PHILOSOPHICAL TRANSACTIONS 
I. On the Effect of Temperature on the Specific Inductive Capacity of a Dielectric. 
By W. Cassie, M.A., Trinity College, Cambridge, Examiner in the Universities 
of Aberdeen and Durham. 
Communicated by J. J. Thomson, M.A., F.R.S., Cavendish Professor of Experimental 
Physics in the University of Cambridge. 
Received May 24,—Read June 20, 1889. 
The object of the experiments described in this paper was to learn how the specific 
inductive capacity of a dielectric is affected by change of temperature. 
Cavendish observed an increase in the capacity of a glass condenser when it was 
heated, but gave no measure of the effect. Dr. Hopkinson observed in light flint 
glass an increase of 2-g per cent, in the capacity between 12° and 83° C. And 
Messrs. Gibson and Barclay showed that there is no appreciable change in the case 
of paraffin between temperatures —12° and 24° C. Except these, no measurements of 
the effect appear to have been hitherto published. 
The present investigation shows an increase of specific inductive capacity with rise 
of temperature in all the solids'* examined, and a decrease in all the liquids except 
one. 
As paraffin, which is a substance comparatively near its melting point at ordinary 
temperatures, shows no change, these results seem to indicate, as far as they go, that 
the specific inductive capacity of a substance has a maximum value about the melting 
point. But it may be questioned whether the data are sufficient as yet to warrant so 
general an Induction. 
The relation which Clerk AIaxwell’s electromagnetic theory of light indicates 
between specific inductive capacity and refractive index makes it interesting to 
compare the effects of temperature on these two quantities. Four of the liquids 
* The results given here for mica, ebonite, and the first sjDecimen of glass are all less than those 
mentioned by Professor J, J. Thomson, in his treatise on “ Applications of Dynamics to Physics and 
Chemistry”; because the results quoted there were obtained from an earlier set of observations, in 
which the precautions for insulation were inferior to those described here. 
MDCCGXO. — A. B 
1.3.90 
