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

Blowing out sparks.

strength of the current, and consequently the reactance e. m. f. of even the smallest coils in the circuit becomes very great, so great as to far exceed the resistance of the air gap in the lightning arrester, hence the current passes

to the ground through the latter path because it encounters less resistance. Lightning may strike a circuit that runs in the open air at any point, and from here the current will run both ways until it finds an escape to the ground. It follows, therefore, that if we desire to properly protect every device in the circuit, we must place

Simple lightning arresters.

arresters on both sides of every one if the connecting lines of both sides are so exposed that they can be struck.

Lightning discharges are prevented from passing through the wire coils wound upon the apparatus in the circuit, owing to enormous reactance of these, but they take the path through the arrester as an alternative simply because this offers less resistance than any other path. If there is a weak point in the insulation of any coil upon which the e. m. f. of the lightning is impressed, and this weak point is not very far from the entering point, then the discharge will break through the insulation at this point and pass to ground, or at least a portion of it will. From this it follows that to make an arrester as effective as possible, the resistance at the air gap must be as low as it can be without being liable to be bridged by the working current flowing in the circuit.

The simple type of lightning arrester illustrated in Fig. 143 cannot be used for circuits in which the currents have any considerable strength, that is, in circuit such as used for operation lights and motors. If such an arrester were placed in a light circuit, the result would be that the first time a lightning discharge passed to the ground, it would form a permanent ground connection, because the generator current would follow the lightning discharge and by producing an arc across the air gap would greatly reduce the resistance, making it so low that it could be readily overcome by the generator e. m. f.

For lightning and power circuits, it is necessary to so construct the lightning arresters, that the generator current cannot follow up the lightning discharge. This result is accomplished in several ways, one of which is illustrated in Fig. 144. In this diagram, b is the wire connecting the upper plate c of the lightning arrester

Non-arcing arresters.

with the circuit. The lower plate is located at e. A casing d, made of porcelain and provided with a number of holes, is placed between c and e. In the holes in d pieces of fuse wire are placed, these being of such length that they will not touch c, but come close to it, say, within a sixteenth of an inch. If, after a lightning discharge passes, the generator current persists in fol

lowing, then the fuse wire through which it passes will be melted and thus the air will widen out and finally become so long that the current will break. After the device has acted a number of times, so many of the fuse wires will be destroyed that it will have to be recharged.

Fig 145 shows a design in which the lightning discharge passes through carbons e and f placed within the chamber, but free to swing around the upper pivots. This arrester acts upon the principle that the heat generated by the arc formed between the carbon points in case the generator current follows up the lightning discharge, ex