A small current through the little coil will warm the wax enough to allow the string to part.  The spring then will ground the line.  Even so simple an apparatus as this operates with considerable accuracy.  All currents below a certain critical amount may flow through the heating coil indefinitely, the heat being radiated rapidly enough to keep the wax from softening and the string from parting.  All currents above this critical amount will operate the arrester; the larger the current, the shorter the time of operating.  It will be remembered that the law of these heating effects is that the heat generated = C^{2}Rt, so that if a certain current operates the arrester in, say 40 seconds, twice as great a current should operate the arrester in 10 seconds.  In other words, the time of operation varies inversely as the square of the current and inversely as the resistance.  To make the arrester more sensitive for a given current—­i.e., to operate in a shorter time—­one would increase the resistance of the coil in the wax either by using more turns or finer wire, or by making the wire of a metal having higher specific resistance.

The present standard sneak-current arrester embodies the two elements of the devices of Fig. 223:  a resistance material to transform the dangerous sneak current into localized heat; and a fusible material softened by this heat to release some switching mechanism.

The resistance material is either a resistance wire or a bit of carbon, the latter being the better material, although both are good.  The fusible material is some alloy melting at a low temperature.  Lead, tin, bismuth, and cadmium can be combined in such proportions as will enable the alloy to melt at temperatures from 140 deg. to 180 deg.  F. Such an alloy is a solder which, at ordinary temperatures, is firm enough to resist the force of powerful springs; yet it will melt so as to be entirely fluid at a temperature much less than that of boiling water.

[Illustration:  Fig. 224.  Heat Coil]

Heat Coil. Fig. 224 shows a practical way of bringing the heating and to-be-heated elements together.  A copper spool is wound with resistance wire.  A metal pin is soldered in the bore of the spool by an easily melting alloy.  When current heats the spool enough, the pin may slide or turn in the spool.  It may slide or turn in many ways and this happily enables many types of arresters to result.  For example, the pin may pull out, or push in, or push through, or rotate like a shaft in a bearing, or the spool may turn on it like a hub on an axle.  Messrs. Hayes, Rolfe, Cook, McBerty, Kaisling, and many other inventors have utilized these combinations and motions in the production of sneak-current arresters.  All of them depend on one action:  the softening of a low-melting alloy by heat generated in a resistance.

When a heat coil is associated with the proper switching springs, it becomes a sneak-current arrester.  The switching springs always are arranged to ground the line wire.  In some arresters, the line wire is cut off from the wire leading toward the apparatus by the same movement which grounds it.  In others, the line is not broken at all, but merely grounded.  Each method has its advantages.