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

by T. O'Conor Sloane

Fig. 103. MAGNETIZING COIL.
Coil, Resistance.
A coil constructed for the purpose of offering a certain resistance to a
steady current. This resistance may be for the purpose of carrying out
quantitative tests, as in Wheatstone bridge work (see Wheatstone's
Bridge), or simply to reduce the intensity of a current. For the first
class of work the coils are wound so as to prevent the creation of a
magnetic field. This is effected by first doubling the wire without
breaking it, and then starting at the bend the doubled wire, which is
insulated, is wound on a bobbin or otherwise until a proper resistance
is shown by actual measurement. The coils are generally contained or set
in closed boxes with ebonite tops. Blocks of brass are placed on the
top, and one end from one coil and one end from the next connect with
the same block. By inserting a plug, P, so as to connect any two blocks,
which have grooves reamed out for the purpose, the coil beneath will be
short circuited. German silver, platinoid or other alloy, q. v., is
generally the material of the wire. A great object is to have a wire
whose resistance will be unaffected by heat.
138 STANDARD ELECTRICAL DICTIONARY.

Fig. 104. RESISTANCE COILS AND CONNECTIONS, SHOWING PLUG.
Coil, Rhumkorff.
The ordinary induction coil with circuit breaker, for use with original
direct and constant current, is thus termed. (See Coil, Induction.)
Synonym--Inductorium.
Coil, Ribbon.
A coil made of copper ribbon wound flatwise, often into a disc-like
shape, and insulated by tape or strips of other material intervening
between the successive turns.
Coils, Compensating.
Extra coils on the field magnets of dynamos or motors, which coils are
in series with the armature windings for the purpose of keeping the
voltage constant. In compound wound machines the regular series-wound
coil is thus termed. In a separately excited dynamo a coil of the same
kind in circuit with the armature may be used as a compensator.
Coils, Henry's.
An apparatus used in repeating a classic experiment in electro-magnetic
induction, due to Prof. Henry. It consists in a number of coils, the
first and last ones single, the intermediate ones connected in pairs,
and one of one pair placed on the top of one of the next pair. On
opening or closing the circuit of an end coil the induced effect goes
through the series and is felt in the circuit of the other end coil.
Prof. Henry extended the series so as to include seven successive
inductions, sometimes called inductions of the first, second, third and
other orders. Frequently ribbon coils (see Coil, Ribbon,) are used in
these experiments.
Coils, Sectioned.
A device for prolonging the range of magnetic attraction. It consists of
a series of magnetizing coils traversed by an iron plunger. As it passes
through them, the current is turned off the one in the rear or passing
to the rear and turned into the next one in advance. The principle was
utilized in one of Page's electric motors about 1850, and later by
others. The port-electric railroad, q. v., utilizes the same principle.
139 STANDARD ELECTRICAL DICTIONARY.
Collecting Ring.
In some kinds of generators instead of the commutator a pair of
collecting rings of metal, insulated from the machine and from each
other, are carried on the armature shaft. A brush, q. v., presses on
each, and the circuit terminals connect to these two brushes. Such rings
are employed often on alternating current generators, where the current
does not have to be changed or commuted. Collecting rings with their
brushes are used also where a current has to be communicated to a
revolving coil or circuit as in the magnetic car wheel, the cut of which
is repeated here. The coil of wire surrounding the wheel and rotating
with it has to receive current. This it receives through the two
stationary brushes which press upon two insulated metallic rings,
surrounding the shaft. The terminals of the coil connect one to each
ring. Thus while the coil rotates it constantly receives current, the
brushes being connected to the actuating circuit.

Fig. 105. MAGNETIC CAR WHEEL SHOWING
COLLECTING RINGS AND BRUSHES.
Collector.
(a) A name for the brush, q. v., in mechanical electric generators, such
as dynamos, a pair of which collectors or brushes press on the
commutator or collecting rings, and take off the current.
(b) The pointed connections leading to the prime conductor on a static
machine for collecting the electricity; often called combs. The points
of the combs or collectors face the statically charged rotating glass
plate or cylinder of the machine.
Colombin.
The insulating material between the carbons in a Jablochkoff candle or
other candle of that type. Kaolin was originally used. Later a mixture
of two parts calcium sulphate (plaster of Paris) and one part barium
sulphate (barytes) was substituted.
The colombin was three millimeters (.12 inch) wide, and two millimeters
(.08 inch) thick. (See Candle, Jablochkoff.)
Column, Electric.
An old name for the voltaic pile, made up of a pile of discs of copper
and zinc, with flannel discs, wet with salt solution or dilute acid,
between each pair of plates.
140 STANDARD ELECTRICAL DICTIONARY.
Comb.
A bar from which a number of teeth project, like the teeth of a comb. It
is used as a collector of electricity from the plate of a frictional or
influence electric machine; it is also used in a lightning arrester to
define a path of very high resistance but of low self-induction, for the
lightning to follow to earth.
Communicator.
The instrument by which telegraph signals are transmitted is sometimes
thus termed.
Commutator.
In general an apparatus for changing. It is used on electric current
generators, and motors, and on induction coils, and elsewhere, for
changing the direction of currents, and is of a great variety of types.
Synonym--Commuter (but little used).

