The form used for intensity coils is the vibrating contact - breaker. It is not desirable to use a separate electro - magnet for intensity coils, as a resistance is offered by it to the passage of the battery current; therefore the iron bundle in the coil, which becomes an electro - magnet, is used instead. This necessitates an alteration in the position of the spring and iron clapper, which, as shown in Fig. 63, are placed vertically. The spring is fixed to a brass block attached to the pedestal, having a vertical plate rising on one side. A screw passes through this plate and comes into contact with the spring a little way above its point of fixation to the block; the use of this screw is to regulate the tension of the spring and its distance from the end of the iron bundle. At the top end of the spring is an iron cylinder or clapper about 1/2 in. long, and of similar diameter; the spring is adjusted so that the face of this cylinder may, when the spring is at rest, be about 1/4 in. from the end of the bundle. A strong brass pillar rises up also from the pedestal, and reaches a little above the centre of the coil.

Through the top of this pillar a strong screw (the platinum screw) passes, carrying on its end a piece of platinum, which comes into contact with the spring where the iron cylinder is attached to it. The spring at this part is armed with platinum, and it is here that the contact is made and broken. The platinum screw is provided with a running boss, so that, when the screw is adjusted, the boss can be brought up tightly against the pillar, and thus prevent the screw from shifting. The surfaces of the platinum require to be smoothed and scraped from time to time, in order to maintain complete contact.

Fig. 63.

Condenser

This is usually shut up in the cavity of the pedestal, though it can be separate. Its purport is to add to the energy of the current that traverses the primary wire, and consequently to increase the force of the secondary discharge. It consists of a number of tinfoil plates, separated by sheets of carefully varnished or rosin - ized paper, the alternate tinfoil plates being joined, thus forming 2 separate insulated series. One is connected with the pillar of the contact - breaker that carries the platinum screw, and the other with the block that holds the vibrating spring: these plates do not form part of the battery circuit, but are, as it were, lateral expansions of that circuit, on each side of the contact - breaker. The insulating sheets between the tinfoil plates thus have their elec trical condition disturbed; when the battery circuit is interrupted, the plates return to their normal state, and in so doing, increase the action of the current circulating in the primary wire. The paper for separating the plates should be moderately thin, not too heavily sized, cut into pieces rather larger than is required, dipped into a solution of 1 oz. shellac dissolved in 6 oz. methylated spirit, hung up to dry for some hours, and examined; if the minutest pinhole be observed in any sheet, it must be rejected.

A second coating of shellac varnish is applied, and when thoroughly dry, the paper is cut to the proper size, and preserved in a portfolio for use. For rosinized paper, ordinary tissue - paper does well, but white demy is better.

The condenser is made thus: - Prepare 50 sheets of tinfoil 5 in. long and wide, 60 pieces of insulating paper 7 in. by 5 in., and 2 thin mahogany boards of rather smaller size, varnished on each side. One board is laid down, and upon it 5 insulating papers are placed, then 1 tin - foil plate, taking care that 1 in. of the latter projects over one side of the varnished paper. Another paper is laid on this, coinciding in position with the first; on this comes a second tinfoil plate, but with the overhanging part at the opposite side. This is covered with Ah insulating paper, and followed by the other plates in similar order. When done, 5 more papers are laid on, then the second mahogany board, and the whole is tied up with guttapercha string. All the projecting tinfoils at one side are pressed together, also those at the other side; the condenser is then ready to be placed in the cavity of the pedestal.

Fedestal

This is made 13 in. long, 8 in. wide, and 2 in. deep. The bottom is movable, and fixed by screws or buttons. The coil is placed horizontally in the centre; holes are made in the top, in order to fix the coil in position. Other holes allow the ends of the primary wires, together with the pillars and binding screws, to be passed through, in order to attach them underneath. The contact - breaker is fixed at one end of the coil, and 2 binding - screws are fitted to the same end of the pedestal; at the other end are 2 ebonite pillars, 6 in. high and about 1/2 in. diameter. If the ebonite be cut off about 1 in. longer than required, the extra can be turned down to a pin 1/4 in. diameter, and a screw cut on its end. The holes in the pedestal are made sufficiently large to allow these pins to pass through, and the pillars can be firmly fixed by putting.a nut on the under side. On top of each pillar is a binding - screw with 2 holes and separate screws to each; one for the reception of an end of the secondary wire of the coil, and the other for attaching any apparatus to be employed in conjunction with the coil.