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

by T. O'Conor Sloane

Fig. 164. END-ON METHOD.
End-on Method.
A method of determining the magnetic moment of a magnet. The magnet
under examination, N S, is placed at right angles to the magnetic
meridian, M O R, and pointing directly at or "end on" to the centre of a
compass needle, n s. From the deflection a of the latter the moment is
calculated.
Endosmose, Electric.
The inflowing current of electric osmose. (See Osmose, Electric.)
End Play.
The power to move horizontally in its bearings sometimes given to
armature shafts. This secures a more even wearing of the commutator
faces. End play is not permissible in disc armatures, as the attraction
of the field upon the face of the armature core would displace it
endwise. For such armatures thrust-bearings preventing end play have to
be provided.
Energy.
The capacity for doing work. It is measured by work units which involve
the exercise of force along a path of some length. A foot-pound,
centimeter-gram, and centimeter-dyne are units of energy and work.
The absolute unit of energy is the erg, a force of one dyne exercised
over one centimeter of space. (See Dyne.)
The dimensions of energy are
force (M * L / T^2) * space (L) = M * (L^2 / T^2).
Energy may be chemical (atomic or molecular), mechanical,
electrical, thermal, physical, potential, kinetic, or actual, and other
divisions could be formulated.
239 STANDARD ELECTRICAL DICTIONARY.
Energy, Atomic.
The potential energy due to atomic relations set free by atomic change;
a form of chemical energy, because chemistry refers to molecular as well
as to atomic changes. When atomic energy loses the potential form it
immediately manifests itself in some other form, such as heat or
electric energy. It may be considered as always being potential energy.
(See Energy, Chemical.)
[Transcriber's note: This item refers to chemical energy, that is
manifest in work done by electric forces during re-arrangement of
electrons. Atomic energy now refers to re-arrangement of nucleons
(protons and neutrons) and the resulting conversion of mass into
energy.]
Energy, Chemical.
A form of potential energy (see Energy, Potential) possessed by elements
in virtue of their power of combining with liberation of energy, as in
the combination of carbon with oxygen in a furnace; or by compounds in
virtue of their power of entering into other combinations more
satisfying to the affinities of their respective elements or to their
own molecular affinity. Thus in a galvanic couple water is decomposed
with absorption of energy, but its oxygen combines with zinc with
evolution of greater amount of energy, so that in a voltaic couple the
net result is the setting free of chemical energy, which is at once
converted into electrical energy in current form, if the battery is on a
closed circuit.
Energy, Conservation of.
A doctrine accepted as true that the sum of energy in the universe is
fixed and invariable. This precludes the possibility of perpetual
motion. Energy may be unavailable to man, and in the universe the
available energy is continually decreasing, but the total energy is the
same and never changes.
[Transcriber's note: If mass is counted a energy (E=m*(c^2)) then energy
is strictly conserved.]
Energy, Degradation of.
The reduction of energy to forms in which it cannot be utilized by man.
It involves the reduction of potential energy to kinetic energy, and the
reduction of kinetic energy of different degrees to energy of the same
degree. Thus when the whole universe shall have attained the same
temperature its energy will have become degraded or non-available. At
present in the sun we have a source of kinetic energy of high degree, in
coal a source of potential energy. The burning of all the coal will be
an example of the reduction of potential to kinetic energy, and the
cooling of the sun will illustrate the lowering in degree of kinetic
energy. (See Energy, Conservation of--Energy, Potential--Energy,
Kinetic.)
Energy, Electric.
The capacity for doing work possessed by electricity
under proper conditions. Electric energy may be either kinetic or
potential. As ordinary mechanical energy is a product of force and
space, so electric energy is a product of potential difference and
quantity. Thus a given number of coulombs of electricity in falling a
given number of volts develop electric energy. The dimensions are found
therefore by multiplying electric current intensity quantity
((M^.5) * (L^.5)),
by electric potential
((M^.5)*(L^1.5) / (T^2)),
giving (M * (L^2)/(T^2)),
the dimensions of energy in general as it should be.
The absolute unit of electric energy in electro-magnetic measure is
(1E-7) volt coulombs.
240 STANDARD ELECTRICAL DICTIONARY.
The practical unit is the volt-coulomb. As the volt is equal to 1E8
absolute units of potential and the coulomb to 0.1 absolute units of
quantity, the volt-coulomb is equal to 1E7 absolute units of energy.
