364 
DR. J. HOPKENSON ON THE ELECTROSTATIC 
Remark. —On account of the small thickness of the equivalent plate of air, inch, 
this result is subject to a greater probable error than the others. No inconvenience 
or uncertainty was experienced from the effect of residual charge. If the screw 
(to, figs. 3, 4, pp. 359, 360) be lowered so that contact with the electrometer is not 
broken, observation becomes at once impossible. 
These results show that my former experiments require no material correction, 
except in the case of plate glass, for which an accurate experiment was formerly 
impossible. They also show that electrostatic capacity does not depend on electro¬ 
motive force up to 200 volts per centimetre for double extra-dense flint, and a some¬ 
what higher electromotive force for the other glasses. It is desirable to show that the 
same is true for a wider range. 
Paraffin. Thickness, 20T9. 
I. 
II. 
III. 
IV. 
Resting on lower .... 
23'82 
12-42 
879 
Ditto .... 
2271 
11-32 
8-80 
Ditto .... 
21-37 
9-96 
8-78 
Contact with both . . 
20-19 
8-78 
8-78 
Mean value of K = 2'29. 
In this case the guard ring condenser was always charged with 700 elements, the 
slide with 300 in order that the same sliding condenser might be used. 
Boltzmann gives 2'32 for paraffin for short times of discharge. 
(III.) To show that K is a constant, as is generally assumed. 
Dr. De La Rue very kindly allowed me to try a few preliminary experiments last 
February with his great chloride of silver battery. A flask of extra-dense flint glass 
was used, insulated with sulphuric acid precisely as in my experiments on residual 
charge. The comparison was made with a large sliding condenser having a scale 
graduated in millimetres. Taking one division of the scale (= about 0'0000026 
microfarad) as a temporary unit of capacity, I found it impossible to say wdiether the 
capacity of the flask was greater or less than 390 divisions, whether the charge in each 
condenser was 20 elements or 1800 elements. Subsequently a similar experiment 
was tried with my own battery and a flask of light flint, with the following results, 
each being the mean of four readings :— 
