Fig. 265. PLUGS FOR RESISTANCE COIL BOX.
Fig. 266. PLUG SWITCH.
Plug, Double.
A spring-jack plug or wedge with two pairs of insulated faces, one
behind the other, so as to simultaneously introduce two loops into a
circuit.
Plug, Grid.
A piece or mass of lead oxide, inserted into the holes in the lead
plates of storage batteries. The holes are often dovetailed or of uneven
section to better retain the plugs.
Plug Infinity.
In a box-bridge or resistance box, a plug whose removal from between two
disconnected discs opens the circuit. All the other discs are connected
by resistance coils of various resistance.
Plug Switch.
A switch composed of two contact blocks, not touching each other and
brought into electrical connection by the insertion of a metallic plug.
The latter is usually provided with an insulating handle, and a seat is
reamed out for it in the two faces of the contact blocks.
421 STANDARD ELECTRICAL DICTIONARY.
Plumbago.
Soft lustrous graphite, a native form of carbon; sometimes chemically
purified. It is used in electro-plating to give a conducting surface to
non-conducting objects, such as wax moulds. The surface, after coating
with plumbago, is sometimes dusted over with iron dust, which
precipitates the metal of the bath and starts the plating. It is
sometimes plated with copper, silver or gold, and is then termed
coppered, silvered, or gilt plumbago. It is gilded by moistening with
etherial solution of gold chloride and exposing to the air, and drying
and igniting.
Plunger.
A movable core which is used in connection with a so-called solenoid
coil, to be drawn in when the coil is excited. (See Coil and Plunger.)
Fig. 267 COIL AND PLUNGER WITH
SCALES TO SHOW ATTRACTION.
P. O.
Abbreviation for Post Office, q.v.
Poggendorf's Solution.
An acid depolarizing and exciting fluid for zinc-carbon batteries. The
following is its formula: Water, 100 parts; potassium bichromate, 12
parts; concentrated sulphuric acid, 25 parts. All parts by weight. Use
cold.
Point, Neutral.
(a) On a commutator of a dynamo the points at the ends of the diameter
of commutation, or where the brushes rest upon the surface of the
commutator, are termed neutral points. At these points there is no
generation of potential, they marking the union of currents of opposite
direction flowing from the two sides of the armature into the brushes.
(b) In electro-therapeutics, a place in the intra-polar region of a
nerve so situated with reference to the kathode and electrode as applied
in treatment, that its condition is unaffected.
Synonym--Indifferent Point.
(c) In a magnet the point of no attraction, situated between the two
poles, at about an equal distance from each, so as to mark the centre of
a magnet of even distribution of polarity.
(d) In thermo-electricity the point of temperature where the
thermo-electric powers of two metals are zero; in a diagram the point
where the lines representing their thermo-electric relations cross each
other; if the metals are arranged in a thermo-electric couple, one end
at a temperature a given amount above, the other at a temperature the
same amount below the neutral point, no current or potential difference
will be produced.
422 STANDARD ELECTRICAL DICTIONARY.
Point, Null.
A nodal point in electrical resonators; a point where in a system of
waves or oscillations, there is rest, the zero of motion being the
resultant of oppositely directed and equal forces. In electrical
resonators it is to be sought for in a point symmetrically situated,
with reference to the spark gap, or in a pair of points, which pair is
symmetrically placed.
The null point in resonators is found by connecting a lead from one of
the secondary terminals of an induction coil to different parts of the
resonator. The null point is one where the connection does not give rise
to any sparks between the micrometer knobs or spark gap, or where the
sparks are of diminished size.
The whole is exactly comparable to loops and nodes in a vibrating string
or in a Chladni plate as described in treatises on sound and acoustics.
(See Resonance, Electrical--Resonator, Electrical.)
Synonym--Nodal Point.
Point Poles.
Magnet poles that are virtually points, or of no magnitude. A long thin
magnet with little leakage except close to the ends may be supposed to
have point poles within itself a short distance back from the ends.
Points, Consequent.
In a magnet with consequent poles, the points where such poles are
situated.
Points, Corresponding.
