Fig. 253. OHMMETER.
Ohmmeter.
An instrument for measuring directly the resistance of a conductor or of
any part of a circuit through which a strong current is passing. It is
the invention of Prof. W. E. Ayrton.
It contains two fixed coils at right angles to each other acting on the
same needle of soft iron. One coil is of thick wire and is placed in
series with the resistance to be measured. The other is of very thin
wire and is placed in parallel with the same resistance. One wire acts
by the total current, the other by the potential difference between the
ends of the resistance. The action on the soft iron needle is due to the
ratio of potential difference to total currents, or to the resistance
itself. By properly designing and proportioning the coils the angular
deflections of the needle are made proportional to the resistance.
In use the thick wire may be kept permanently in circuit. On connecting
the binding posts of the thin wire coil to any two parts of the circuit
its resistance is at once given by the deflection of the needle.
When no current is passing the needle rests in any position. A current
in the thick coil brings it to zero. A current simultaneously passing
through the thin high resistance coil brings about the deflection.
The instrument is a commercial rather than a scientific one.
Ohm's Law.
The fundamental law expressing the relations between current,
electro-motive force and resistance in an active electric circuit. It
may be expressed thus:
(a) The current strength is equal to the electro-motive force divided by
the resistance.
(b) The electro-motive force is equal to the current strength multiplied
by the resistance.
(c) The resistance is equal to the electro-motive force divided by the
current strength. All these are different forms of the same statement.
Algebraically the law is usually expressed thus, (a) C = E/R. It may
also be expressed thus: (b) E = C*R and (c) R= E/C, in which R denotes
resistance, C denotes current strength, and E denotes electro-motive
force.
Ohm, True.
The true ohm is the resistance of a column of mercury 1 square
millimeter in cross-sectional area, and 106.24 centimeters long. (See
Ohm.)
Synonym-Rayleigh Ohm.
Oil Insulation.
Oil insulation has received several applications in electrical work. It
has been proposed for use in underground conduits. These it was proposed
to fill with oil after the insertion of the conductors, the latter
properly wrapped with cotton or other covering. For induction coils it
has been very successfully used. Its principal utility depends on the
fact that it is liquid, so that if pierced by a spark it at once closes
again. A solid insulator if pierced is permanently injured. It is also
used in telegraph insulators (see Insulator, Liquid) to prevent surface
leakage.
397 STANDARD ELECTRICAL DICTIONARY.
Olefiant Gas.
A compound gas; C2H4; composed of carbon, 24; hydrogen, 4; molecular
weight, 28; specific gravity, .981.
It is a dielectric of about the resistance of air. Its specific
inductive capacity at atmospheric pressure is 1.000722 (Boltzman.)
Synonym--Ethene; heavy carburetted hydrogen.
[Transcriber's note: Also called ethylene. A primary use is polyethylene
plastic.]
Open. adj.
An electric circuit is said to be open when it is cut or broken so that
no current can pass through it. The term may be recollected by thinking
of a switch; when open no current can pass through it. The same
adjective is applied to magnetic circuits, an air gap implying an open
circuit.
Open Circuit Oscillation.
An oscillation of current in open circuit so that a spark discharge
accompanies it. It is produced by electric resonance in a simple circle
or loop of wire with ends placed near together but not touching, if the
circuit is of such size that its period of oscillation corresponds with
that of the inducing discharge. (See Resonance, Electric.) Its period
depends entirely on the self-induction of the circuit.
Ordinate.
In a system of plane co-ordinates (see Co-ordinates), the distance of
any point from the axis of abscissas measured parallel to the axis of
ordinates.
Ordinates, Axis of.
The vertical axis in a system of co-ordinates, q. v.
Synonym--Axis of Y.
Organ, Electric.
An organ in which the air blast is admitted or excluded from the
different pipes by electric mechanism.
The outlines of the system are a series of contacts worked by the keys
and stops, which cause, when operated by the organist, a current to pass
through electro-magnets, opening the valves of the different pipes. Thus
the manual may be at any distance from the organ, and a number of organs
may be worked upon the same manual. As many as five in a single
cathedral are thus connected to a manual in the chancel.
Orientation of a Magnetic Needle.
The acquirement by a magnetic needle of its position of rest, with its
magnetic axis in the magnetic meridian.
