The excitation of magnetism in the core is the principal aim of the primary coil, and as a strong current is essential to that object, it is made of thick wire and of moderate length. In the secondary coil; the tension of 'the induced cukrtnt alone is aimed at, and with this view it is made of as thin wire as can be made, so as to admit of as many turns as possible being brought within the influence of the core and primary coil. The electric conformation of the secondary coil is sometimes looked upon in the saner light as that of a galvanic battery. The total electromotive force of the coil is the sun of that of all the turns in it, in the same way that the electromotive force of the bat tery is proportionate to the number of cells.
Extra Current —Not only does a galvanic current induce electricity in a neighbor ing circuit, but it also acts inductively on itself. When contact is broken in a battery circuit, the galvanic spark is seen. See GALVANISM. When the wire is short, the spark is feeble, but it increases in brilliancy with the length of the circuit, and this becomes particularly observable when the wire is wound round in a coil. This certainly does not arise from the current being strong with the long wire, and weak with the short one, for quite the reverse is the case, as might be shown with the aid of a galvanometer. The real cause of the superior brilliancy of the galvanic spark with the long circuit is to be found in the induction of the primary current on the various parts of itself, exciting, as they are called, extra currents in the primary wire. It has been fully attested by experi ment, that at the instant a galvanic current begins and ends, extra currents arc induced by the action of the several parts of its circuit upon each other, that at the beginning of the cur rent being inverse, and that at the end direct. As the extra current inverse acts opposite to the main current, it does not appear as a separate current, but only retards the instan taneous passage of the main current. The extra current direct succeeds the main cur rent, and has consequently a separate existence. It is what is generally referred to when the extra current is spoken of. This extra current is of much higher tension than the original current. The effect of the extra current on the direct induced current of the secondary coil is to lessen very decidedly its tension. If a way be made for the extra current, the tension of the induced current falls prodigiously. In a large coil-machine, which gives freely sparks of 1 or 2 in. in length, when the two portions of the break are joined by a thin wire, so as to allow the extra current to pass, sparks will not travel between the two poles, however near they are brought. When no such communication exists, a portion of the extra corrent leaps over between the separating parts of the break, and in so far diminishes the intensity of the secondary current. The condenser of the coil-machine, to be afterwards described, has for its object the absorption or suppression of the extra current, but the manner in which it effects this is not yet properly explained. The prejudicial effect of the extra current on the induced current is easily understood, when we bear in mind that it prolongs the cessation of the magnetism of the core and of the current in the primary coil, and thus impairing the suddenness of this chanp, reduces the tension of the induced current.
Induction Coll.--The essential parts of this apparatus have been already described in detail. A primary coil with its core of iron wire, and a secondary coil exterior to and insulated from a primary coil, form the main portion of the instrument. The primary coil is connected with the poles of a galvanic battery, and in the circuit a rheotom is introduced, to effect the interruptions of the current essential to its inductive action. The only parts not yet referred to are the condenser and the commutator. The condenser consists of several sheets of tinfoil and oiled silk, laid alternately the one above the other. The first, third, fifth, etc., sheata of tinfoil are connected by strips of the same material; so are the second, fourth, and sixth, etc. ; the whole forming a condensing apparatus like a Leyden jar, the odd sheets forming the one coating, and the even sheets the other. Each set of sheets is connected with one of the wires of the primary coil. '1 he condenser is generally placed in the sole of the instrument, and does not meet the eye. The com mutator consists of an ivory cylinder covered with conducting plates on two sides, and is so constructed that it can break contact, or transmit the current through the coil in either direction.
An induction coil, as constructed by Ladd of London, is represented In the figure. The forms under which the instrument appears are very various, and the one in the figure only serves to show the general requirements in its tion. The two binding-screws, p and a, are for the battery wires; C is the commutator. The two coils, W, lie horizontally on the sole of the ment, S. The secondary coil alone is seen, the primary being within it and out of view. The breaking hammer, being behind the coil, is likewise not shown. The condenser is contained by the box which constitutes the sole, and a conducting connection is lished between its coatings and the wires of the primary coil. The terminations of the secondary coil are fixed to the heads of the glass pillars, P, V, which are furnished with pointed .ids capable of universal motion. The excellence of the instrument depends on the insulation of the'secondtiry coil. The bobbin, must be'niade of glass, gutta percha, or (but of all) vulcanite, so as to prevent the induced electricity from reaching the ,round by the primary coil Care must also be taken to insulate the different parts of the secondary coil from each other. If this were not done, the spark which com pletes the secondary current, instead of taking place at the rods, the place at which it is wanted, would pass within the coil itself. It is necessary, in consequence,•to have each layer of the coil insulated from the other, by interposing gutta-percha paper, and cement ing it with a hot iron to the sides of the bobbin. The induced current must thus pass through all the turns of the wire, and is prevented from shortening its course by leaping over one or more layers of the coil.