In bound electrostatic charges the points of equal charges of opposite
potentials; the points at opposite extremities of electrostatic lines of
force. This definition implies that the bound charges shall be on equal
facing areas of conductors, as otherwise the spread or concentration of
the lines of force would necessitate the use of areas of size
proportionate to the spreading or concentrating of the lines of force.
At the same time it may figuratively be applied to these cases, the
penetration of the surface by a single line of force including the area
fixed by its relation to the surrounding lines.
Points, Isoelectric.
In electro-therapeutics, points of equal potential in a circuit.
423 STANDARD ELECTRICAL DICTIONARY.
Points of Derivation.
The point where a single conductor branches into two or more conductors,
operating or acting in parallel with each other.
Polar Angle.
The angle subtended by one of the faces of the pole pieces of the field-
magnet of a dynamo or motor. The centre of the circle of the angle lies
in the axis of the armature.
Synonym--Angle of Polar Span.
Polar Extension.
An addition made of iron to the poles of magnets. Various forms have
been experimented with. The pole pieces of dynamo field magnets are
polar extensions.
Synonyms--Pole Piece--Polar Tips.
Polarity, Diamagnetic.
The induced polarity of diamagnetic substances; it is the reverse of
paramagnetic polarity, or of the polarity of iron. A bar of diamagnetic
material held parallel with the lines of force in a magnetic field has a
like pole induced in the end nearest a given pole of the field magnet,
and vice versa. This theory accounts for the repulsion by a magnet of a
diamagnetic substance. The existence of this polarity is rather an
assumption. It originated with Faraday.
Polarity, Paramagnetic.
The induced polarity of paramagnetic substances, such as iron, nickel,
or cobalt.
When such a substance is brought into a magnetic field the part nearest
a specific pole of a magnet acquires polarity opposite to that of such
pole and is thereby attracted.
Another way of expressing it, in which the existence of a pole in or
near to the field is not implied, is founded on the conventional
direction of lines of force. Where these enter the substance a south
pole is formed and where they emerge a north pole is formed.
Such polarity tends always to be established in the direction of
greatest length, if the body is free to rotate.
424 STANDARD ELECTRICAL DICTIONARY.
Polarization.
(a) The depriving of a voltaic cell of its proper electro-motive force.
Polarization may be due to various causes. The solution may become
exhausted, as in a Smee battery, when the acid is saturated with zinc
and thus a species of polarization follows. But the best definition of
polarization restricts it to the development of counter-electro-motive
force in the battery by the accumulation of hydrogen on the negative
(carbon or copper) plate. To overcome this difficulty many methods are
employed. Oxidizing solutions or solids are used, such as solution of
chromic acid or powdered manganese dioxide, as in the Bunsen and
Leclanché batteries respectively; a roughened surface of platinum black
is used, as in the Smee battery; air is blown through the solution to
carry off the hydrogen, or the plates themselves are moved about in the
solution.
(b) Imparting magnetization to a bar of iron or steel, thus making a
permanent magnet, is the polarization of the steel of which it is made.
Polarization may be permanent, as in steel, or only temporary, as in
soft iron.
(c) The strain upon a dielectric when it separates two oppositely
charged surfaces. The secondary discharge of a Leyden jar, and its
alteration in volume testify to the strain put upon it by charging.
(d) The alteration of arrangement of the molecules of an electrolyte by
a decomposing current. All the molecules are supposed to be arranged
with like ends pointing in the same direction, positive ends facing the
positively-charged plate and negative ends the negatively-charged one.
(e) The production of counter-electro-motive force in a secondary
battery, or in any combination capable of acting as the seat of such
counter-electro-motive force. (See Battery, Secondary--Battery, Gas.)
The same can be found often in organized cellular tissue such as that of
muscles, nerves, or of plants. If a current is passed through this in
one direction, it often establishes a polarization or potential
difference that is susceptible of giving a return current in the
opposite direction when the charging battery is replaced by a conductor.
Polarization Capacity.
A voltaic cell in use becomes polarized by its negative plate
accumulating hydrogen, or other cause. This gradually gives the plate a
positive value, or goes to set up a counter-electro-motive force. The
quantity of electricity required to produce the polarization of a
battery is termed its Polarization Capacity or Capacity of Polarization.
Polarization of the Medium.
