UNITS IN THE ELECTROMAGNETIC UNIT OF ELECTRICITY. 
715 
Substituting the values of a—b and b given above we find 
log y= *07705 
& b 
—^=•00027 
a —o- 
and the electrostatic measure of the capacity of the condenser is consequently 396’8. 
Part II. 
The comparison of the capacity of this condenser with that of one without a 
guard ring. 
As this condenser could not be worked by a commutator on account of the guard 
ring, it was necessary to compare its capacity with the capacity of a condenser 
without a guard ring. At first it was intended to compare the guard ring condenser 
with one of considerably greater capacity. Such a condenser was constructed, having 
a number of brass discs separated by thin pieces of shellac, the alternate discs being 
electrically connected, a wmight was placed upon the disc at the top to keep the 
system steady; and the system was placed in a vessel formed by putting a bell-jar 
on a surface plate. There w r ere two openings into this vessel, one of these was 
connected with a water pump ; the other with the air outside the jar by a series of 
tubes filled with cotton wool and chloride of calcium, to free the air passing through 
them from dust and moisture; air was then pumped through the vessel for about 
24 hours, and both openings were then closed. The capacity of this condenser was 
compared with that of the guard ring condenser, by connecting one plate of each 
condenser to earth, and the other with two points, P and Q, of a battery circuit; 
resistance boxes being placed between P and Q. A point O of the circuit between P 
and Q was then put to earth, and the resistance in the parts O P, O Q, so adjusted 
that when the charges of the two condensers were sent simultaneously into an electro¬ 
meter there was no deflection of the needle, showing that the charges in the two 
condensers were then equal and opposite. In this case, the capacities of the condensers, 
whose plates were connected with P and Q respectively, would bear the same ratio to 
each other as the resistance in O Q bears to the resistance in O P. With the battery- 
power obtainable, this method however was found not to be sufficiently sensitive, as the 
resistance in either of the arms O P, O Q, could be altered by about *75 per cent, 
without appreciably disturbing the equilibrium of the needle of the electrometer when 
the charges of the condenser were sent into it. It was therefore decided to make a 
condenser without a guard ring equal in capacity to the guard ring condenser, and 
employ the method given in § 229 of Maxwell’s ‘ Electricity and Magnetism,’ to 
determine when the two condensers were of equal capacity; this method can be made 
