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

by T. O'Conor Sloane

332 STANDARD ELECTRICAL DICTIONARY.
M.
(a) Symbol of gaseous pressure equal to one-millionth of an atmosphere.
(b) The Greek m, µ, is used as the symbol of magnetic permeability.
333 STANDARD ELECTRICAL DICTIONARY.
Machine, Cylinder Electric.
A frictional electric machine whose rotating glass is in the shape of a
cylinder instead of a disc as in the more recent machines.

Fig. 217. PLATE FRICTIONAL ELECTRIC MACHINE.
Machine, Frictional Electric.
An apparatus for development of high tension electricity by contact
action, brought about by friction.
It consists of a plate or cylinder of glass mounted on insulating
standards and provided with a handle for turning it. One or more
cushions of leather are held on an insulated support, so as to rub
against the plate or cylinder as it is turned. A metal comb or combs are
held on another insulating support so as to be nearly in contact with
the surface of the glass plate at a point as far removed as possible
from the rubbers. The combs are attached to a brass ball or round-ended
cylinder, which is termed the prime conductor.
In use either the prime conductor or cushions are connected by a chain
or otherwise with the earth. Assume it to be the cushions. As the
machine is worked by turning the plate, the glass and cushion being in
contact develop opposite electricities. The glass is charged with
positive electricity, and as it turns carries it off and as it reaches
the prime conductor by induction and conduction robs it of its negative
electricity. Meanwhile the cushions negatively excited deliver their
charge to the earth. The action thus goes on, the prime conductor being
charged with positive electricity.
334 STANDARD ELECTRICAL DICTIONARY.
If the prime conductor is connected to the earth and the cushions are
left insulated, negative electricity can be collected from the cushions.
In some machines both prime conductor and cushions are kept insulated
and without ground contact. Electrodes connecting with each are brought
with their ends close enough to maintain a sparking discharge.
Machine Influence.
A static electric machine working by induction to build up charges of
opposite nature on two separate prime conductors. In general they are
based on the principle of the electrophorous. Work is done by the
operator turning the handle. This rotates a disc and draws excited parts
of it away from their bound charges. This represents a resistance to
mechanical motion. The work absorbed in overcoming this mechanical
resistance appears as electric energy. There are various types of
influence machines, the Holtz, Toeppler-Holtz and Wimshurst being the
most used. The electrophorous, q. v., is a type of influence machine.
Machine, Holtz Influence.
A static electric machine. It includes two plates, one of which is
rapidly rotated in front of the other. Two armatures of paper are
secured to the back of the stationary plate at opposite ends of a
diameter. To start it one of these is charged with electricity. This
charge by induction acts through the two thicknesses of glass upon a
metal bar carrying combs, which lies in front of the further side of the
movable plate. The points opposite the armature repel electrified air,
which strikes the movable disc and charges it. A second rod with comb at
the opposite end of the same diameter acts in the reverse way. Thus
opposite sections of the disc are oppositely charged and the combs with
them. By induction these portions of the disc react upon the two
armatures. The opposite electricities escape from the armatures by paper
tongues which are attached thereto and press against the back of the
movable plate. As the plate rotates the opposite electricities on its
face neutralize the electricity repelled from the combs. The charges on
the back strengthen the charges of the armatures and brass combs. Thus
the machine builds up, and eventually a discharge of sparks takes place
from the poles of the brass combs.
335 STANDARD ELECTRICAL DICTIONARY.
Machine, Toeppler-Holtz.
A modification of the Holtz machine. The priming charge of the armatures
is produced by friction of metallic brushes against metallic buttons on
the face of the rotating plate. (See Machine, Holtz.)
Machine, Wimshurst.
A form of static influence machine. It consists of two plates of glass,
on which radial sectors of tinfoil are pasted. Both plates are rotated
in opposite directions. The sectors of the two plates react one upon the
other, and electric charges of opposite sign accumulate on the opposite
sides of the plates and are collected therefrom by collecting combs.
Mack.
A name, derived from Maxwell, and suggested for the unit of inductance.
It is due to Oliver Heaviside, but has never been adopted. (See Henry.)
Magne-Crystallic Action.
The action of a supposed force of the same name, proposed by Faraday. It
relates to the different action of a magnetic field upon crystalline
bodies, according to the position of their axes of crystallization. A
needle of tourmaline, normally paramagnetic, if poised with its axis
horizontal, is diamagnetic. Bismuth illustrates the same phenomenon. The
subject is obscure. Faraday thought that he saw in it the action of a
specific force.
Magnet.
A body which tends when suspended by its centre of gravity to lay itself
in a definite direction, and to place a definite line within it, its
magnetic axis, q. v., in a definite direction, which, roughly speaking,
lies north and south. The same bodies have the power of attracting iron