The volt-coulomb is very seldom used, and the unit of Electric Activity
or Power (see Power, Electric), the volt-ampere, is universally used.
This unit is sometimes called the Watt, q. v., and it indicates the rate
of expenditure or of production of electric energy.
The storing up in a static accumulator or condenser of a given charge of
electricity, available for use with a given change of potential
represents potential electric energy.
The passing of a given quantity through a conductor with a given fall of
potential represents kinetic electric energy.
In a secondary battery there is no storage of energy, but the charging
current simply accumulates potential chemical energy in the battery,
which chemical energy is converted into electric energy in the discharge
or delivery of the battery.
It is customary to discuss Ohm's law in this connection; it is properly
treated under Electric Power, to which the reader is referred. (See
Power, Electric.)
[Transcriber's note: A volt-ampere or watt is a unit of power. A
volt-coulomb-second or watt-second is a unit of energy. Power multiplied by
time yields energy.]
Energy, Electric Transmission of.
If an electric current passes through a conductor all its energy is
expended in the full circuit. Part of the circuit may be an electrical
generator that supplies energy as fast as expended. Part of the circuit
may be a motor which absorbs part of the energy, the rest being expended
in forcing a current through the connecting wires and through the
generator. The electric energy in the generator and connecting wires is
uselessly expended by conversion into heat. That in the motor in great
part is utilized by conversion into mechanical energy which can do
useful work. This represents the transmission of energy. Every electric
current system represents this operation, but the term is usually
restricted to the transmission of comparatively large quantities of
energy.
A typical installation might be represented thus. At a waterfall a
turbine water wheel is established which drives a dynamo. From the
dynamo wires are carried to a distant factory, where a motor or several
motors are established, which receive current from the dynamo and drive
the machinery. The same current, if there is enough energy, may be used
for running lamps or electroplating. As electric energy (see Energy,
Electric,) is measured by the product of potential difference by
quantity, a very small wire will suffice for the transmission of a small
current at a high potential, giving a comparatively large quantity of
energy. It is calculated that the energy of Niagara Falls could be
transmitted through a circuit of iron telegraph wire a distance of over
1,000 miles, but a potential difference of 135,000,000 volts would be
required, something quite impossible to obtain or manage.
[Transcriber's note: Contemporary long distance power transmission lines
use 115,000 to 1,200,000 volts. At higher voltages corona discharges
(arcing) create unacceptable losses.]
241 STANDARD ELECTRICAL DICTIONARY.
Energy, Kinetic.
Energy due to matter being actually in motion. It is sometimes called
actual energy. The energy varies directly with the mass and with the
square of the velocity. It is represented in formula by .5 *M * (v^2).
Synonyms--Actual Energy--Energy of Motion--Dynamic Energy.
Energy, Mechanical.
The energy due to mechanical change or motion, virtually the same as
molar energy. (See Energy, Molar.)
Energy, Molar.
The energy of masses of matter due to movements of or positions of
matter in masses; such as the kinetic energy of a pound or of a ton in
motion, or the potential energy of a pound at an elevation of one
hundred feet.
Energy, Molecular.
The potential energy due to the relations of molecules and set free by
their change in the way of combination. It is potential for the same
reason that applies to atomic and chemical energy, of which latter it is
often a form, although it is often physical energy. The potential energy
stored up in vaporization is physical and molecular energy; the
potential energy stored up in uncombined potassium oxide and water, or
calcium oxide (quicklime) and water is molecular, and when either two
substances are brought together kinetic, thermal or heat energy is set
free, as in slaking lime for mortar.
Energy of an Electrified Body.
An electrified body implies the other two elements of a condenser. It is
the seat of energy set free when discharged. (See Dielectric, Energy
of.) The two oppositely charged bodies tend to approach. This tendency,
together with the distances separating them, represents a potential
energy.
Energy of Stress.
Potential energy due to stress, as the stretching of a spring. This is
hardly a form of potential energy. A stressed spring is merely in a
position to do work at the expense of its own thermal or kinetic energy
because it is cooled in doing work. If it possessed true potential
energy of stress it would not be so cooled.
Energy of Position.
Potential energy due to position, as the potential energy of a pound
weight raised ten feet (ten foot lbs.). (See Energy, Potential.)
Energy, Physical.
