﻿Mr. Rollo Appleyard on Dielectrics. 403 



11,103 scale-divs. Tlie instantaneous discharge from the 

 celluloid sheet (5 J in. diain., *00615 inch thick) with the same 

 voltage, was 115 divs., whicb is the mean of the two readings 

 obtained by reversing the battery. The test can scarcely be 

 regarded as a capacity test under the above conditions. It is 

 recorded here simply to show the phenomenon of slow dis- 

 charge, and the existence of the permanent initial E.M.F. 



Gutta-Pereha. 



Appended are the results of some tests upon a sheet of 

 pure gutta-percha, in the mercury apparatus, at high voltages. 

 The thickness of this sheet was about 2 mils ; the diameter of 

 the circular mercury surface in contact with it, bounded by 

 the insulating rings, was, as before, 5-g- inches. Curves drawn 

 from the tabulated results exhibit the phenomenon of dielectric 

 hysteresis, and show that the effect of the current, up to a 

 certain point, is to increase the resistance ; there is a subse- 

 quent tendency in the opposite direction, which would probably 

 end in the breaking-down of the sample. There w T as no initial 

 electromotive force to be observed with the gutta-percha. 



Four separate tests were made upon the sheet, two with a 

 cycle commencing at maximum volts, and two commencing 

 at the minimum. In each case, whether beginning at the 

 maximum or minimum volts, there was a decided increase of 

 resistance as the result of a cycle. With such a thin sheet as 

 2 mils the test was a very severe one for the gutta-percha ; 

 hence it is not surprising that the curves are somewhat broken. 

 Two-thousandths of an inch of material is here effectively 

 contending against 1200 volts, and setting up a barrier of 

 3000 million ohms over an area of 5J inches. 



A very interesting point about these four tests is that when 

 the cycle commences at maximum voltage the subsequent read- 

 ings are far steadier, and the curve of resistance and voltage 

 smoother, than when minimum volts are started with ; this is 

 clearly indicated by a comparison of the four curves which 

 may be drawn from these tables. 



There is no absolute need to short-circuit the apparatus in 

 order to remove the residual charge of the sheet ; but short- 

 circuiting probably quickens the process. This is a matter 

 for further experiment. It was found that the sample of 

 gutta-percha completely lost its charge after twenty-four 

 hours, whether short-circuited or not, and during that time it 

 regained its normal condition as regards insulation resistance. 

 Twenty-four hours was therefore allowed between each of the 

 four tests upon the gutta-percha sheet. As a rule, about 

 ten minutes' rest was given between the consecutive voltages 

 of individual tests ; this was found to be time enough to 

 practically discharge the sheet when short-circuited. 



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