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

Arrester for alternating circuits.

pands the air within the chamber and thus forces the carbons out and in that way make the air gap so long that the e. m. f. of the generator cannot maintain the arc. Fig. 146 shows a way in which lightning arresters

are combined with a reactive, or choking coil, to render them more effective. The reactance of the coil k acts to prevent the lightning from passing through and thus helps the discharge to break across the air gap in the

Railway arrester.

arrester and also takes the strain off the apparatus beyond it in the circuit.

Fig 147 shows what is commonly called a magnetic blow-out lightning arrester. The magnet M, which is wound with rather large wire, is connected with the main ine. When a lightning discharge passes to ground it

Figure 150

Lightning arresters for direct-current circuits.
Tank lightning arrester for railway service, Type T.

bridges the points e e. If the generator current follows the discharge the line of force of the magnet force the current out of the position d, and when the arc is forced so far out as to make the length greater than the e. m. f. of the circuit can surmount, the current breaks.

Fig. 148 shows a form of lightning arrester that is used extensively in connection with alternating currents.

Non-arcing metals.

The construction is clearly shown. It consists of a number of short cylinders made of non-arcing metal which are held close to each other in a porcelain casing. The lightning discharge strikes across from cylinder to cylinder until it reaches the ground.

One of the choke coils of an arrester, used in connection with alternating current circuits of very high e. m. f., is shown in Fig. 149, by connecting a large number in series, and combining these with a corresponding number of reactive coils. These arresters and coils are assembled in a frame with the arresters arranged at the top of the frame, and the coils piled side by side under them.

Fig. 150 shows a type of arrester for direct current circuits, especially electric railways.

A number of theories have been advanced to explain the action of so-called non-arcing metals. We know from experience that with some metals the length of arc that can be sustained with an e. m. f. is greater than with other, and a question naturally arises as to what causes this difference. If we investigate the action from the instant when the arc begins to form we will be able to discover what occurs, and thus be able to form some idea of the cause of the difference in action with different metals.

Suppose we take two rods of metal pressing against each other end to end. If we separate them the current will jump across the intervening air space. At the instant the arc is formed the resistance between the points is very high, being due to the length of the column of air between the points. The current passing over this resistance generates a large amount of heat, and this vaporizes the metal, thus filling the space between the ends of the