The dielectric polarization, q. v., of a dielectric, implying the
arrangement of its molecules in chains or filaments; a term due to
Faraday. He illustrated it by placing filaments of silk in spirits of
turpentine, and introduced into the liquid two conductors. On
electrifying one and grounding (or connecting to earth) the other one,
the silk filaments arranged themselves in a chain or string connecting
the points of the conductors.
Polar Region.
That part of the surface of a magnet whence the internal magnetic lines
emerge into the air. (S. P. Thompson.) As such lines may emerge from
virtually all parts of its surface, the polar regions are indefinite
areas, and are properly restricted to the parts whence the lines emerge
in greatest quantity.
Polar Span.
A proportion of the circle which represents the transverse section of
the armature space between the pole pieces of the field magnet in a
dynamo or motor; it is the proportion which is filled by the faces of
the pole pieces.
425 STANDARD ELECTRICAL DICTIONARY.
Pole, Analogous.
The end of a crystal of a pyroelectric substance, such as tourmaline,
which end when heated become positively electrified. On reduction of
temperature the reverse effect obtains.
Pole, Antilogous.
The end of a crystal of a pyroelectric substance, such as tourmaline,
which end, while increasing in temperature, becomes negatively
electrified. During reduction of its temperature the reverse effect
obtains.
Pole Changer.
(a) An automatic oscillating or vibrating switch or contact-breaker
which in each movement reverses the direction of a current from a
battery or other source of current of fixed direction, as such current
goes through a conductor.
(b) A switch moved by hand which for each movement effects the above
result.
Pole, Negative.
(a) In a magnet the south pole; the pole into which the lines of force
are assumed to enter from the air or outer circuit.
(b) In a current generator the pole or terminal into which the current
is assumed to flow from the external circuit. It is the negatively
charged terminal and in the ordinary voltaic battery is the terminal
connected to the zinc or positive plate.
Pole Pieces.
The terminations of the cores of field or other electro-magnets, or of
permanent magnets. These terminations are variously shaped, sometimes
being quite large compared to the core proper of the magnet.
They are calculated so as to produce a proper distribution of and
direction of the lines of force from pole to pole. As a general rule the
active field should be of uniform strength and the pole pieces may be of
contour calculated to attain this end.
Pole, Positive.
(a) In a magnet the north pole; the pole from which lines of force are
assumed to emerge into the air.
(b) In a current generator the pole or terminal whence the current is
assumed to issue into the outer circuit. It is the positively charged
terminal, and in the ordinary voltaic battery is the terminal connected
to the copper or carbon plate, termed the negative plate.
Poles.
(a) The terminals of an open electric circuit, at which there
necessarily exists a potential difference, produced by the generator or
source of electro-motive force in the circuit.
(b) The terminals of an open magnetic circuit; the ends of a magnetized
mass of steel, iron or other paramagnetic substance.
(c) The ends in general of any body or mass which show electric or
magnetic properties more developed than those of the central sections of
the body.
426 STANDARD ELECTRICAL DICTIONARY.
Pole, Salient.
In dynamo and motor field magnets, salient poles are those projecting
from the base or main body of the field magnet, as distinguished from
consequent poles formed by coils wound on the main body itself.
Fig. 268. SALIENT POLES OF FIELD MAGNET.
Poles, Compensating.
A device for avoiding the cross-magnetizing effect on the commutator
core due to the lead of the brushes. It consists in maintaining a small
bar electro-magnet perpendicularly between the pole pieces. This
compensates the cross-magnetizing effect.
Poles of Intensity.
The locus of highest magnetic force on the earth's surface. One such
pole is in Siberia, another is about lat. 52° N., long. 92° W.
[Transcriber's note: 52° N., long. 92° W is about 250 miles Northeast of
Winnipeg.]
Poles of Verticity.
The magnetic poles of the earth. (See Magnetic Poles.)
Pole Tips.
The extreme ends of the expanded poles of a field magnet. In some
machines some of the pole tips are made of cast iron, to alter the
distribution of the lines of force and resulting magnetic pull upon the
armatures. This is done to take off the weight of the armature from its
bearings.
Pole, Traveling.
A term applied to the poles produced in the action of a rotatory field,
whose poles constantly rotate around the circle of the field. (See
Field, Rotatory.)
417 STANDARD ELECTRICAL DICTIONARY.
Porous Cup.
