Fig. 275. BOX BRIDGE.
437 STANDARD ELECTRICAL DICTIONARY.
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.
438 STANDARD ELECTRICAL DICTIONARY.
This establishes a Torricellian vacuum in B. The cock n is now turned,
giving the position X, when air is at once exhausted from the vessel
connected to C. This process is repeated until full exhaustion is
obtained. In practice the first exhaustion is often effected by a
mechanical pump. By closing the cock on the outlet tube c but little air
need ever find its way to the chambers o and B.

Fig. 276. GEISSLER AIR PUMP.
439 STANDARD ELECTRICAL DICTIONARY.
Pumping.
In incandescent lamps a periodical recurring change in intensity due to
bad running of the dynamos, or in arc lamps to bad feeding of the
carbons.

Fig. 277. SPRENGEL AIR PUMP.
Pump, Sprengel.
A form of mercurial air pump. A simple form is shown in the cut. Mercury
is caused to flow from the funnel A, through c d to a vessel B. A side
connection x leads to the vessel R to be exhausted. As the mercury
passes x it breaks into short columns, and carries air down between
them, in this way exhausting the vessel R. In practice it is more
complicated. It is said to give a better vacuum than the Sprengel pump,
but to be slower in action.
440 STANDARD ELECTRICAL DICTIONARY.
Pump, Swinburne.
A form of mechanical air pump for exhausting incandescent lamp bulbs.
Referring to the cut, A is a bulb on the upper part of a tube G; above A
are two other bulbs C and D. From the upper end a tube runs to the bulb
E. Through the cock L, and tube F connection is made with a mechanical
air pump. The tube H leads to a drying chamber I, and by the tube J
connects with the lamp bulbs or other objects to be exhausted. The tube
G enters the bottle B through an airtight stopper, through which a
second tube with stopcock K passes. In use a vacuum is produced by the
mechanical pumps, exhausting the lamp bulbs to a half inch and drawing
up the mercury in G. The bent neck in the bulb E, acts with the bulb as
a trap to exclude mercury from F. When the mechanical pumps have
produced a vacuum equal to one half inch of mercury, the cock L is
closed and K is opened, and air at high pressure enters. This forces the
mercury up to the vessel D, half filling it. The high pressure is now
removed and the mercury descends. The valve in D closes it as the
mercury falls to the level G. Further air from the lamps enters A, and
by repetition of the ascent of the mercury, is expelled, through D. The
mercury is again lowered, producing a further exhaustion, and the
process is repeated as often as necessary.

Fig. 278. SWINBURNE'S AIR PUMP.
Push-Button.
A switch for closing a circuit by means of pressure applied to a button.
The button is provided with a spring, so that when pushed in and
released it springs back. Thus the circuit is closed only as long as the
button is pressed. The electric connection may be made by pressing
together two flat springs, each connected to one of the wires, or by the
stem of the button going between two springs, not in contact, forcing
them a little apart to secure good contact, and thereby bridging over
the space between them.
441 STANDARD ELECTRICAL DICTIONARY.
Pyro-electricity.
A phenomenon by which certain minerals when warmed acquire electrical
properties. (Ganot.) The mineral tourmaline exhibits it strongly. It was
originally observed in this mineral which was found to first attract and
then to repel hot ashes.
The phenomenon lasts while any change of temperature within certain
limits is taking place. In the case of tourmaline the range is from
about 10º C. (50º F.) to 150º C. (302º F.) Above or below this range it
shows no electrification.
The effect of a changing of temperature is to develop poles, one
positive and the other negative. As the temperature rises one end is
positive and the other negative; as the temperature becomes constant the
polarity disappears; as the temperature falls the poles are reversed.
If a piece of tourmaline excited by pyro-electricity is broken, its
broken ends develop new poles exactly like a magnet when broken.
The following minerals are pyro-electric: Boracite, topaz, prehnite,
zinc silicate, scolezite, axenite. The following compound substances are
also so: Cane sugar, sodium- ammonium racemate and potassium tartrate.
The list might be greatly extended.
The phenomenon can be illustrated by sifting through a cotton sieve upon
the excited crystal, a mixture of red lead and flowers of sulphur. By
the friction of the sifting these become oppositely electrified; the
sulphur adheres to the positively electrified end, and the red lead to
the negatively electrified end. (See Analogous Pole-Antilogous Pole.)
Pyromagnetic Motor.
A motor driven by the alternation of attraction and release of an
armature or other moving part, as such part or a section of it is
rendered more or less paramagnetic by heat.
Thus imagine a cylinder of nickel at the end of a suspension rod, so
mounted that it can swing like a pendulum. A magnet pole is placed to
one side to which it is attracted. A flame is placed so as to heat it
when in contact with the magnet pole. This destroys its paramagnetism
and it swings away from the magnet and out of the flame. It cools,
becomes paramagnetic, and as it swings back is reattracted, to be again
released as it gets hot enough. This constitutes a simple motor.
A rotary motor may be made on the same lines. Nickel is particularly
available as losing its paramagnetic property easily.
442 STANDARD ELECTRICAL DICTIONARY.
Various motors have been constructed on this principle, but none have
attained any practical importance. Owing to the low temperature at which
it loses its paramagnetic properties nickel is the best metal for
paramagnetic motors.
In Edison's motor, between the pole pieces of an electro-magnet a
cylinder made up of a bundle of nickel tubes is mounted, so as to be
free to rotate. A screen is placed so as to close or obstruct the tubes
farthest from the poles. On passing hot air or products of combustion of
a fire or gas flame through the tubes, the unscreened ones are heated
most and lose their paramagnetism. The screened tubes are then attracted
and the armature rotates, bringing other tubes under the screen, which
is stationary. Then the attracted tubes are heated while the others
cool, and a continuous rotation is the result.

Fig. 279. EDISON'S PYROMAGNETIC MOTOR.
Pyromagnetic Generator.
A current generator producing electric energy directly from thermal
energy by pyromagnetism.
Edison's pyromagnetic generator has eight electro-magnets, lying on
eight radii of a circle, their poles facing inward and their yokes
vertical. Only two are shown in the cut. On a horizontal iron disc are
mounted eight vertical rolls of corrugated nickel representing
armatures. On each armature a coil of wire, insulated from the nickel by
asbestus is wound. The coils are all in series, and have eight
connections with a commutator as in a drum armature. There are two main
divisions to the commutator. Each connects with an insulated collecting
ring, and the commutator and collecting rings are mounted on a spindle
rotated by power. Below the circle of vertical coils is a horizontal
screen, mounted on the spindle and rotating with it.
A source of heat, or a coal stove is directly below the machine and its
hot products of combustion pass up through the coils, some of which are
screened by the rotating screen. The effect is that the coils are
subjecting to induction owing to the change in permeability of the
nickel cores, according as they are heated, or as they cool when the
screen is interposed. The two commutator segments are in constant
relation to the screen, and current is collected therefrom and by the
collecting rings is taken to the outside circuit.
443 STANDARD ELECTRICAL DICTIONARY.
Pyromagnetism.
The development of new magnetic properties or alteration of magnetic
sensibility in a body by heat. Nickel and iron are much affected as
regards their paramagnetic power by rise of temperature.