The potential energy stored up in physical position or set free in
physical change. Thus a vapor or gas absorbs energy in its vaporization,
which is potential energy, and appears as heat energy when the vapor
liquefies.
242 STANDARD ELECTRICAL DICTIONARY.
Energy, Potential, or Static Energy.
The capacity for doing work in a system due to advantage of position or
other cause, such as the stress of a spring. A pound weight supported
ten feet above a plane has ten foot lbs. of potential energy of position
referred to that plane. A given weight of an elementary substance
represents potential chemical energy, which will be liberated as actual
energy in its combination with some other element for which it has an
affinity. Thus a ton of coal represents a quantity of potential chemical
energy which appears in the kinetic form of thermal energy when the coal
is burning in a furnace. A charged Leyden jar represents a source of
potential electric energy, which becomes kinetic heat energy as the same
is discharged.
Energy, Thermal.
A form of kinetic molecular energy due to the molecular motion of bodies
caused by heat.
Entropy.
Non-available energy. As energy may in some way or other be generally
reduced to heat, it will be found that the equalizing of temperature,
actual and potential, in a system, while it leaves the total energy
unchanged, makes it all unavailable, because all work represents a fall
in degree of energy or a fall in temperature. But in a system such as
described no such fall could occur, therefore no work could be done. The
universe is obviously tending in that direction. On the earth the
exhaustion of coal is in the direction of degradation of its high
potential energy, so that the entropy of the universe tends to zero.
(See Energy, Degradation of.)
[Transcriber's note: Entropy (disorder) INCREASES, while AVAILABLE
ENERGY tends to zero.]
Entropy, Electric.
Clerk Maxwell thought it possible to recognize in the Peltier effect, q.
v., a change in entropy, a gain or loss according to whether the
thermo-electric junction was heated or cooled. This is termed Electric
Entropy. (See Energy, Degradation of.)
243 STANDARD ELECTRICAL DICTIONARY.

Fig. 165. EPINUS' CONDENSER,
Epinus' Condenser.
Two circular brass plates, A and B, are mounted on insulating supports,
and arranged to be moved towards or away from each other as desired.
Between them is a plate of glass, C, or other dielectric. Pith balls may
be suspended back of each brass plate as shown. The apparatus is charged
by connecting one plate to an electric machine and the other to the
earth. The capacity of the plate connected to the machine is increased
by bringing near to it the grounded plate, by virtue of the principle of
bound charges. This apparatus is used to illustrate the principles of
the electric condenser. It was invented after the Leyden jar was
invented.

Fig. 166. EPINUS' CONDENSER.
E. P. S.
Initials of Electrical Power Storage; applied to a type of secondary
battery made by a company bearing that title.

Fig. 167. CAM EQUALIZER.
244 STANDARD ELECTRICAL DICTIONARY.
Equalizer.
In electro-magnetic mechanism an arrangement for converting the pull of
the electro-magnet varying in intensity greatly over its range of
action, into a pull of sensibly equal strength throughout. The use of a
rocking lever acting as a cam, with leverage varying as the armature
approaches or recedes from the magnet core is one method of effecting
the result. Such is shown in the cut. E is an electro-magnet, with
armature a. A and B are the equalizer cams. The pull on the short end of
the cam B is sensibly equal for its whole length.
Many other methods have been devised, involving different shapes of pole
pieces, armatures or mechanical devices other than the one just shown.
Equipotential. adj.
Equal in potential; generally applied to surfaces. Thus every magnetic
field is assumed to be made up of lines of force and intersecting those
lines, surfaces, plane, or more or less curved in contour, can be
determined, over all parts of each one of which the magnetic intensity
will be identical. Each surface is the locus of equal intensity. The
same type of surface can be constructed for any field of force, such as
an electrostatic field, and is termed an equipotential surface.
Equipotential Surface, Electrostatic.
A surface in an electrostatic field of force, which is the locus of all

points of a given potential in such field; a surface cutting all the
lines of force at a point of identical potential. Lines of force are cut
perpendicularly by an equipotential surface, or are normal thereto.
Equipotential Surface, Magnetic and Electro-magnetic.
A surface bearing the same relation to a magnetic or electro-magnetic
field of force that an electrostatic equipotential surface (see
Equipotential Surface, Electrostatic,) does to an electrostatic field of
force.
Equivalent, Chemical.
