This done, let us see the arrangement of all the apparatus:
H H ([Plate VIIa], Fig. 1) is the ceiling of a well-situated lofty room, with an opening, o o, at the upper part.
M M, a bracket or table fastened against the wall, about a metre distant from the ceiling, H H.
N N, a wooden platform for the observer.
A, the bifilar electrometer.
B, Bohnenberger's electroscope.
a a, a movable conductor formed of a brass rod 15 to 18 millimetres in diameter, insulated below by means of a glass rod, well varnished with gum lac, having a suspending pulley, c, and a wooden guide-rod underneath it, l, within the guiding tube, k. At the upper part of this conductor, a a, there is a sliding roof, b, which can be adjusted so as to prevent rain entering at the opening, o o. The conductor terminates in a disc made of a sheet of thin brass, d, 24 centimetres in diameter. Upon this disc, or even in place of it, we may use metallic points.
As a support to the conductor at the upper part, I have made use of a triangular ring, x, drawn at its full size in Fig. 2. The conductor passes between three springs, and the triangular ring is held in place by three silk cords, m m m. Their material should not be mixed with any cotton, and it may be advisable to saturate them with an alcoholic solution of gum lac.
f f f is a hempen cord, which is used to raise and lower the conductor.