Diagram of connections.

tended for high e. m. f. currents in which the switch contacts are immersed in oil, and we also showed designs for large capacity switches in which the movement is effected by means of compressed air. Switches are also made so

as to be immersed in oil, and at the same time so as to be actuated by compressed air. This construction, however, is only used for high e. m. f. and strong currents. Switches of this type are not placed on switchboards as

Oil switch operation.

they are too bulky for such location but are mounted on independent stands.

These oil switches are provided with three magnets, one to operate a valve that admits air to the cylinder, at the upper end, and thus force the piston down; one to operate the valve which admits air to the lower end of the cylinder and thus force the piston upward. The third magnet acts upon a latch which holds either one of the two valves in the open position, except when the third magnet is energized. From the diagram Fig. 142, the way in which the circuit of these three magnets is arranged can be understood. When the controlling switch on the switchboards is in the position F, the current passes through magnet B which actuates the catch. When the switch is moved to position E the current passes through magnets B and A and air is admitted to the upper end of the cylinder. When the switch is in position G, magnets B and C are connected, and the valve at the lower end of the cylinder is operated. It can be seen that to make this switch automatic, or in other words, a circuit breaker, all that is required is to provide a magnet that will move the controlling switch at the upper end of the diagram.

Lightning Arresters.

A lightning arrester is a device used to protect the apparatus connected in an electric circuit from the destructive effects of lightning. For circuit in which the current is very small, as for example, telegraph and telephone lines, the lightning arrester is a simple device consisting of a pair of sheet brass plates having saw teeth on the edge; these being set so as to leave a space of about one-sixteenth of an inch between the ends of the teeth. One of the plates is connected with the circuit, and the other one with the ground. Fig. 143 shows the way in which such an arrester is arranged and connected. When it is desired to make the protection more complete, two sets of plates are provided and connected as shown. The first diagram forms a single pole arrester, and the second is a double pole arrester.

The e. m. f. of the current that normally flows through the circuit is not sufficient to strike across the space separating the teeth of the lightning arrester, but when lightning strikes the line wires the e. m. f. is at once raised to an enormous magnitude, and for such an e. m. f. the short air space between the plates is an utterly insignificant resistance. Not only will the lightning discharge pass through the arrester, and to the ground, but none of it will go through the coils of wire on the magnets of the telephone or telegraph instruments. Now inasmuch

Preventing damage from lightning.

as the resistance of these coils is practically nothing in comparison with that of the air gap between the plates of the arrester, it may be asked, Why will lightning pass through the air gap and not through the wire coils?

This is not difficult to show. Suppose that the e. m.

f. of the lightning discharged is one million volts, then the instant it strikes the circuit, the e. m. f. is increased to one million volts. This instantaneous increase in e. m. f. produces an enormous instantaneous increase in the