ON THE ELECTRICAL PROPERTIES OF PURE SULPHUR, 
139 
i + 8 X 10 8 in one case, 
and 1 + 2 X 10“ 7 in the other. 
Consequently our results are quite independent of leakage. Absorption, unless it 
be instantaneous, is guarded against by observing the point at which the kick 
vanishes. 
Some doubt having arisen as to the correctness of the formula obtained by Glaze- 
brook, Mr. A, Pollock was kind enough to look into the matter for us. His con¬ 
clusion is that instead of the formula quoted we ought to have 
G being the galvanometer resistance, the other letters retaining their meanings. 
This does not lead to any appreciable correction of our values. 
Now, when the sulphur condenser is heated, the difference in the coefficients of 
expansion of sulphur and aluminium must lead either to the film separating from the 
aluminium or to both it and the aluminium being thrown into a state of strain. We 
know that the plates remained firmly attached to the film, from a subsequent exarni- 
tion, and, consequently, we are driven to admit that heating strains both the film and 
the plates. Now the coefficients of linear expansion as obtained from Sir W. Thom¬ 
son’s ‘Collected Papers’ (vol. 3, p. 209), which numbers were taken from “Clark’s 
Constants of Nature,” are for sulphur, say, ’000063, and for aluminium, ’000022. To 
be quite satisfactory, of course, the linear expansion of this particular kind of sulphur 
ought to be observed, but the values given (mean of those of Ftzeau and Kopp) are 
probably not very far wrong. 
It is probable that when the aluminium plates are heated they are quite free to 
expand laterally and also vertically ; in fact, the gold dish is so weak and soft, it may 
be set on one side, and the sulphur round the aluminium plates will, if anything, tend 
to drag them outwards. We may assume, therefore, that when the condenser is 
heated the aluminium plates increase in area, and are subject to some little stress, 
while the sulphur film increases freely in thickness, but is prevented, by sticking to 
the plates, from expanding beyond their limits. The film being thin and weak, 
compared to the plates, nearly the whole of the resulting strain is borne by it. 
Consequently, any change in condenser capacity observed on heating will be due 
either to the direct effect of temperature change in altering dimensions or dielectric 
constant, or to the stresses thereby set up. We will assume the former alternative 
and see to what results it leads. 
The facts are, that when the sulphur is heated, say from 20° C. to 70° C., the mica 
standard and resistance boxes being at constant temperature, no change in the 
balance is observed; for instance, it remains steady at 2463 X 10 -G microfarad within 
one figure in the last place. 