Origin of Co-ordinates.
In a system of linear co-ordinates the point of intersection of the
axes; the point whose co-ordinates are both zero.
398 STANDARD ELECTRICAL DICTIONARY.
Oscillating Needle.
A small light bar magnet suspended by a filament and employed in
determining the intensity of a magnetic field by the oscillations it
completes in a given time after a given disturbance.
Oscillations, Electric.
In static electricity the sudden and very rapid alternations in the
discharge of a static condenser. This discharge of the disruptive order
seems a single one, but is really composed of a number of discharges
alternating in direction and producing electro-magnetic ether waves,
probably identical with light waves except that they are longer and far
less rapid.
Oscillatory Electro-motive Force.
Electro-motive force rapidly changing in sense or in direction, so that
it presents an oscillatory character. The alternating current and the
telephone current as used in practice are actuated by this type of
electro-motive force.
Osmose, Electric.
When two liquids are separated by a porous diaphragm, and a strong
current of electricity is passed through from the liquid on one side,
through the diaphragm, to the liquid on the other side, the liquid on
the side towards which the current is passing rises in level. The
process is termed electric osmose. When a liquid is forced through a
diaphragm a current is produced; in other words electric osmose is
reversible. The current thus produced is termed a diaphragm current.
Oscillation, Electric.
The phase of discharge of a static condenser in one direction. It is
usually followed by a discharge in the opposite direction constituting a
second oscillation, and so on, so that a great number of exceedingly
short oscillations are comprised. Thus, in the discharge of the Leyden
jar a large number of oscillations of current back and forth are
produced, the current alternating like the swings of a pendulum.
These oscillations are supposed to affect the ether, producing waves in
it identical with light waves, except that we have not been able yet to
produce them short enough to affect the visual organs. The waves thus
produced can be reflected or refracted; some substances are transparent
for them and others opaque. There is a possibility that man may yet
succeed in producing electric oscillations of sufficient frequency to
bring about the direct production of light.
Oscillatory Displacement.
Hypothetical displacement currents of rapidly alternating direction
produced in the oscillatory discharge of a Leyden jar or static
condenser.
Oscillatory Induction.
Induction produced by sympathetic action of an oscillatory discharge or
by electric resonance. (See Oscillations, Electric--Resonance,
Electric--Resonator, Electric.)
399 STANDARD ELECTRICAL DICTIONARY.
Outlet.
The part of an electrolier or electric light fixture out of which the
wires are led for attachment of an incandescent light socket.
Output.
The rate of energy delivered or of work done by a machine. In the case
of a current generator it is the volt-coulombs per given second, or
better the volt-amperes delivered at its outer circuit terminals.
Output, Magnetic.
The analogue in a magnetic circuit of the output of an electric circuit.
It is the product of the magnetizing force by the induced magnetism.
Output, Unit of.
As a unit of output of a dynamo Prof. Sylvanus P. Thompson has proposed
1,000 watts, or one kilowatt. This unit is now frequently used. To
completely define the dynamo, however, the amperage or the voltage must
also be given, as a 10 kilowatt--110 volt machine, or a 10 kilowatt--99
ampere machine.
[Transcriber's note: 10 kilowatt at 110 volts is 91 amperes.]
Over-Compounding.
A proportioning of the series and shunt windings of a compound dynamo,
so that the voltage of the terminals rises with the load or output
enough to allow for the drop in mains, thus maintaining the potential
for full load at distant points in a district. It is carried out by an
increase of ampere-turns in the series winding.
Overload.
In an electric motor a mechanical load put upon it so great as to
prevent economical working. One effect of such a load is to make the
armature run so slowly as to unduly reduce the counter-electro-motive
force and hence to permit so much current to pass through the coils as
to heat them, perhaps injuriously. In this case the production of heat
implies the waste of energy.
Overtype Dynamo or Motor.
A dynamo or motor whose armature is placed above or in the upper part of
the field magnets, the yoke piece of the magnets being in or resting
upon the base of the machine.
Ozone.
An allotropic form of oxygen. It possesses much more energetic chemical
properties than oxygen. It is supposed to contain three atoms of oxygen
in its molecule, represented thus:
O
/ \
O---O
It is produced by electric discharges and it is its peculiar odor which
is noticed about an electric machine, and sometimes in a thunderstorm
near the path of a lightning flash.
