The Standard Electrical Dictionary / A Popular Dictionary of Words and Terms Used in the Practice of Electrical Engineering

by T. O'Conor Sloane

Fig. 260. DIAGRAM OF PRINCIPLE
OF THE DISPERSION PHOTOMETER.
The cut, Fig. 261, gives a perspective view of Ayrton's Dispersion
Photometer. C is the standard candle, L the concave lens, R the rod for
producing the two shadows on the screen S.

Fig. 261. AYRTON'S DISPERSION PHOTOMETER.
The mirror M is fixed at an angle of 45° with the stem on which it
rotates. The light of the arc lamp is received by the mirror and is
reflected through the lens. The candle holder slides along a graduated
bar C, and at D is an index plate to show the angle at which the spindle
carrying the mirror is set.
414 STANDARD ELECTRICAL DICTIONARY.
Dr. J. Hopkinson in his dispersion photometer uses a double convex lens.
This gives a focal image of the arc-lamp between the lens and screen,
whence the rays diverge very rapidly, thus giving the desired dispersion
effect.
It is principally for arc lamps that dispersion photometers are used.
Photometer, Shadow.
A photometer in which the relative intensity of the two lights is
estimated by the intensity or strength of shadows of the same object
which they respectively cast.

Fig. 262. RUMFORD'S SHADOW PHOTOMETER.

Fig. 263. RUMFORD'S SHADOW PHOTOMETER ARRANGED FOR TESTING
INCANDESCENT LAMPS.
415 STANDARD ELECTRICAL DICTIONARY.
A rod is supported in a vertical position. Back of it is a screen of
white paper. The two lights to be compared are arranged in front of the
rod and at a little distance from each other. They are shifted about
until the two shadows appear of equal darkness. The relative intensity
of the lights varies inversely with the square of their distances from
the shadows cast respectively by them on the screen.
The cut, Fig. 262, shows the simplest type of the shadow photometer. In
the cut, Fig. 263, a shadow photometer for testing incandescent lamps is
shown. In it E is the lamp under trial supported by a clamp H. A is an
ampere meter in circuit with the lamp, and V is a voltmeter. A candle C
can be moved along a graduated scale G G. R is the vertical rod, and S
is the screen on which the shadows fall.
Photophore.
An instrument for medical examination of the cavities of the body. It
includes an incandescent lamp mounted in a tube with a concave mirror
and convex lens.
Photo-voltaic Effect.
The change in resistance of some substances effected by light. Selenium,
of all substances, is most susceptible to this effect. (See Selenium.)
Piano, Electric.
A piano whose manual or key-board operates to close electric circuits,
whereby electro-magnets are caused to operate to drive the hammers
against the strings.
Pickle.
An acid solution for cleaning metal surfaces before electro-plating,
galvanizing or other deposition of metal upon them.
Picture, Electric.
A picture produced by passing a strong discharge through a piece of gold
leaf clamped or firmly pressed upon a sheet of paper. The gold leaf is
cut out of the desired shape, or else a stencil of paper overlays it.
The discharge dissipates the gold, and produces a purple colored
reproduction of the design upon the paper. The design is due to the
deposition of an exceedingly thin film of metallic gold.
Synonym--Electric Portrait.
Pile.
A galvanic or voltaic battery. It is sometimes restricted to a number of
voltaic couples connected. It should be only applied to batteries with
superimposed plates and no containing vessel such as the Dry Pile, q.
v., or Volta's Pile, q. v.
Pilot Transformer.
In alternating current distribution a small transformer placed at any
part of the system and connected to a voltmeter in the central station,
to indicate the potential difference of the leads.
Pilot Wires.
Wires brought from distant parts of electric light or power mains, and
leading to voltmeters at the central station, so that the potential of
distant parts of the system can be watched. The wires can be very small,
as they have but little current to transmit.
416 STANDARD ELECTRICAL DICTIONARY.
Pistol, Electric.
An experimental apparatus for exhibiting the power of electric
incandescence or of the electric spark. A tube is mounted with a handle
like a pistol. A plug is provided to screw in and out of its side. The
plug carries two wires connected on its inner side by a fine platinum
wire, or else disconnected but with their ends brought near together to
act as terminals for the production of a spark. To use it the tube is
filled with a mixture of air and gas, the latter either hydrogen,
hydro-carbon or other combustible gas. The tube when full is corked. The
wire is heated to incandescence by a current, or a spark is passed from
a Leyden jar or other source of electrostatic excitation. The mixture,
if properly proportioned, explodes and expels the cork violently.

