An electrified wire is capable of exciting a current In another wire placed near it, but nut In contact, and such a current is termed an induced current. Induced currents generally have a very high electromotive force, and are capable of sparking across far greater spaces than can be accomplished by ordinary battery currents. An induction coil consists of a cylindrical bobbin with an iron core, surrounded first by a primary coil of stout wire, and then by a secondary coil of very fine wire, carefully insulated between the different parts. The primary coil is joined to the terminals of some Bunsen or Grove cells, and includes an interrupter (contact breaker) and a commutator. The object of the former is to repeatedly and rapidly make and break the primary circuit. The primary coil, destined to carry strong currents, and produce a powerful magnetic field at the centre, is made in few turns, so as to lessen resistance and avoid self - induction of the primary current. The iron core, whose value depends upon its great co - efficient of magnetic induction, is best made of a bundle of fine wires to avoid induction currents.
The secondary coil is made in many turns that the coefficient of mutual induction may be large, its increased resistance being immaterial in the presence of such great electro - motive force. With these genera] explanations, the construction of induction coils may be entered upon, the information being mainly condensed from Dyer's practical little book.
Prepare a paper tube, about 4 in. long and 3/4 in. diameter, and wind on it 2 or 3 layers of copper wire covered with cotton, and of the size of ordinary bell wire. A binding - screw is attached to each end of the wire, as shown in Fig. 58, by which means it can be united to a battery. The paper tube upon which the wire is wound it tilled up with a bundle of iron wire a.
Prepare a second paper tube of similar length to that in the centre of the primary coil, but large enough.in diameter to slide over it. Fit 2 discs of wood on the ends of this tube, and then wind on 5 or 6 layers of cotton - covered copper wire about the site of stout packthread, and attach binding - screws to its beginning and end (Fig. 59). If the primary coil be attached to a battery, and contact be broken rapidly, distinct shocks may be felt from the induction coil. Thus are constructed the coils of electro - magnetic machines for medical and experimental purposes. But such are not intensity coils, for their mode of construction involves the loss of nearly ail the electrical current excited in the wires of which they are composed.