Coefficient, Economic.

In machinery, electric generators, prime motors and similar structures, the number expressing the ratio between energy absorbed by the device, and useful, not necessarily available, work obtained from it. It is equal to work obtained divided by energy absorbed, and is necessarily a fraction. If it exceeded unity the doctrine of the conservation of energy would not be true. The economic coefficient expresses the efficiency, q. v., of any machine, and of efficiencies there are several kinds, to express any one of which the economic coefficient may be used.

Thus, let W--energy absorbed, and w = work produced ; then w/W is the economic coefficient, and for each case would be expressed numerically.

(See Efficiency, Commercial--Efficiency, Electrical--Efficiency of Conversion.)

The distinction between useful and available work in a dynamo is as follows: The useful work would include the work expended by the field, and the work taken from the armature by the belt or other mechanical connection. Only the latter would be the available work.

131 STANDARD ELECTRICAL DICTIONARY.

Coercive or Coercitive Force.

The property of steel or hard iron, in virtue of which it slowly takes up or parts with magnetic force, is thus termed ("traditionally"; Daniell). It seems to have to do with the positions of the molecules, as jarring a bar of steel facilitates its magnetization or accelerates its parting, when not in a magnetic field, with its permanent or residual magnetism. For this reason a permanent magnet should never be jarred, and permitting the armature to be suddenly attracted and to strike against it with a jar injures its attracting power.

Coercive force is defined also as the amount of negative magnetizing force required to reduce remnant magnetism to zero.

By some authorities the term is entirely rejected, as the phenomenon does not seem directly a manifestation of force.

Coil and Coil Plunger.

A device resembling the coil and plunge, q. v., except that for the plunger of iron there is subst.i.tuted a coil of wire of such diameter as to enter the axial aperture of the other, and wound or excited in the same or in the opposite sense, according to whether attraction or repulsion is desired.

Coil and Plunger.

A coil provided with a core which is free to enter or leave the central aperture. When the coil is excited, the core is drawn into it. Various forms of this device have been used in arc lamp regulators.

Synonym--Sucking coil.

Fig. 95. COIL AND COIL PLUNGER OF MENGIES ARC LAMP.

Fig. 96. COIL AND PLUNGER EXPERIMENT.

132 STANDARD ELECTRICAL DICTIONARY.

Coil and Plunger, Differential.

An arrangement of coil and plunger in which two plungers or one plunger are acted on by two coils, wound so as to act oppositely or differentially on the plunger or plungers. Thus one coil may be in parallel with the other, and the action on the plunger will then depend on the relative currents pa.s.sing through the coils.

Coil, Choking.

A coil of high self-induction, used to resist the intensity of or "choke" alternating currents. Any coil of insulated wire wound around upon a laminated or divided iron core forms a choking coil. The iron coil is usually so shaped as to afford a closed magnetic circuit.

A converter or transformer acts as a choking coil as long as its secondary is left open. In alternating current work special choking coils are used. Thus for theatrical work, a choking coil with a movable iron core is used to change the intensity of the lights. It is in circuit with the lamp leads. By thrusting in the core the self-induction is increased and the current diminishes, lowering the lamps; by withdrawing it the self-induction diminishes, and the current increases.

Thus the lamps can be made to gradually vary in illuminating power like gas lights, when turned up or down.

Synonyms--Kicking Coil--Reaction Coil.

Fig. 97. DIFFERENTIAL COILS AND PLUNGERS.

Fig. 98. BISECTED COILS.

133 STANDARD ELECTRICAL DICTIONARY.

Coils, Bisected.

Resistance coils with connections at their centers, as shown in the diagram. They are used for comparing the resistances of two conductors.

The connections are arranged as shown in the coil, each coil being bisected. For the wires, movable knife-edge contacts are employed. The principle of the Wheatstone bridge is used in the method and calculations.

Coil, Earth.

A coil of wire mounted with commutator to be rotated so as to cut the lines of force of the earth's magnetic field, thereby generating potential difference. The axis of rotation may be horizontal, when the potential will be due to the vertical component of the earth's field, or the axis may be horizontal, when the potential will be due to the vertical component, or it may be set at an intermediate angle.

Synonym--Delezenne's Circle.

Fig. 99. DELEZENNE'S CIRCLE OR EARTH COIL.

Coil, Electric.

A coil of wire used to establish a magnetic field by pa.s.sing a current through it. The wire is either insulated, or so s.p.a.ced that its convolutions do not touch.

Coil, Flat.

A coil whose windings all lie in one plane, making a sort of disc, or an incomplete or perforated disc.

Coil, Induction.

A coil in which by mutual induction the electromotive force of a portion of a circuit is made to produce higher or lower electro-motive force, in an adjoining circuit, or in a circuit, part of which adjoins the original circuit, or adjoins part of it.

An induction coil comprises three princ.i.p.al parts, the core, the primary coil and the secondary coil. If it is to be operated by a steady current, means must be provided for varying it or opening and closing the primary circuit. A typical coil will be described.

134 STANDARD ELECTRICAL DICTIONARY.

The core is a ma.s.s of soft iron preferably divided to prevent extensive Foucault currents. A cylindrical bundle of soft iron wires is generally used. Upon this the primary coil of reasonably heavy wire, and of one or two layers in depth, is wrapped, all being carefully insulated with sh.e.l.lac and paper where necessary. The secondary coil is wrapped upon or over the primary. It consists of very fine wire; No. 30 to 36 is about the ordinary range. A great many turns of this are made. In general terms the electro-motive force developed by the secondary stands to that of the primary terminals in the ratio of the windings. This is only approximate.

The greatest care is required in the insulating. The secondary is sometimes wound in sections so as to keep those parts differing greatly in potential far from each other. This prevents sparking, which would destroy the insulation.

A make and break, often of the hammer and anvil type, is operated by the coil. (See Circuit Breaker, Automatic.) As the current pa.s.ses through the primary it magnetizes the core. This attracts a little hammer which normally resting on an anvil completes the circuit. The hammer as attracted is lifted from the anvil and breaks the circuit. The soft iron core at once parts with its magnetism and the hammer falls upon the anvil again completing the circuit. This operation goes on rapidly, the circuit being opened and closed in quick succession.

Every closing of the primary circuit tends to produce a reverse current in the secondary, and every opening of the primary circuit tends to produce a direct current in the secondary. Both are of extremely short duration, and the potential difference of the two terminals of the secondary may be very high if there are many times more turns in the secondary than in the primary.

The extra currents interfere with the action of an induction coil. To avoid their interference a condenser is used. This consists of two series of sheets of tin foil. Leaves of paper alternate with the sheets of tin-foil, the whole being built up into a little book. Each sheet of tin-foil connects electrically with the sheet next but one to it. Thus each leaf of a set is in connection with all others of the same set, but is insulated from the others. One set of leaves of tin-foil connects with the hammer, the other with the anvil. In large coils there may be 75 square feet of tin-foil in the condenser.