(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 gla.s.s 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 subst.i.tuted.

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 bra.s.s 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.

Compa.s.s.

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 compa.s.s, 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 compa.s.s. (See Compa.s.s, Mariner's--Compa.s.s, Spirit, and other t.i.tles under compa.s.s, also Magnetic Axis--Magnetic Elements.) The needle in good compa.s.ses carries for a bearing at its centre, a little agate cup, and a sharp bra.s.s pin is the point of support.

Compa.s.s, Azimuth.

A compa.s.s with sights on one of its diameters; used in determining the magnetic bearing of objects.

142 STANDARD ELECTRICAL DICTIONARY.

Compa.s.s Card.

The card in a compa.s.s; 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 compa.s.s, at the ends generally of star points. (See Compa.s.s, 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.

Compa.s.s, Declination.

An instrument by which the magnetic declination of any place may be determined. It is virtually a transit instrument and compa.s.s 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.

FIG. 108. DECLINATION COMPa.s.s.

Compa.s.s, Inclination.

A magnetic needle mounted on a horizontal axis at its centre of gravity, so as to be free to a.s.sume the dip, or magnetic inclination, when placed in the magnetic meridian. It moves over the face of a vertical graduated circle, and the frame also carries a spirit level and graduated horizontal circle. In use the frame is turned until the needle is vertical. Then the axis of suspension of the needle is in the magnetic meridian. The vertical circle is then turned through 90? of the horizon, which brings the plane of rotation of the needle into the magnetic meridian, when it a.s.sumes the inclination of the place.

143 STANDARD ELECTRICAL DICTIONARY.

Compa.s.s, Mariner's.

A compa.s.s distinguished by the card being attached to and rotating with the needle. A mark, the "lubber's mark" of the sailors is made upon the case. This is placed so that the line connecting it, and the axis of rotation of the card is exactly in a plane, pa.s.sing through the keel of the ship. Thus however the ship may be going, the point of the card under or in line with the "lubber's mark," shows how the ship is pointing. The case of the mariner's compa.s.s is often bowl-shaped and mounted in gimbals, a species of universal joint, so as to bc always horizontal. (See Compa.s.s, Spirit-Gimbals.)

FIG. 109. MARINER'S COMPa.s.s.

Compa.s.s, Points of the.

The circle of the horizon may bc and is best referred to angular degrees. It has also been divided into thirty-two equiangular and named points. A point is 11.25?. The names of the points are as follows: North, North by East, North North-east, North-east by North, North-east, North-east by East, East North-east, East by North, East, East by South, East South-east, South-east by East, South-east, South-east by South, South South-east, South by East, South, South by West, South South-west, South-west by South, South-west, South-west by West, West South-west, West by South, West, West by North, West North-west, North-west by West, North-west, North West by North, North North-west, North by West. They are indicated by their initials as N. N. W., North North-west, N. by W., North by West.