In the electrolysis of water some ozone may be produced, thus
diminishing the volume of the oxygen or of the mixed gases given off.
This is a source of inaccuracy in a gas voltameter.
400 STANDARD ELECTRICAL DICTIONARY.
Pacinotti's Inductor.
The Pacinotti or Gramme Ring. (See Pacinotti's Ring.)
Pacinotti's Ring.
A ring of iron wire wound with coils of insulated wire at right angles
to its circular axis, and used as the armature of a dynamo or motor. A
number of connections are taken from the coils to a central commutator.
Fig. 254. PACINOTTI'S MACHINE, WITH RING ARMATURE.
If such a ring with its coils is rotated in a field, current can be
taken from points of the commutator on a line at right angles to the
lines of force entering the ring.
The ring was discovered in 1862 by Pacinotti, and later was
independently discovered by Gramme. It is often known as the Gramme
ring.
Pacinotti Teeth.
Projections on a cylindrical or drum armature, between which in the
grooves formed thereby, the wire is wound. The teeth being of iron tend
to diminish the reluctance or magnetic resistance of the interpolar
space, or interval between the poles of the field magnet.
Synonym--Pacinotti Projections.
Paillard Alloys.
Non-magnetic palladium alloys, invented by Paillard, of
Switzerland, used in anti-magnetic watches. The following are given as
the compositions of several such alloys:
I. II.
Palladium, 60 to 75 parts 50 to 75 parts
Copper, I5 to 25 " 20 to 30 "
Iron. 1 to 5 " 5 to 20 "
401 STANDARD ELECTRICAL DICTIONARY.
The following are more complex:
I. II.
Palladium, 65 to 75 parts 45 to 50 parts
Copper, 15 to 25 " 15 to 25 "
Nickel, 1 to 5 " 2 to 5 "
Silver, 3 to 10 " 20 to 25 "
Gold, 1 to 2-1/2 " 2 to 5 "
Platinum, 1/2 to 2 " 2 to 5 "
Steel, 1 to 5 " 2 to 5 "
These alloys are used for balance springs, as well as for the balance
wheels and escapement parts of watches. The elasticity of recently
produced springs has been found to be very satisfactory.
Page Effect.
The sounds produced by magnetizing and demagnetizing a bar of iron or
steel; the magnetic tick. The sounds are strong enough to produce a
telephonic effect. (See Magnetic Tick.)
Palladium.
A metal of the platinum series. It has the highest power of occlusion,
q.v., of all metals. It is the characteristic ingredient of non-magnetic
watch alloys.
Palladium used as an electrode in the electrolysis of water will occlude
936 volumes of hydrogen, and the hydrogen-palladium alloy will exceed in
size the original electrode.
Fig. 255. LUMINOUS PANE.
Pane, Luminous.
A pane of glass, one side of which has pasted to it a long zigzag strip
of tinfoil. A design is made by cutting through the strip. On
discharging a Leyden jar or an electric machine through the strip sparks
appear where the tinfoil is severed, thus producing the design in a
luminous effect. Many variations can be employed in their construction.
402 STANDARD ELECTRICAL DICTIONARY.
Pantelegraphy.
A system of telegraphy for transmitting designs, maps, drawing, and the
like by telegraphy. (See Telegraphy, Facsimile.)
Paper Filaments.
Filaments of carbon for incandescent lamps made from paper.
This is one of the earliest materials practically used. The paper is cut
out of proper shape, and is carbonized in a close vessel, while embedded
in powdered charcoal or some other form of carbon to absolutely cut off
access of air. It is then placed in the lamp chamber and flashed or
subjected to the regular treatment.
Parabola.
A curve; one of the conic sections. It is approximately represented by a
small arc of a circle, but if extended becomes rapidly deeper than a
half circle.
If, from a point within called the focus, lines are drawn to the curve
and then other lines are drawn from these points parallel to the axis,
the angles of incidence will he equal to the angles of reflection as
referred to tangents at the points where the lines touch the curve.