The quotient obtained by dividing the atomic weight of an element by its
valency.
Equivalents, Electro-chemical.
The weight of any substance set free by one coulomb of electricity. The
following give some equivalents expressed in milligrams:
Hydrogen .0105 Mercury (mercurous) 2.10
Gold .6877 Iron (ferric) .1964
Silver 1.134 Iron (ferrous) .294
Copper (cupric) .3307 Nickel .3098
Mercury (mercuric) 1.05 Zinc .3413
Lead 1.0868 Chlorine .3728
Oxygen .89
245 STANDARD ELECTRICAL DICTIONARY.
Equivalent, Electro-mechanical.
The work or energy equivalent to unit quantities of electric energy, q.
v.; or equivalent to a unit current in a conductor whose ends differ one
unit of potential. The unit of electric energy taken is the watt-second
or volt-coulomb. One volt-coulomb is equal to
Ergs 1E7 [10000000]
Foot Pound .737337
Gram-degree C. .24068
Horse Power Second .0013406
Pound-degree F. .000955
One horse power is equal to 745.943 volt coulombs per second.
Equivalent, Electro-thermal.
The heat produced by a unit current passing through a conductor with
unit difference of potential at its ends; the heat equivalent of a
volt-coulomb or watt-second. It is equal to
Gram-degree C. .24068
Pound-degree F. .000955
Equivalent, Thermo-chemical.
The calories evolved by the combination of one gram of any substance
with its equivalent of another substance being determined, the product
obtained by multiplying this number by the equivalent (atomic or
molecular weight / valency) of the first element or substance is the
thermo-chemical equivalent. If expressed in kilogram calories, the
product of the thermo-chemical equivalent by 0.43 gives the voltage
required to effect such decomposition.
The following are thermo-chemical equivalents of a few combinations:
Water 34.5
Zinc oxide 43.2
Iron protoxide 34.5
Iron Sesquioxide 31.9 X 3
Copper oxide 19.2
Equivolt.
"The mechanical energy of one volt electro-motive force exerted under
unit conditions through one equivalent of chemical action in grains."
(J. T. Sprague.) This unit is not in general use as the unit of electric
energy, the volt-coulomb and (for rate of electric energy) the
volt-ampere being always used.
Erg.
The absolute or fundamental C. G. S. unit of work or energy. The work
done or energy expended in moving a body through one centimeter against
a resistance of one dyne.
Erg-ten.
Ten millions of ergs, or ten meg-ergs.
Escape.
A term applied to leakage of current.
Etching, Electric.
A process of producing an etched plate. The plate is coated with wax,
and the design traced through as in common etching. It is then placed in
a bath and is connected to the positive terminal from a generator, whose
negative is immersed in the same bath, so that the metal is dissolved by
electrolytic action. By attaching to the other terminal and using a
plating bath, a rough relief plate may be secured, by deposition in the
lines of metal by electroplating.
Synonym--Electric Engraving.
246 STANDARD ELECTRICAL DICTIONARY.
Ether.
The ether is a hypothetical thing that was invented to explain the
phenomena of light. Light is theoretically due to transverse vibrations
of the ether. Since the days of Young the conception of the ether has
extended, and now light, "radiant heat," and electricity are all treated
as phenomena of the ether. Electrical attraction and repulsion are
explained by considering them due to local stresses in the ether;
magnetic phenomena as due to local whirlpools therein. The ether was
originally called the luminiferous ether, but the adjective should now
be dropped. Its density is put at 936E-21 that of water, or equal to
that of the atmosphere at 210 miles above the earth's surface. Its
rigidity is about 1E-9 that of steel (see Ten, Powers of); as a whole it
is comparable to an all-pervading jelly, with almost perfect elasticity.
The most complete vacuum is filled with ether.
All this is a hypothesis, for the ether has never been proved to exist.
Whether gravitation will ever be explained by It remains to be seen.
[Transcriber's note: The Michelson-Morley experiment in 1887 (five years
before this book) cast serious doubt on the ether. In 1905 Einstein
explained electromagnetic phenomenon with photons. In 1963 Edward M.
Purcell used special relativity to derive the existence of magnetism and
radiation.]
Eudiometer.
A graduated glass tube for measuring the volumes of gases. In its
simplest form it is simply a cylindrical tube, with a scale etched or
engraved upon it, closed at one end and open at the other. The gas to be
measured is collected in it over a liquid, generally water, dilute
sulphuric acid in the gas voltameter, or mercury. Many different shapes
have been given them by Hoffmann, Ure, Bunsen and others.
