3S6 Description and Use of a very sensible Electro'ineter, 
much nearer the ball, and brought into contact with it, if ne- 
cessary, till the gold-leaf is put in action. This degree of elec- 
tricity is so weak, that it would be absolutely insensible in the 
ordinary electrometer of Bennet. 
When the electricity is still feebler, we may advantageously 
employ a condenser adapted to the instrument. The eircular 
plate, on the margin of which is screwed the ball of the electro- 
meter replaces the cover of the condenser, and a plate or disc 
furnished with a glass handle, and which is placed above the 
first, represents the base. These plates are covered with a thin 
coating of amber-varnish on the faces which are brought into 
contact. If we wish to try a very weak electricity, we first 
touch, in order to deprive it of its electricity, the inferior plate, 
or the wire which carries the ball ; we then place above it the 
other plate, and afterwards touch the lower plate or its wire, 
with the body whose electricity we wish to examine, touching, 
at the same time, the upper plate, in order to deprive it of its 
electricity. The upper disc is then removed by its glass handle, 
and we observe towards which of the two small columns the 
gold-leaf is carried, and the sign marked upon this column will 
indicate the kind of electricity. If, for example, we put in con- 
tact with the lower surface of the lower plate of the condenser a 
a small disc of zinc, about fths of an inch in diameter, and press 
it against this plate, without touching the plate with the finger, 
and if we touch at the same time the upper disc of the con- 
denser, to deprive it of its electricity, and if we afterwards re- 
move the disc of zinc on one side, and on the other side the 
upper plate, we shall observe the gold-leaf approach the column 
marked minus, A similar effect will be observed, if we put in 
contact with the disc of the apparatus the metallic side of a 
piece of silvered paper. 
It will often be more convenient to put the body we wish to 
examine in contact with the upper and moveable plate, and 
touching the inferior plate, to deprive it of its electricity, pro- 
ceeding in other respects as we have already described. The 
electricity, however, which the instrument now indicates, will be 
opposite to that which is communicated to the upper plate, be- 
cause, by this method, the plate united to the instrument forms 
the base of the condenser. 
