256
XIV
RESIDUAL CHARGE
formed the inner coating, and the zinc canister the outer
coating of a Leyden jar, of which the benzene formed the
dielectric. The inner coating was connected with one pair of
quadrants—not earthed-of an electrometer; and by means
TE
FIG. 31.
B
of the key a this coating could be con-
nected to earth. The outer coating
could either be connected by y to the
earth as well, or by B to one pole of a
constant battery of 100 small Daniell
cells, the other pole of which was kept
at zero potential by an earth-connection.
If we now suppose the circuit to be
closed at a and ß, and open at y, the
needle of the electrometer will clearly
stand at zero, and the circuit will be traversed by a current
whose strength will depend upon the resistance of the benzene.
If the circuit is now broken at a, the inner coating strives to
charge itself to the potential of the outer, and hence the
electrometer needle is deflected in the direction in which it
would move if the unearthed quadrants were directly connected
with the insulated pole of the battery. This we shall call
the positive direction. The rate of deflection of the needle
enables us to measure the resistance of the benzene. The
capacity of the electrometer was to that of the benzene con-
denser in the ratio of 4.5:1.¹ The whole potential of the
battery would have deflected the needle 5500 scale-divisions
from its position of rest. Now suppose that the connection
to the electrometer at 8 was broken one second after opening
the circuit at a, and that the electrometer was found to give
a deflection of a scale-divisions. The difference of potential
of the coatings would then have sunk in a second through
a/5500 of its value. In the absence of the electrometer it
would have fallen (4.5+ 1) times as rapidly, i.e. through
a/1000 in a second, or to 1/e¹ of its value in 4. 1000/a
seconds. This latter time is to be divided by the specific
inductive capacity of benzene in order to obtain the specific
resistance in absolute electrostatic measure.
In this way,
then, we can measure the resistance. In order to observe
1 The large apparent capacity of the quadrants is due to the strong charge on
the electrometer needle.