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.

[Illustration:  Fig. 227.  Self-Soldering Heat-Coil Arrester]

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.

[Illustration:  Fig. 228.  Cook Arrester]

Another central-office protector, which uses a mass of special metal composition for its heat producing element is that designed by Frank B. Cook and shown in Fig. 228.  In this the carbon blocks are cylindrical in form and specially treated to make them “self-cleaning.”  Instead of employing a self-soldering feature in the sneak-current arrester of this device, Cook provides for electrically resoldering them after operation, a clip being designed for holding the elements in proper position and passing a battery current through them to remelt the solder.

In small magneto exchanges it is not uncommon to employ combined fuse and air-gap arresters for central-office line protection, the fuses being of the mica-mounted type already referred to.  A group of such arresters, as manufactured by the Dean Electric Company, is shown in Fig. 229.

[Illustration:  Fig. 229.  Mica Fuse and Air-Gap Arresters]

Types of Subscribers’ Station Protectors.  Figs. 230 and 231 show types of subscribers’ station protectors adapted to the requirements of central-battery and magneto systems.  These, as has been said, should be mounted at or near the point of entrance of the subscriber’s line into the premises, if the line is exposed outside of the premises.  It is possible to arrange the fuses so that they will be safe and suitable for their purposes if they are mounted out-of-doors near the point of entrance to the premises.  The sneak-current arrester, if one exists, and the carbon arrester also, must be mounted