CAPACITY OF GLASS, AND OF LIQUIDS. 
359 
I shall not now minutely describe its details, it is sufficient to say that by connecting 
its middle to earth two condensers can be charged to equal and opposite potentials of 
500 elements. 
The guard-ring condenser .—This is the instrument of my former experiments, with 
the switch removed and some slight improvements in mechanical detail. It is by no 
means perfect in workmanship, and the irregularities of the results now to be given 
are to be attributed to such imperfections. It was not worth while to make a new 
instrument, as, for any present interest, determinations of capacities of glasses, correct 
to 1 per cent., are as valuable as if they were absolutely accurate. 
The sliding condensers .—Two sliding condensers were constructed possessing 
together a very considerable range of capacity. Each has a single scale and is 
used as before merely as a balance to the guard ring condenser, excepting in one 
experiment, the subject of the next section. 
The switch .—The switch formerly used performed the following operations:— 
Initially, the quadrant of the electrometer was to earth, the guard ring and the plates 
of the guard ring condenser were connected to one pole of the battery, the sliding 
condenser to the other pole. On turning the handle the quadrant and the condensers 
were insulated ; next, the charges of the condensers were mixed, the guard ring being 
put to earth at the same time; and, finally, the connected condensers were connected to 
the quadrant of the electrometer; they remained so connected until the handle of the 
switch was turned back into its first position. This instrument could not be used to 
Fig. 3. 