Fig. 264. ELECTRIC PISTOL.
Pith.
A light and soft cellular tissue forming the central core of exogenous
trees and plants. In the older parts of the tree the woody tissue often
encroaches in and partly obliterates it.
For electrical pith-balls, the pith of the elder, of corn, or, best of
all, of sun-flower stems is used.
Pith-balls.
Ball made of pith. They are used in the construction of electroscopes
and for other experiments in static electricity.
They are cut out with a sharp knife and their shape may be improved by
gentle rolling in the hand or between the fingers.
Pivot Suspension.
Suspension poising or supporting of an object on a sharp pivot. This is
used for the needle in the ordinary compass. A cavity or inverted cup,
which may be made of agate, is attached to the middle of the needle
which has a hole for its reception. The centre of gravity of the needle
comes below the bottom of the cup.
Pivot suspension is not perfect, as it has considerable friction. There
is no restitution force, as with torsion filaments.
417 STANDARD ELECTRICAL DICTIONARY.
Plant.
The apparatus for commercial manufacturing or technical works. An
electric lighting plant includes the boilers, engines and dynamos for
producing the current, and the electric mains and subsidiary apparatus.
Plant Electricity.
Electricity manifested by plant life. By means of a galvanometer
potential differences are found to exist in different parts of trees or
fruits. The roots and interior portions are negative, and the flowers,
smaller branches and fruit are positive.
In some cases a contraction of the tissue of plants can be produced by
an electric current. The sensitive plant and others exhibit this
phenomenon, exactly analogous to the action of muscular tissue.
Plate, Arrester.
In a lightning arrester the plate connected to the circuit. Sometimes
both plates are designated arrester plates.
Plate Condenser.
A static condenser having a flat plate of glass for dielectric. (See
Epinus' Condenser.)
Plate Electrical Machine.
A frictional electric machine, in which a circular plate of glass is
excited by friction with the cushions. It is the most recent type of
frictional machine and has superseded the old cylinder machines. In its
turn it is superseded by influence machines, really plate machines, but
not so termed in practice.
Plate, Ground.
In a lightning arrester, the plate connected to the earth.
Plate, Negative.
In a voltaic battery, either primary or secondary, the plate which is
unattacked by the oxygen or negative radical or element of the fluid. It
corresponds to the carbon plate in the ordinary voltaic battery, and is
the one charged with positive electricity.
Plate, Positive.
In a voltaic battery, either primary or secondary, the plate which is
dissolved or attacked by the oxygen or negative radical or element of
the fluid. It is the plate corresponding to the zinc plate in the
ordinary voltaic battery, and is the one charged with negative
electricity.
Plating Balance.
A balance or scales to which articles in an electroplater's bath are
suspended. A weight exceeding by a known amount that of the article as
immersed overbalances the article. When the plating is being deposited
as soon as it exceeds the excess of weight of the counterpoise the
balance tips, the article descends a little, the electric circuit is
broken and the plating ceases. Thus the plating is automatically stopped
when a predetermined amount of metal is deposited.
418 STANDARD ELECTRICAL DICTIONARY.
Plating Bath.
A vessel of solution for the deposition of metal by electrolysis as used
in electro-plating.
Plating, Electro-.
The deposition of metal by electrolysis so as to coat the conducting
surface of objects therewith. The full details of the many processes are
very lengthy and cannot be given here.
The general principle includes a battery or source of electric current.
The object to be plated is connected to the negative terminal and is
immersed in the solution. Thus with a battery the object is in
electrical connection with the zinc plate. To the other terminal a
metallic plate is connected. The object and the plate termed the anode
being introduced into a suitable bath, the metal whose solution is in
the bath is deposited upon the surface of the object.
The bath is a solution of the metal in some form that will lend itself
to the electrolytic action. The anode is often a plate of the metal of
the bath, so that it dissolves as fast as metal is deposited on the
