XIV
259
RESIDUAL CHARGE
Scarcely a trace of residual charge can be detected, and its
insulating power is not far short of that of our best insulators.
On account of the evaporation which took place, the experi-
ments could not be extended much beyond twenty-four hours,
and at the end of this time a definite limit to the resistance
had not yet been reached. But the insulating power may be
judged from the fact that in a minute after breaking the
earth-connection at a, the electrometer needle had only moved
six scale divisions from its position of rest. From this, and
from the ratio of the capacities given above, it follows that a
Leyden jar containing this benzene as dielectric would require
two hours to lose half its charge.
6. The residual charge exhibited by impure benzene arises
from polarisation, which produces some kind of after-effect,
and not from an absorption of free electricity. This can be
proved by the following experiments. After breaking the
circuit at a, just at the instant when the residual charge
begins to exhibit itself, the benzene is allowed to run out
through an opening in the bottom as quickly and quietly
as possible; the residual charge now makes its appearance
suddenly, and its sign is the same as it would have been if
the benzene had not been run out. If the residual charge
were due to an absorption of electricity by the dielectric, the
removal of this dielectric would certainly be followed by a
sudden deflection; but the sign of this would have been
opposite to that of the electricity removed with the dielectric,
and therefore opposite to that of the original residual charge.
In another experiment I polarised impure benzene in a large
vessel between two plates, A and B, which were several centi-
metres apart. A and B were then brought to the potential
zero; and a system of three other plates, 1, 2, and 3, was
immediately introduced into the space between them. The
outer plates 1 and 3 were connected to earth and to one pair
of quadrants of an electrometer. Plate 2, which was connected
to the other pair of quadrants, was closely attached to 3, but
insulated from it. Thus no benzene could enter between 2
and 3, whereas there was a layer a few centimetres thick
between 1 and 2. On introducing this system there was an
immediate deflection in the galvanometer, the direction of
which changed with the direction of polarisation of the benzene;