(Daniell), also nickel and cobalt.
Magnets are substances which possess the power of attracting iron.
(Ganot.)
[Transcriber's note: Edward Purcell and others have explained magnetic
and electromagnetic phenomenon as relativistic effects related to
electrostatic attraction. Magnetism is caused by Lorentz contraction of
space along the direction of a current. Electromagnetic waves are caused
by charge acceleration and the resulting disturbance of the
electrostatic field. (Electricity and Magnetism: Berkeley Physics
Course Volume 2, 1960)]
Magnet, Anomalous.
A magnet possessing more than the normal number (two) of poles. If two
straight magnets are placed end to end with their south poles in
juxtaposition the compound bar will seem to possess three poles, one at
each end and one in the middle. The apparent pole in the middle is
really made up of two consequent poles, q. v. It sometimes happens that
when a single long thin bar is magnetized consequent poles are produced,
although such magnet is in one piece. This may be accidental, as in such
case it is quite hard to avoid anomalous poles, or, as in the field
magnets of some forms of dynamos, anomalous poles may be purposely
produced.
Magnet, Artificial.
A magnet formed artificially by any method of magnetization (see
Magnetism) applicable to permanent magnets, electro-magnets and
solenoids. It expresses the distinction from the natural magnets or
lodestone, q. v. It is made of steel in practice magnetized by some of
the methods described under Magnetization.
336 STANDARD ELECTRICAL DICTIONARY.
Magnet, Axial.
A straight-solenoid with axial core.
Magnet, Bar.
A bar magnet is one in the shape of a bar, i. c., straight with parallel
sides and considerably longer than wide or deep.
Magnet, Bell-shaped.
A form of permanent magnet used in some galvanometers. In shape it is a
thick-sided cylindrical box with two slots cut out of opposite sides, so
as to make it represent a horseshoe magnet. Its shape enables it to be
surrounded closely by a mass of copper, for damping its motion, to
render the instrument dead-beat. Such a magnet is used in Siemens &
Halske's galvanometer.
Magnet Coil.
A coil to be thrust over an iron core, to make an electro-magnet. They
are often wound upon paper or wooden bobbins or spools, so as to be
removable from the core if desired.
Magnet, Compensating.
(a) A magnet fastened near a compass on an iron or steel ship to
compensate the action of the metal of the ship upon the magnetic needle.
The ship itself always has some polarity and this is neutralized by one
or more compensating magnets.
(b) See below.
Magnet, Controlling.
A magnet attached to a galvanometer by which the directive tendency of
its magnetic needle is adjusted. In the reflecting galvanometer it often
is a slightly curved magnet carried by a vertical brass spindle rising
from the center of the instrument, and which magnet may be slid up and
down on the spindle to regulate or adjust its action.
Synonym--Compensating Magnet.
Magnet, Compound.
A permanent magnet, built up of a number of magnets. Small bars can be
more strongly magnetized than large. Hence a compound magnet may be made
more powerful than a simple one.
Magnet Core.
The iron bar or other mass of iron around which insulated wire is wound
for the production of an electro-magnet. The shapes vary greatly,
especially for field magnets of dynamos and motors. For these they are
usually made of cast iron, although wrought iron is preferable from the
point of view of permeability.
Magnet, Damping.
A damping magnet is one used for bringing an oscillating body to rest.
The body may be a metallic disc or needle, and the action of the magnet
depends on its lines of force which it establishes, so that the body has
to cut them, and hence has its motion resisted.
337 STANDARD ELECTRICAL DICTIONARY.
Magnet, Deflection of.
The change of position of a magnet from the plane of the earth's
meridian in which it normally is at rest into another position at some
angle thereto, by the effect of an artificial magnetic field, as the
deflection of a galvanometer needle.
Magnet, Electro-.
A magnet consisting of a bar of iron, bundle of iron wires, iron tube or
some equivalent, around which a coil of insulated wire is wound. Such
combination becomes polarized when a current is passed through it and is
an active magnet. On the cessation of the current its magnetism in part
or almost completely disappears. (See Electro-magnet.)
Magnet, Equator of.
In a magnet the locus of points of no attractive power and of no
polarity. In a symmetrical, evenly polarized magnet it is the imaginary
line girdling the centre. The terms Neutral Point or Neutral Line have
displaced it.
Synonyms--Neutral Line--Neutral Point.
Magnet, Field.
A magnet, generally an electro-magnet, used to produce the field in a
dynamo or motor.
Magnet, Haarlem.
Celebrated magnets made in Haarlem, Holland. Logeman, Van Wetteren,
Funckler and Van der Willigen were the makers who gave the celebrity to
the magnets. They were generally horseshoe magnets, and would carry
about twenty times their own weight.
Magnet, Horseshoe.
A magnet of U shape--properly one with the poles brought a little closer
together than the rest of the limbs. For direct lifting and attractive
effects it is the most generally adopted type. Its advantage as regards
lifting effect is due to small reluctance, q. v., offered by a complete
iron circuit, such as the armature and magnet together produce. As the
term is now used it is applied to any U shaped magnet.

