Proportionate Arms. In general terms the arms of a Wheatstone bridge
whose proportion has to be known to complete the measurement. There is a
different system of naming them. Some designate by this title the two
arms in parallel with each other branching at and running from one end
of the bridge to the two galvanometer connections. In the cut of the Box
Bridge, A C and A B are the proportionate arms. The third arm is then
termed the Rheostat arm. (Stewart & Gee.)
Others treat as proportionate arms the two side members of the bridge in
parallel with the unknown resistance and third or rheostat arm.
Synonym--Ratio Arms.
Prostration, Electric.
Too great exposure to the voltaic arc in its more powerful forms causes
symptoms resembling those of sunstroke. The skin is sometimes affected
to such a degree as to come off after a few days. The throat, forehead
and face suffer pains and the eyes are irritated. These effects only
follow exposure to very intense sources of light, or for very long
times.
[Transcriber's note: Arcs emit ultraviolet rays.]
Protector, Comb.
A lightning arrester, q. v., comprising two toothed plates nearly
touching each other.
Protector, Electric.
A protective device for guarding the human body against destructive or
injurious electric shocks. In one system, Delany's, the wrists and
ankles are encircled by conducting bands which by wires running along
the arms, back and legs are connected. A discharge it is assumed
received by the hands will thus be short circuited around the body and
its vital organs. India rubber gloves and shoe soles have also been
suggested; the gloves are still used to some extent.
Pull.
A switch for closing a circuit when pulled. It is used instead of a push
button, q.v., in exposed situations, as its contacts are better
protected than those of the ordinary push button.
Pump, Geissler.
A form of mercurial air pump. It is used for exhausting Geissler tubes,
incandescent lamp bulbs and similar purposes.
Referring to the cut, A is a reservoir of mercury with flexible tube C
connected to a tube at its bottom, and raised and lowered by a windlass
b, the cord from which passes over a pulley a. When raised the mercury
tends to enter the chamber B, through the tube T. An arrangement of
stopcocks surmounts this chamber, which arrangement is shown on a larger
scale in the three figures X, Y and Z. To fill the bulb B, the cocks are
set in the position Z; n is a two way cock and while it permits the
escape of air below, it cuts off the tube, rising vertically from it.
This tube, d in the full figure connects with a vessel o, pressure gauge
p, and tube c, the latter connecting with the object to be exhausted.
The bulb B being filled, the cock m is closed, giving the position Y and
the vessel A is lowered until it is over 30 inches below B.