Types of Central-Office Protectors. A form of combined heat coil and air-gap arrester, widely used by Bell companies for central-office protection, is shown in Fig. 226. The two inner springs form the terminals for the two limbs of the metallic-circuit line, while the two outside springs are terminals for the continuation of the line leading to the switchboard. The heat coils, one on each side, are supported between the inner and outer springs. High-tension currents jump to ground through the air-gap arrester, while sneak currents permit the pin of the heat coil to slide within the sleeve, thus grounding the outside line and the line to the switchboard.

Fig. 226. Sneak-Current and Air-Gap Arrester
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Self-Soldering Heat Coils. Another form designed by Kaisling and manufactured by the American Electric Fuse Company is shown in Fig. 227. In this the pin in the heat coil projects unequally from the ends of the coil, and under the action of a sneak current the melting of the solder which holds it allows the outer spring to push the pin through the coil until it presses the line spring against the ground plate and at the same time opens the path to the switchboard. When the heat-coil pin assumes this new position it cools off, due to the cessation of the current, and resolders itself, and need only be turned end for end by the attendant to be reset. Many are the variations that have been made on this self-soldering idea, and there has been much controversy as to its desirability. It is certainly a feature of convenience.

Fig. 227. Self-Soldering Heat-Coil Arrester
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Instead of using a wire-wound resistance element in heat-coil construction some manufacturers employ a mass of high-resistance material, interposed in the path of the current. The Kellogg Company has long employed for its sneak-current arrester a short graphite rod, which forms the resistance element. The ends of this rod are electroplated with copper to which the brass terminal heads are soldered. These heads afford means for making the connection with the proper retaining springs.