Fig. 218. JOULE'S ELECTRO-MAGNET.
Magnet, Joule's Electro.
An electro-magnet of the shape of a cylinder with a longitudinal segment
cut-off. It is wound with wire as shown. The segment cut-off is a piece
of the same shape as the armature. It is of high power.
338 STANDARD ELECTRICAL DICTIONARY.
Magnetic Adherence.
The tendency of a mass of iron to adhere to the poles of a magnet. It is
best figured as due to the virtual shortening of lines of force, as the
more permeable iron gives a better path for them than the air can
afford, and consequently a virtually shorter one.
Magnetic Attraction and Repulsion.
The attraction of a magnet for iron, steel, nickel and cobalt and of
unlike poles of magnets for each other. It is identical with
electro-magnetic attraction, q.v. (Also see Electro-magnetism.)
Magnetic Attraction and Repulsion, Coulomb's Law of.
Magnetic attraction and repulsion are inversely as the square of the
distance. (Ganot.)
While theoretically true in the case of isolated poles, in practise it
does not generally apply on account of the large diameter and relative
shortness of magnets.
Magnetic Axis.
The line connecting the poles of a magnet. It does not generally
coincide exactly with any symmetrical axis of figure. In such cases an
error is introduced into the indications of the needle which must be
determined and allowed for in compasses. To determine it with a magnetic
needle the suspension cup is made removable, so that the needle can be
reversed. Readings are taken with one side of the needle and then with
the other side of the needle up, and the average corresponds with the
position of the magnetic axis in both positions of the needle.
Magnetic Azimuth.
The angle, measured on a horizontal circle, between the magnetic
meridian and a great circle of the earth passing through the observer
and any observed body. It is the astronomical azimuth of a body referred
to the magnetic meridian and therefore subject to the variation of the
compass. The angle is the magnetic azimuth of the observed body.
Magnetic Battery.
A name for a compound permanent magnet; one made up by bolting or
clamping together, or to single soft iron pole pieces, a number of
single permanent magnets. There are a number of forms of compound
magnets. In making them care has to be taken to have them of even
strength. It is also well to have them slightly separated. The object of
both these precautions is to prevent a stronger element or magnet from
depolarizing its neighbor.
Synonym--Compound Magnet.
Magnetic Bridge.
An apparatus for testing the relative permeability of iron. It consists
of a rectangular system of iron cores. Three of the sides are wound with
wire as shown. The other side is built up of double bars, and from the
centre two curved arms rise, as shown in the cut. The arms do not touch.
Between them a short magnet is suspended by a filament, which also
carries a mirror and an index.
339 STANDARD ELECTRICAL DICTIONARY.

Fig. 219. MAGNETIC BRIDGE.
A lamp and scale are provided as in the reflecting galvanometer. When
adjusted the magnetic needle hangs as shown in the cut, Fig. 219,
without any tendency to turn towards either curved pole piece. If all
iron parts are symmetrical and of similar metal, a current through the
coils will make no difference. It will work in magnetic opposition upon
the two arms, or, in other words, will maintain both arms at identical
potential.

Fig. 220. POLE PIECES, MAGNETIC NEEDLE
AND MIRROR OF MAGNETIC BRIDGE.
If there is the least difference in permeability, length or thickness
between any of the iron bars the magnetic potential of the two curved
arms will differ, and the magnetic needle will turn one way or the
other. In practical use different samples of iron are substituted for
the unwound members of the fourth side of the parallelogram, and the
needle by its motions indicates the permeability.
In the cut, Fig. 220, D D are the ends of the curved pole pieces; A the
wire carrying the mirror B and magnetic needle N, and E is the index
which shows the larger deflections.
340 STANDARD ELECTRICAL DICTIONARY.
Magnetic Circuit.
A magnetic field of force is characterized by the presence of lines of
force, which, while approximately parallel, curve around and tend to
form closed curves. The polarity of a field of force is referred to an
imaginary direction of the lines of force from the north pole through
space to the south pole, and in the part of the field corresponding to
the body of the magnet, from the south to the north pole. The cut
indicates these features. Hence the magnetic field of force is termed
the magnetic circuit, and to it are attributed a species of resistance
termed reluctance, q. v., and the producing cause of the field or lines
of force is termed sometimes magneto-motive force, q. v.) corresponding
to the electro-motive force. The modern treatment of the magnetic
circuit is similar to the application of Ohm's law and the laws of
resistance and conductivity to the electric circuit.
Magnetic Circuit, Double.
A magnetic circuit which virtually represents two horseshoe magnets
placed with their like poles in contact. It is used for field magnets,
the armatures occupying a place between the consequent poles.