[Transcriber's note; The general equation of a parabola is
A*x^2 + B*x*y + C*y^2 + D*x + E*y + F = 0
such that B^2 = 4*A*C, all of the coefficients are real, and A and C are
not zero. A parabola positioned at the origin and symmetrical on the y
axis is simplified to y = a*x^2 ]
Parabolic Reflector.
A reflector for a light, a paraboloid or surface of revolution whose
section is a parabola. A light placed at its focus has its rays
reflected parallel to each other.
Examples of parabolic reflectors are seen in electric search lights and
in locomotive head-lights. They are employed in electric search lights.
The arc light must be of such construction as to maintain its ignited
points always at the same point, the focus of the paraboloid.
Paraffine. v.
To coat or saturate with paraffine wax. Paper may be paraffined by
dipping in the wax, or by being sprinkled with fragments of wax,
subsequently melted in with a hot iron or otherwise. The tops of battery
carbons are often paraffined to prevent the acid from rising in the
pores by capillary attraction and rusting the connections.
403 STANDARD ELECTRICAL DICTIONARY.
Paraffine Wax.
A hydro-carbon composed principally of mixtures of the higher members of
the paraffine series C n H2 n + 2. It is made from cannel coal, coal
tar, or petroleum by distillation. It is an insulator. Its resistance at
46° C. (114.8° F.) per centimeter cube is 3.4E16 ohms, or about the
highest resistance known.
Its specific inductive capacity (for milky wax) is 2.47 (Schiller). For
clear wax it is given as follows by different authorities:
1.92 Ayrton.
1.96 Wüllner.
1.977 Gibson & Barclay.
2.32 Baltzmann.
It is extensively used in condensers and other electric apparatus as a
dielectric and insulator.
Paragrêles.
Protectors against hail; lightning rods used to guard fields against
hail; of little or no real utility.
Parallax.
The apparent change in position of an object when looked at from two
points of view. By looking at an object a few feet distant first with
one eye and then with the other, the shifting in apparent position is
seen.
In reading the position of an indicator or needle over a scale parallax
introduces an error unless the eye is held vertically over the needle.
By making the dial of looking- glass and holding the eye so that the
reflection of its pupil is bisected by the needle this verticality is
ensured.
Parallel.
(a) In the nomenclature of electric circuits two or more conductors
leading from one point to another, are said to be in parallel.
(b) When two or more conductors connect two main leads of comparatively
large size and low resistance they are said to be in parallel or in
multiple arc. This order is easiest pictured as the rungs of a ladder in
parallel connecting its two sides representing the main leads.
It may be used as a noun as "arranged in parallel," or as an adjective
as "a parallel circuit," the opposite of series, q. v.
Paramagnetic. adj.
Possessing paramagnetic properties; tending to occupy a position with
the longer axis parallel to the lines of force of a magnetic field;
having magnetism; attracted by a magnet.
"If a homogeneous isotropic substance is placed in a magnetic field it
becomes magnetized at every point in the direction of the magnetic
intensity at that point, and with an intensity of magnetization
proportional to the magnetic intensity. When the positive direction of
the induced magnetization is the same as that of the magnetic intensity
the substance is called Magnetic or Paramagnetic; when it is opposite,
the substance is called Diamagnetic." (Emtage.)
A paramagnetic substance has high permeability or multiplying power for
lines of force, hence in a magnetic field a bar of iron, etc., is in
unstable equilibrium unless its longer axis is parallel with the lines
of force in order to reduce as much as possible the reluctance of the
circuit.
404 STANDARD ELECTRICAL DICTIONARY.
Iron is the most paramagnetic of all substances. Other paramagnetic
metals are: Nickel, cobalt, manganese, platinum, cerium, osmium,
palladium. Diamagnetic metals are bismuth, antimony, zinc, tin, mercury,
lead, silver, copper, gold, arsenic. Bismuth is the most diamagnetic of
all metals.
Of gases oxygen is most paramagnetic. Becquerel calculated that a cubic
yard of oxygen condensed would act on a magnetic needle as powerfully as
5.5 grains of metallic iron. Liquefied oxygen will adhere to the poles
of a magnet.
Changes of temperature and of other conditions may affect a body's
magnetism. Thus hot oxygen is diamagnetic, and a substance paramagnetic
in a vacuum may be diamagnetic in air.
Of liquids, solutions of iron or cobalt are paramagnetic; water, blood,
milk, alcohol, ether, oil of turpentine and most saline solutions are
diamagnetic.
