conds, 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