366 
DR. J. HOPKINSON ON THE ELECTROSTATIC 
It was assumed that on cooling the surface of the flask would continue to conduct to 
the level at which the acid had been. 
The following table gives the results of the experiment:— 
Temperature Centigrade. 
Capacity. 
o 
81 
2694 
11th Nov. 
48 
266 
55 
27 
2634 
55 
12 
262 
12th Nov. 
394 
2664 
55 
67| 
2684 
55 
83 
2714 
55 
60 
268 
55 
504 
267 
55 
13 
264 
13th Nov. 
We may conclude, I think safely, that the specific inductive capacity of light flint 
does increase slightly, but that the increase from 12° to 83° does not exceed 2^ per 
cent. The conductivity of the same glass'”' increases about 100-fold between the same 
temperature, and the residual charge also increases greatly. 
(IV.) Examination of the method of the fve-plate induction balance. 
The theoretical accuracy of this method rests on the assumption that the distance 
between the plates may be considered small in comparison with their diameter. 
When this condition is not sufficiently considered, it is easy to see that it is not 
likely that correct results will in all cases be obtained, for suppose that in lieu of the 
plate of glass a thin sheet of metal of considerable size is interposed between the 
fourth and fifth plates of the balance, it ought to be needful to withdraw the fifth 
plate by an amount equal to the thickness of the sheet. One can apprehend that it 
will be actually necessary to push it in, but to an extent which it would not be easy 
to calculate. 
Some doubt is also thrown upon the practical accuracy of the method by the fact 
that Mr. Gordon has arrived at the very unexpected result that the specific inductive 
capacities of glasses change with the lapse of time. 
In order to satisfy myself on the point I had a rough model of a five-plate induction 
balance made. The instrument is far too rough to give minutely accurate results if 
the method were good, but I believe it is sufficient to show rapidly that it cannot be 
used with safety. The insulation was not perfect, and no attempt was made to 
enclose the instrument or shield the connexions from casual inductive action. The 
plates are all 4 millims. thick ; they are, as in Mr. Gordon’s apparatus, 6 and 4 inches 
diameter. Each plate is suspended in a vertical plane by two rods and hooks from 
* “ Residual Chai’ge of the Leyden Jar,” Phil. Trans., 1877. 