Evaporation, Electric.
The superficial sublimation or evaporation of a substance under the
influence of negative electricity. It is one of the effects investigated
by Crookes in his experiments with high vacua. He found that when a
metal, even so infusible as platinum, was exposed to negative
electrification in one of his high vacuum tubes, that it was volatilized
perceptibly. A cadmium electrode heated and electrified negatively was
found to give a strong coating of metal on the walls of the tube. Even
in the open air the evaporation of water was found to be accelerated by
negative electrification.
Exchange, Telephone.
The office to which telephone wires lead in a general telephone system.
In the office by a multiple switch board, or other means, the different
telephones are interconnected by the office attendants, so that any
customers who desire it may be put into communication with each other.
The exchange is often termed the Central Office, although it may be only
a branch office.
Excitability, Faradic.
The action produced in nerve or muscle of the animal system by an
alternating or intermitting high potential discharge from an induction
coil.
247 STANDARD ELECTRICAL DICTIONARY.
Excitability, Galvanic.
The same as Faradic excitability, except that it refers to the effects
of the current from a galvanic battery.
Excitability of Animal System, Electric.
The susceptibility of a nerve or muscle to electric current shown by the
effect produced by its application.
Exciter.
A generator used for exciting the field magnet of a dynamo. In
alternating current dynamos, e. g., of the Westinghouse type, a special
dynamo is used simply to excite the field magnet. In central station
distribution the same is often done for direct current dynamos.
Exosmose, Electric.
The outflowing current of electric osmose. (See Osmose, Electric.)
Expansion, Coefficient of.
The number expressing the proportional increase in size, either length,
area or volume, of a substance under the influence generally of heat.
There are three sets of coefficients, (1) of linear expansion, (2) of
superficial expansion, (3) of cubic expansion or expansion of volume.
The first and third are the only ones much used. They vary for different
substances, and for the same substance at different temperatures. They
are usually expressed as decimals indicating the mixed number referred
to the length or volume of the body at the freezing point as unity.
Expansion, Electric.
(a) The increase in volume of a condenser, when charged
electrostatically. A Leyden jar expands when charged, and contracts when
discharged.
(b) The increase in length of a bar of iron when magnetized.
This is more properly called magnetic expansion or magnetic elongation.
Exploder.
(a) A small magneto-generator for producing a current for heating the
wire in an electric fuse of the Abel type (see Fuse, Electric), and
thereby determining an explosion.
(b) The term may also be applied to a small frictional or influence
machine for producing a spark for exploding a spark fuse.
Explorer.
A coil, similar to a magnetizing coil (see Coil, Magnetizing), used for
investigating the electro-magnetic circuit and for similar purposes. If
placed around an electro-magnet and connected with a galvanometer, it
will produce a deflection, owing to a momentary induced current, upon
any change in the magnet, such as removing or replacing the armature. It
is useful in determining the leakage of lines of force and for general
investigations of that nature. It is often called an exploring coil.
Hughes' Induction Balance (see Induction Balance, Hughes') is sometimes
called a Magnetic Explorer. The exploring coil may be put in circuit
with a galvanometer for quantitative measurements or with a telephone
for qualitative ones.
248 STANDARD ELECTRICAL DICTIONARY.
Extension Bell Call.
A system of relay connection, q. v., by which a bell is made to continue
ringing after the current has ceased coming over the main line. It is
designed to prolong the alarm given by a magneto call bell, q. v., which
latter only rings as long as the magneto handle is turned. A vibrating
electric bell (see Bell, Electric,) is connected in circuit with a local
battery and a switch normally open, but so constructed as to close the
circuit when a current is passed and continue to do so indefinitely. The
distant circuit is connected to this switch. When the magneto is worked
it acts upon the switch, closes the local battery circuit and leaves it
closed, while the bell goes on ringing until the battery is exhausted or
the switch is opened by hand.
Eye, Electro-magnetic.
An apparatus used in exploring a field of electro-magnetic radiations.
It is a piece of copper wire 2 millimeters (.08 inch) in diameter, bent
into an almost complete circle 70 millimeters (.28 inch) in diameter,
with terminals separated by an air gap. This is moved about in the
region under examination, and by the production of a spark indicates the
locality of the loops or venters in systems of stationary waves.