A cup of pipe clay, unglazed earthenware or other equivalent material
used in voltaic cells to keep two liquids separate and yet to permit
electrolysis and electrolytic conduction.
They are necessarily only an expedient, as their porous nature permits
considerable diffusion, and were they not porous electrolytic action
would be impossible.
Synonym--Porous Cell.
Porret's Phenomenon.
In electro-physiology, an increase in the diameter of a nerve produced
by the positive pole of a voltaic circuit, when placed in contact with
the tissue and near to the nerve in question, the other pole being
connected to a more or less remote part of the body.
Portelectric Railroad.
A railroad worked by solenoidal attraction, the car forming the core of
the solenoids. It includes a series of solenoids or hollow coils of
copper wire distributed all along the road and inclosing within
themselves the track. On this a cylindrical car with pointed ends moves
on wheels. Current is supplied to the solenoid in advance of the car,
and attracts it. As it advances it breaks the contacts of the attracting
solenoid and turns the current into the one next in advance. This
operation is repeated as the car advances.
The solenoids are placed close together, each including in the trial
track 630 turns of No. 14 copper wire. The car was of wrought iron, 12
feet long, 10 inches in diameter and weighing 500 lbs. It was proposed
to employ the system for transportation of mail matter and similar uses.
Position Finder.
An instrument for determining the position of objects which are to be
fired at from forts. It is designed for use from forts situated on the
water.
Fiske's position finder may be thus generally described. On a chart the
channel is divided into squares, and the position finder determines the
square in which a vessel lies. For each square the direction and
elevation of the guns is calculated beforehand. The enemy can therefore
be continuously located and fired at, although from smoke or other cause
the object may be quite invisible to the gunner.
It comprises two telescopes situated at distant extremities of as long a
base line as is obtainable. These telescopes are kept directed upon the
object by two observers simultaneously. The observers are in constant
telephonic communication. As each telescope moves, it carries a contact
over an arc of conducting material. Below each telescope is an arm also
moving over an arc of conducting material. These arcs enter into a
Wheatstone bridge and are so connected that when the arm and the distant
telescope are at the same angle or parallel a balance is obtained. Thus
each observer has the power of establishing a balance. A chart is
provided for each of them, and over it the arm connected with the
distant telescope and an arm or indicator attached to the telescope at
that station move so that as long as both telescopes point at the object
and each observer maintains the electric balance, the intersection of
the arms shows the position on the chart.
The Position Finder is a simplification and amplification of the Range
Finder, q. v. In practice the observers may be placed far from the
forts, and may telephone their observations thereto. It has been found
accurate within one-third of one per cent.
428 STANDARD ELECTRICAL DICTIONARY.
Positive Direction.
The direction which lines of force are assumed to take in the air or
outer circuit from a positive to a negative region. It applies to
electrostatic, to magnetic and to electro-magnetic lines of force.
Positive Electricity.
The kind of electricity with which a piece of glass is charged when
rubbed with silk; vitreous electricity.
In a galvanic cell the surface of the copper or carbon plate is charged
with positive electricity. (See Electrostatic Series.)
According to the single fluid theory positive electrification consists
in a surplus of electricity.
[Transcriber's note: "Positive electricity" is a deficiency of electrons.]
Post Office. adj.
Many pieces of electric apparatus of English manufacture are thus
qualified, indicating that they are of the pattern of the apparatus used
by the British Post Office in its telegraph department.
Potential.
Potential in general may be treated as an attribute of a point in space,
and may express the potential energy which a unit mass would have if
placed at that point.
This conception of potential is that of a property attributable to a
point in space, such that if a unit mass were placed there the forces
acting upon it would supply the force factor of energy, while the body
would supply the mass factor. This property is expressible in units,
which produce, if the supposed mass is a unit mass, units of work or
energy, but potential itself is neither.
Thus taking gravitation, a pound mass on the surface of the earth
(assuming it to be a sphere of 4,000 miles radius) would require the
expenditure of 21,120,000 foot pounds to remove it to an infinite
distance against gravity. The potential of a point in space upon the
surface of the earth is therefore negative and is represented by
-21,120,000*32.2 foot poundals (32.2 = acceleration of gravity). (See
Poundal.) In practice and conventionally all points on the earth's
surface are taken as of zero potential.