object, thus keeping up the strength of the solution.
The objects to be plated must be scrupulously clean, and great care must
be taken to keep the bath uncontaminated.
When the object has a non-conducting surface, it is made conducting by
being brushed over with plumbago q.v. In addition iron dust is sometimes
dusted over it. This acts by precipitating the metal of the bath
directly and thus giving a conducting basis for the metal to deposit on.
To avoid getting iron in a bath the object may be dipped in copper
sulphate solution. This precipitates copper in place of the iron and
leaves the article in good shape for silver or other plating.
Electro-plating, if made thick enough, gives a reverse of the article
when separated therefrom. A direct copy can be got by a second plating,
on the first plating after separation, or a wax impression can be
employed.
Under the different metals, formulae for the baths will be found. (See
also Quicking--
Steeling--Plating Balance.)
Platinoid.
An alloy of copper, nickel, zinc in the proportions of German silver
with 1 or 2 per cent of tungsten. It is used for resistances. It has a
specific resistance (or resistance per centimeter cube) of about 34
microhms. Its percentage variation in resistance per degree C. (1.8° F.)
is only about .021 per cent., or less than half that of German silver.
This is its most valuable feature.
419 STANDARD ELECTRICAL DICTIONARY.
Platinum.
A metal; one of the elements; symbol, Pt; atomic weight, 197.4;
equivalent, 49.35; valency, 4; specific gravity, 21.5.
It is a conductor of electricity.
The following data refer to the annealed metal at 0° C. (32° F.)
Relative Resistance (Silver annealed = 1), 6.022
Specific Resistance, 9.057 microhms.
Resistance of a wire,
(a) 1 foot long, weighing 1 grain, 2.779 ohms.
(b) 1 foot long, 1/1000 inch thick, 54.49 "
(c) 1 meter long, weighing 1 gram, 1.938 "
(d) 1 meter long, 1 millimeter thick, .1153 "
Resistance of a 1 inch cube, 3.565
Electro-chemical equivalent (Hydrogen = .0105), 0.5181.
The coefficient of expansion by heat is almost the same as that of
glass. It can be passed through holes in glass and the latter can be
melted about it so as to hermetically seal its place of passage through
the glass. It is used in incandescent lamps for leading-in wires and
other similar uses.
Platinum Black.
Finely divided platinum. It is made by boiling a solution of platinic
chloride with excess of sodium carbonate and a quantity of sugar, until
the precipitate is perfectly black and the supernatant liquid is
colorless. It seems to possess a great power of occluding oxygen gas.
When heated to redness it becomes spongy platinum. The negative plates
of a Smee battery are coated with platinum black.
Platinum-silver Alloy.
An alloy of 1 part platinum and 2 parts silver, used for resistance coils.
Relative Resistance (silver annealed = 1 ), 16.21 microhms.
Specific Resistance at 0°C. (32° F.), 24.39
Resistance of a wire,
(a) 1 foot long, weighing 1 grain, 4.197 ohms.
(b) 1 foot long, 1/1000 inch diameter, 146.70 "
(c) 1 meter long weighing 1 gram, 2.924 "
(d) 1 meter long, 1 millimeter diameter, 0.3106 "
Resistance of a 1 inch cube, 9.603 microhms.
Percentage Variation per degree C. (1.8° F.)
at about 20° C. (68° F.), 0.031 per cent.
Synonym--Platinum Alloy.
Platinum Sponge.
Finely divided platinum obtained by igniting platinum black, q.v., and
also by igniting salts of platinum. It has considerable power of
condensing or occluding oxygen. It will, if in good condition, set fire
to a jet of hydrogen impinging upon it.
Plow.
Contact arms projecting downwards from the motors, trucks, or bodies of
electric street cars, which enter the underground conduit through the
slot and carry contact pieces or brushes, to take the current for
driving the motors from the leads within the conduit.
420 STANDARD ELECTRICAL DICTIONARY.
Plücker Tubes.
A special form of Geissler tube designed for the production of
stratification and for observing the effects produced in the space
surrounding the negative electrode.
Plug.
(a) A piece of metal with a handle and a somewhat tapered end, used to
make connections by insertions between two plates or blocks of metal
slightly separated and with grooves to receive it.
(b) A plug or wedge with two metallic faces, insulated from each other
with a separate wire connected to each one. It is used in spring-jacks
q. v., to introduce a loop in a circuit.
Synonym--Wedge.
Plug. v.
To connect by inserting a plug, as in a resistance box.