248 STANDARD ELECTRICAL DICTIONARY.
F.
Abbreviation for Fahrenheit, as 10º F., meaning 10º Fahrenheit. (See
Fahrenheit Scale.)
Fahrenheit Scale.
A thermometer scale in use in the United States and England. On this
scale the temperature of melting ice is 32°; that of condensing steam is
212°; the degrees are all of equal length. Its use is indicated by the
letter F., as 180° F. To convert its readings into centigrade, subtract
32 and multiply by 5/9. (b) To convert centigrade into F. multiply by
9/5 and add 32. Thus 180° F. = ((180-32) * 5/9)° C. = 82.2° C. Again
180° C. = (180 * 9/5) + 32 = 324° F.
[Transcribers note: 180° C. = (180 * 9/5) + 32 = 356° F. ]
The additions and subtractions must be algebraic in all cases. Thus when
the degrees are minus or below zero the rules for conversion might be
put thus: To convert degrees F. below zero into centigrade to the number
of degrees F. add 32, multiply by 5/9 and place a minus sign (-) before
it. (b) To convert degrees centigrade below zero into Fahrenheit,
multiply the number of degrees by 9/5, subtract from 32 if smaller; if
greater than 32 subtract 32 therefrom, and prefix a minus sign, thus:
-10° C. = 32 - (10 * 9/5) = 14°. Again, -30°C. = (30 * 9/5) - 32 = 22 =
-22° F.
249 STANDARD ELECTRICAL DICTIONARY.
Farad.
The practical unit of electric capacity; the capacity of a conductor
which can retain one coulomb of electricity at a potential of one volt.
The quantity of electricity charged upon a conducting surface raises its
potential; therefore a conductor of one farad capacity can hold two
coulombs at two volts potential, and three coulombs at three volts, and
so on. The electric capacity of a conductor, therefore, is relative
compared to others as regards its charge, for the latter may be as great
as compatible with absence of sparking and disruptive discharge. In
other words, a one farad or two farad conductor may hold a great many
coulombs. Charging a conductor with electricity is comparable to pumping
air into a receiver. Such a vessel may hold one cubic foot of air at
atmospheric pressure and two at two atmospheres, and yet be of one cubic
foot capacity however much air is pumped into it.
The farad is equal to one fundamental electrostatic unit of capacity
multiplied by 9E11 and to one electro-magnetic unit multiplied by 1E-9.
The farad although one of the practical units is far too large, so the
micro-farad is used in its place. The capacity of a sphere the size of
the earth is only .000636 of a farad.
[Transcriber's note: Contemporary calculations give about .000720
farad.]
Faraday, Effect.
The effect of rotation of its plane produced upon a polarized beam of
light by passage through a magnetic field. (See Magnetic Rotary
Polarization.)
Faraday's Cube.
To determine the surface action of a charge, Faraday constructed a room,
twelve feet cube, insulated, and lined with tinfoil. This room he
charged to a high potential, but within it he could detect no excitement
whatever. The reason was because the electricity induced in the bodies
within the room was exactly equal to the charge of the room-surface, and
was bound exactly by it. The room is termed Faraday's cube.
Faraday's Dark Space.
A non-luminous space between the negative and positive glows, produced
in an incompletely exhausted tube through which a static discharge, as
from an induction coil, is produced. It is perceptible in a rarefaction
of 6 millimeters (.24 inch) and upwards. If the exhaustion is very high
a dark space appears between the negative electrode and its discharge.
This is known as Crookes' dark space.
Faraday's Disc.
A disc of any metal, mounted so as to be susceptible of rotation in a
magnetic field of force, with its axis parallel to the general direction
of the lines of force. A spring bears against its periphery and another
spring against its axle. When rotated, if the springs are connected by a
conductor, a current is established through the circuit including the
disc and conductor. The radius of the disc between the spring contacts
represents a conductor cutting lines of force and generating a potential
difference, producing a current. If a current is sent through the
motionless wheel from centre to periphery it rotates, illustrating the
doctrine of reversibility. As a motor it is called Barlow's or
Sturgeon's Wheel. If the disc without connections is rapidly rotated it
produces Foucault currents, q. v., within its mass, which resist its
rotation and heat the disc.
250 STANDARD ELECTRICAL DICTIONARY.