718 
MR. J. J. THOMSON ON THE NUMBER OP ELECTROSTATIC 
The connexions are made in the following way. Let A denote the outer cylinder 
of the guard ring condenser, B the guard ring pieces, and C the inner cylinder; 
let A' and Cf denote the plates of the other condenser; T the armature of the 
Leyden-jar, which is not connected with the earth. Then A is connected with the 
earth; B to A 7 and to cl of the key, C to e of the key, Cf to f and T to a ; b and l 
are connected with the earth, and h is connected with the electrometer. 
Before the plunger is pushed down A is put to earth ; B and C to T; A' to T; 
C' to earth. 
When the plunger is pushed down, before it reaches e and g, A is to earth. B and 
C are charged and disconnected. A' and Cf have equal and opposite charges. 
When the plunger is pushed down a little further, so that cl comes into contact 
with l, B and A' are put to earth, so that the charges on C and O' are free to flow 
into the electrometer when the plunger goes a little further and strikes h. 
If the capacities of the two condensers are equal, the charges on C and Cf are equal, 
and of opposite signs, and when they flow together into the electrometer, their 
combined effect will be zero. The distance between the plates of the plate-condenser 
was altered, until the needle of the electrometer was not deflected when the plunger 
of the key was pushed down. This method was found to be very sensitive ; if after a 
balance had been obtained, the capacity of one condenser was altered by 1 per cent., 
the quantity of electricity sent to the electrometer was sufficient to drive the spot of 
light off the scale. 
The insulation of the two condensers and the key was tested several times, both 
electrostatically and by attempting to pass a current through them. If either con¬ 
denser was charged, and the key put in electrical connexion with it, the loss of 
charge in five minutes was not quite 2 per cent., so that the loss in the small time 
required to push the plunger down is quite negligible. When the condensers and 
the key were put in circuit with a battery of 150 Daniell’s elements, no current could 
be detected with a galvanometer whose resistance was 11,000 ohms. 
Part III. 
The determination of the electromagnetic measure of the capacity of the condenser 
without the guard ring. 
This was effected by the method described at the commencement of this paper. 
The galvanometer used had a resistance of about 11,000 ohms. It was insulated 
by placing it upon pieces of glass coated with paraffin. 
The battery consisted of 150 Daniell’s cells, coupled up with 25 Leclanche. The 
Daniell’s cells were put into trays, containing 10 each. The resistance of the battery 
was about 2500 ohms. The insulation of the battery gave a considerable amount of 
trouble, but the following plan was found successful. The case containing the trays 