[Transcriber's note; 21,120,000 foot pounds is about 8 KWh.]
429 STANDARD ELECTRICAL DICTIONARY.
Potential, Absolute.
The absolute electrical potential at a point possesses a numerical value
and measures the tendency which the existing electric forces would have
to drive an electrified particle away from or prevent its approach to
the point, if such a particle, one unit in quantity, were brought up to
or were situated at that point. It is numerically equal to the number of
ergs of work which must be done to bring a positive unit of electricity
from a region where there is absolutely no electric force up to the
point in question. (Daniell.) Two suppositions are included in this. The
region where there is an electric force has to be and only can be at an
infinite distance from all electrified bodies. The moving of the
particle must take place without any effect upon the distribution of
electricity on other particles.
Potential, Constant.
Unchanging potential or potential difference.
The ordinary system of incandescent lighting is a constant potential
system, an unvarying potential difference being maintained between the
two leads, and the current varying according to requirements.
Potential Difference, Electric.
If of any two points the absolute potentials are determined, the
difference between such two expresses the potential difference.
Numerically it expresses the quantity of work which must be done to
remove a unit of electricity from one to the other against electric
repulsion, or the energy which would be accumulated in moving it the
other way.
A positively charged particle is driven towards the point of lower
potential. A negatively charged body is driven in the reverse direction.
Potential Difference, Electro-motive.
A difference of potential in a circuit, or in part of a circuit, which
difference produces or is capable of producing a current, or is due to
the flow of such current.
It may be expressed as the fall in potential or the electro-motive force
included between any two points on a circuit. The current in an active
circuit is due to the total electro-motive force in the circuit. This is
distributed through the circuit in proportion to the resistance of its
parts. Owing to the distribution of electro-motive force throughout a
circuit including the generator, the terminals of a generator on closed
circuit may show a difference of potential far lower than the
electro-motive force of the generator on closed circuit. Hence potential
difference in such a case has been termed available electro-motive
force.
Potential, Electric Absolute.
The mathematical expression of a property of a point in space, measuring
the tendency which existing electric forces would have to drive an
electrified unit particle away from or prevent its approach to the point
in question, according to whether the point was situated at or was at a
distance from the point in question.
Potential is not the power of doing work, although, as it is expressed
always with reference to a unit body, it is numerically equal to the
number of ergs of work which must be done in order to bring a positive
unit of electricity from a region where there is no electric
force--which is a region at an infinite distance from all electrified
bodies--up to the point in question. This includes the assumption that
there is no alteration in the general distribution of electricity on
neighboring bodies. (Daniell.)
In practice the earth is arbitrarily taken as of zero electric potential.
430 STANDARD ELECTRICAL DICTIONARY.
Potential, Fall of.
The change in potential between any two points on an active circuit. The
change in potential due to the maintenance of a current through a
conductor.
The fall in potential multiplied by the current gives work or energy
units.
The fall of potential in a circuit and its subsequent raising by the
action of the generator is illustrated by the diagram of a helix. In it
the potential fall in the outer circuit is shown by the descent of the
helix. This represents at once the outer circuit and the fall of
potential in it. The vertical axis represents the portion of the circuit
within the battery or generator in which the potential by the action of
the generator is again raised to its original height.
In a circuit of even resistance the potential falls evenly throughout
it.
A mechanical illustration of the relation of fall of potential to
current is shown in the cut Fig. 269. A vertical wire is supposed to be
fixed at its upper end and a lever arm and cord at its lower end, with
weight and pulley imparts a torsional strain to it. The dials and
indexes show a uniform twisting corresponding to fall of potential. For
each unit of length there is a definite loss of twisting, corresponding
to fall of potential in a unit of length of a conductor of uniform
resistance. The total twisting represents the total potential
difference. The weight sustained by the twisting represents the current
maintained by the potential difference. For a shorter wire less twisting
would be needed to sustain the weight, as in a shorter piece of the
conductor less potential difference would be needed to maintain the same
current.
Fig. 269. MECHANICAL ILLUSTRATION OF
FALL OF POTENTIAL AND CURRENT STRENGTH.
431 STANDARD ELECTRICAL DICTIONARY.
