118 
]\rESSRS. .T. HOPKINSON AND E. WILSON ON THE 
Capacity, 
(a.) Loiv Freqvency. 
Fig. 3 gives a diagram of connections, showing how the apparatus is arranged for 
the purpose of determining the capacity of poor insulators, such as window glass or 
ice, at varying temperatures. I'his is a bridge method, the flask F being placed in 
series with a condenser of known capacity K, and on the other side non-inductive 
resistances H^, By means of keys h,, the bridge can be connected to the poles 
of a Siemens alternator A; its potential difference is measured on a Kelvin multi¬ 
cellular voltmeter V. On the shaft of the alternator is fixed the revolving contact- 
maker B, which makes contact once in a period, and the epoch can be chosen. 
The Kelvin quadrant electrometer Q has one pair of quadrants connected to a pole 
of the revolving contact-maker B, and the other to a mercury cup 4 in a block of 
paraffin. The other terminal of B is connected to the junction between F and K ; by 
means of mercury cups 1, 2, 3, the electrometer can be connected through the contact- 
maker to either end, or to the middle of the bridge. 
The compensating resistance B is the resistance due to pencil lines drawn on fine 
obscured glass strip,'^' about 12 inches long and f inch wide, contact being made at 
each end by means of mercury in a small paraffin cup, and the whole varnished whilst 
hot with shellac varnish. A series of these resistances was made, ran^iner in value 
from a few megohms to a few tens of thousands of ohms. For the purpose of these 
experiments a knowledge of their actual resistance is of no moment, although for the 
purpose of manipulation their resistances are known. 
The method of experiment is as follows ;—Mercury cups 1 and 4 are connected bv 
a wire, placing the electrometer and contact-maker across F, and the contact-maker is 
moved until it indicates no potential. Cups 3, 4 are now connected, and resistance B 
is adjusted until the electrometer again reads zero. After a few trials, alternately 
* See ‘Phil. Mag.,’ ]\[arcli, 1S79 