Paramagnetism.
(a) The science or study of paramagnetic substances and phenomena.
(b) The magnetic property of a paramagnetic substance; that of being
attracted by a magnet, and of arranging itself with its longer axis
parallel with the lines of force of a magnetic field.
Parchmentizing.
If cellulose is treated with a mixture of two parts of sulphuric acid
and one part of water perfectly cold, it becomes like parchment. It
should at once be washed with water, and then with ammonia and water.
The Swan incandescent light fibres are made of parchmentized cotton
thread, which is afterward carbonized.
Partial Earth.
A fault in a conductor caused by imperfect connection with the earth,
where insulation from the earth is desired.
Passive State.
A state of a substance in virtue of which it is unattacked by a solvent
which ordinarily would dissolve or attack it. Iron in strong nitric acid
is unattacked or assumes the passive state. This particular case is
supposed to be due to a coating of magnetic oxide, so that there would
be properly speaking no question of a passive state, but only one of
superficial protection.
The existence of a true passive state of any substance is very doubtful.
P. D.
Abbreviation for potential difference or difference of potential, or for
electro-motive force.
405 STANDARD ELECTRICAL DICTIONARY.
Peltier Effect.
The thermal effect produced by the passage of a current through the
junction of two unlike conductors. Such junction is generally the seat
of thermo-electric effects, and a current is generally produced by
heating such a junction. If an independent current is passed in the same
direction as that of the thermoelectric current, it cools the junction,
and warms it if passed in the other direction. In general terms,
referring to thermo-electric couples, if passed through them it tends to
cool the hot and heat the cool junction. The phenomenon does not occur
in zinc-copper junctions.
Peltier's Cross.
A bar of bismuth and a bar of antimony soldered centre to centre at
right angles, being notched or halved there to receive or to set into
each other. It is used to demonstrate the Peltier effect, q. v. To one
pair of ends are connected the terminals of a battery circuit; to the
other pair are connected the terminals of a galvanometer.
The galvanometer by its deflections in one and then in the other
direction indicates that the junction is heated when the current passes
from antimony to bismuth and vice versa. It thus illustrates the heating
and cooling of a thermo-electric junction by a current of electricity.
The current from the battery by the Peltier effect either heats or cools
the junction, as the case may be. This heating or cooling them produces
a thermo-electric current in the galvanometer circuit. The battery has
no direct influence on the galvanometer.
Pendant Cord.
A double conductor or pair of conductors, insulated from each other and
covered with a worsted, silk, or cotton covering and used to suspend
incandescent lamps and at the same time to conduct the current to them.
It is also used for other similar service, such as acting as conductors
for small motors. Often each conductor is composed of a number of thin
wires laid together. This gives flexibility to the cord.
Synonym--Flexible Cord.
Pendulum, Electric.
(a) A pendulum operated by the intermittent action of an electro-magnet,
whose circuit is opened and closed by the pendulum itself. A point at
the lower end of the pendulum swinging through a globule of mercury may
close and open the circuit. Various other methods of accomplishing the
same end are employed ..
(b) A pith ball suspended by a thread from an insulating stand. It is
used to show the attraction exercised by a piece of sealing wax or other
substance excited by rubbing.
406 STANDARD ELECTRICAL DICTIONARY.
Pen, Electric.
A stylus for producing a series of perforations in paper, so that the
paper may act as a stencil for the reproduction of a great number of
copies of the original matter. Various kinds of electric pens have been
invented. One kind, invented by Edison, consists of a handle carrying an
electric motor actuating a needle, which is driven in and out of the
other end of the handle with high rapidity. It is used by being held
vertically on the paper with the needle end downward, and is moved so as
to describe perforated letters or designs. The paper is then used as a
stencil with an ink roller to reproduce the writing or design ad
libitum. A simpler kind dispenses with the motor and depends on the
perforations produced by the electric spark. As shown in the cut the
stylus is one terminal of an induction coil circuit. The support on
which the paper rests is the other terminal and must be a conductor. In
use the induction coil is started, and the stylus is moved over the
paper; a series of sparks pass through the paper from stylus to the
supporting tablet, perforating the paper and producing a stencil to be
used for reproduction.
