n I took that which corresponded to a single pair, that is, the
thirtieth part of the total tension. Other electrometers may be compared
with one already properly adjusted, without always having recourse to
the pile.

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.

_i_ is a copper wire covered with silk, by means of which the triangular
ring, _x_, and through that and its springs the conductor communicates
with either the electrometer or the electroscope.

Quickly raising the conductor by pulling the cord, _f_, the index of the
electrometer will describe a more or less large impulsive arc, and,
after two oscillations, will stop at the definite arc. Having thus
measured the electric tension of the air, and having lowered the
conductor, I next place the wire, _i_, in communication with the
electroscope, _B_, and by again raising the conductor, I ascertain
whether the electricity be positive or negative. It is scarcely
necessary to say that the conductor, when raised, gives electricity of
the same nature as that prevailing at the moment in the atmosphere; and
when lo