The action of the condenser is to dispose of the direct extra current.

When the primary circuit is opened this current pa.s.ses into the condenser, which at once discharges itself in the other direction through the coil. This demagnetizes the core, and the action intensifies and shortens the induced current. The condenser prevents sparking, and in general improves the action of the coil.

Many details enter into the construction of coils, and many variations in their construction obtain. Thus a mercury cup into which a plunger dips often replaces the anvil and hammer.

135 STANDARD ELECTRICAL DICTIONARY.

The induction coil produces a rapid succession of sparks, which may spring across an interval of forty inches. The secondary generally ends in special terminals or electrodes between which the sparking takes place. A plate of gla.s.s, two inches in thickness, can be pierced by them. In the great Spottiswoode coil there are 280 miles of wire in the secondary, and the wire is about No. 36 A.W.G.

Fig. 100. VERTICAL SECTION OF INDUCTION COIL.

Fig. l01. PLAN OF INDUCTION COIL CONNECTIONS.

Induction coils have quite extended use in electrical work. They are used in telephone transmitters, their primary being in circuit with the microphone, and their secondary with the line and receiving telephone.

In electric welding, and in the alternating current system they have extended application. In all these cases they have no automatic circuit breaker, the actuating current being of intermittent or alternating type.

136 STANDARD ELECTRICAL DICTIONARY.

In the cuts the general construction of an induction coil is shown. In the sectional elevation, Fig. 100, A, is the iron core; B is the primary of coa.r.s.e wire; C is a separating tube, which may be of pasteboard; D is the secondary of fine wire; E, E are the binding posts connected to the secondary; H, H are the heads or standards; K, K are the terminals of the primary; F is the vibrating contact spring; G, a standard carrying the contact screw; J is the condenser with wires, L, M, leading to it.

Referring to the plan, Fig. 101, H represents the primary coil; B and A are two of the separate sheets of the condenser, each sheet with projecting ears; G, G are the heads of the coil; the dark lines are connections to the condenser. One set of sheets connects with the primary coil at C, and also with the vibrating spring shown in plan and in the elevation at F. The other set of sheets connects with the post, carrying the contact screw. The other terminal of the primary runs to a binding post E. F, in the plan is a binding post in connection with the standard and contact screw.

Coil, Induction, Inverted.

An induction coil arranged to have a lower electro-motive force in the secondary than in the primary. This is effected by having more convolutions in the primary wire than in the secondary. Such coils in practice are used with the alternating current and then do not include a circuit breaker or condenser. They are employed in alternating current system and in electric welding. (See Welding, Electric--Converter.)

In the cut an inverted coil, as constructed for electric welding is shown. In it the primary coil is marked P; the secondary, merely a bar of metal, is marked E, with terminals S, S; the heavy coils, I, of iron wire are the core; K is a screw for regulating the clamps; J, Z is a second one for the same purpose, while between D and D' the heat is produced for welding the bars, B, B', held in the clamps, C, C'. It will be seen how great may be the difference in turns between the single circle of heavy copper rod or bar which is the secondary of the coil, and the long coil of wire forming the primary.

Fig. 102. INVERTED INDUCTION COIL FOR ELECTRIC WELDING.

137 STANDARD ELECTRICAL DICTIONARY.

Coil, Induction, Telephone.

An induction coil used in telephone circuits. It is placed in the box or case near the transmitter. The primary is in circuit with the microphone. The secondary is in circuit with the line and receiving telephone. In the Bell telephone apparatus the primary of the induction coil is wound with No. 18 to 24 A. W. G. wire to a resistance of 1/2 ohm; the secondary, with No. 36 wire to a resistance of 80 ohms. The Edison telephone induction coil was wound with similar wires to a resistance of 3 to 4 ohms and of 250 ohms respectively.

Coil, Magnetizing.

A coil of insulated wire for making magnets; and for experimental uses; it has a short axis and central aperture of as small size as consistent with the diameter of the bar to be magnetized, which has to pa.s.s through it readily. The wire may be quite heavy, 2 or 3 millimeters (.08--.12 inch) thick, and is cemented together with carpenter's glue, or with sh.e.l.lac or ethereal solution of gum copal. In use it is pa.s.sed over the bar a few times while a heavy current is going through it. It is used for magic circles also. (See Circle, Magic.)

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 quant.i.tative tests, as in Wheatstone bridge work (see Wheatstone's Bridge), or simply to reduce the intensity of a current. For the first cla.s.s 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 bra.s.s 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 cla.s.sic 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 pa.s.ses through them, the current is turned off the one in the rear or pa.s.sing 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.