Fig. 106. DYNAMO OR MOTOR COMMUTATOR.
Commutator Bars.
The metallic segments of a dynamo or motor commutator.
Commutator, Flats in.
A wearing away or lowering in level of one or more metallic segments of
a commutator. They are probably due in many cases to sparking, set up by
periodic springing in the armature mounting, or by defective commutator
connections.
Commutator of Current Generators and Motors.
In general a cylinder, formed of alternate sections of conducting and
non-conducting material, running longitudinally or parallel with the
axis. Its place is on the shaft of the machine, so that it rotates
therewith. Two brushes, q. v., or pieces of conducting material, press
upon its surface.
141 STANDARD ELECTRICAL DICTIONARY.
As a part of electric motors and generators, its function is to collect
the currents produced by the cutting of lines of force so as to cause
them all to concur to a desired result. The cut shows the simplest form
of commutator, one with but two divisions. Its object may be to enable a
current of constant direction to be taken from a rotating armature, in
which the currents alternate or change direction once in each rotation.
It is carried by the shaft A of the armature and rotates with it. It
consists of two leaves, S S, to which the terminals of the armature are
connected. Two springs, W W, the terminals of the outer circuit, press
against the leaves. The springs which do this take off the current. It
is so placed, with reference to the springs and armature, that just as
the current changes in direction, each leaf changes from one spring to
the other. Thus the springs receive constant direction currents. The
changing action of this commutator appears in its changing the character
of the current from alternating to constant. Were two insulated
collecting rings used instead of a commutator, the current in the outer
circuit would be an alternating one. On some dynamos the commutator has
a very large number of leaves.
Taking the Gramme ring armature, there must be as many divisions of the
commutator as there are connections to the coils. In this case the
function of the commutator is simply to lessen friction, for the brushes
could be made to take current from the coils directly outside of the
periphery of the ring.
Commutator, Split Ring.
A two-division commutator for a motor; it consists of two segments of
brass or copper plate, bent to arcs of a circle, and attached to an
insulating cylinder. They are mounted on the revolving spindle, which
carries the armature, and acts as a two part commutator. For an example
of its application, see Armature, Revolving, Page's. (See also Fig.
107.)

Fig. 107. SECTION OF SPLIT RING COMMUTATOR, WITH BRUSHES.
Compass.
An apparatus for utilizing the directive force of the earth upon the
magnetic needle. It consists of a circular case, within which is poised
a magnetized bar of steel. This points approximately to the north, and
is used on ships and elsewhere to constantly show the direction of the
magnetic meridian. Two general types are used. In one the needle is
mounted above a fixed "card" or dial, on which degrees or points of the
compass, q. v., are inscribed. In the other the card is attached to the
needle and rotates with it. The latter represents especially the type
known as the mariner's compass. (See Compass, Mariner's--Compass,
Spirit, and other titles under compass, also Magnetic Axis--Magnetic
Elements.) The needle in good compasses carries for a bearing at its
centre, a little agate cup, and a sharp brass pin is the point of
support.
Compass, Azimuth.
A compass with sights on one of its diameters; used in determining the
magnetic bearing of objects.
142 STANDARD ELECTRICAL DICTIONARY.
Compass Card.
The card in a compass; it is circular in shape, and its centre coincides
with the axis of rotation of the magnetic needle; on it are marked the
points of the compass, at the ends generally of star points. (See
Compass, Points of the.) It may be fixed, and the needle may be poised
above it, or it may be attached to the needle and rotate with it.
Compass, Declination.
An instrument by which the magnetic declination of any place may be
determined. It is virtually a transit instrument and compass combined,
the telescope surmounting the latter. In the instrument shown in the
cut, L is a telescope mounted by its axis, X, in raised journals with
vernier, K, and arc x, for reading its vertical angle, with level n. The
azimuth circle, Q, R, is fixed. A vernier, V is carried by the box, A,
E, and both turn with the telescope. A very light lozenge-shaped
magnetic needle, a, b, is pivoted in the exact centre of the graduated
circles, Q R, and M. The true meridian is determined by any convenient
astronomical method, and the telescope is used for the purpose. The
variation of the needle from the meridian thus determined gives the
magnetic declination.