* It may be pointed out that when I first put a guard-wire into 

 practical use, at Mr. Price's suggestion, he mentioned to me that one of 

 its advantages would be that in sheet-tests the surface leakage could be 

 eliminated in the manner here described. 



