e impedance
coils are wound on insulating rods, which hold them near, but not
touching, the ground carbon. The coils are arranged so that they may
be turned when discharges roughen the surfaces of the wires.

[Illustration: Fig. 214. Holtzer-Cabot Arrester]

Metallic Electrodes:--Copper or other metal blocks with roughened
surfaces separated by an insulating slip may be substituted for the
carbon blocks of most of the arresters previously described. Metal
blocks lack the advantage of carbon in that the latter allows
discharges at lower potentials for a given separation, but they have
the advantage that a conducting dust is not thrown off from them.

[Illustration: Fig. 215. Carbon Air-Gap Arrester]

Provision Against Continuous Arc:--For the purpose of short-circuiting
an arc, a globule of low-melting alloy may be placed in one carbon
block of an arrester. This feature is not essential in an arrester
intended solely to divert lightning discharges. Its purpose is to
provide an immediate path to ground if an arc arising from artificial
electricity has been maintained between the blocks long enough to melt
the globule. Fig. 215 is a plan and section of the Western Electric
Company's arrester used as the high potential element in conjunction
with others for abnormal currents and sneak currents; the latter are
currents too small to operate air-gap arresters or substantial fuses.

Protection Against Strong Currents. _Fuses._ A fuse is a metal
conductor of lower carrying capacity than the circuit with which it is
in series at the time it is required to operate. Fuses in use in
electrical circuits generally are composed of some alloy of lead,
which melts at a reasonably low temperature. Alloys of lead have lower
conductivity than copper. A small copper wire, however, may fuse at
the same volume of current as a larger lead alloy wire.

Proper Functions:--A fuse is not a good lightning arrester. As
lightning damage is caused by current and as it is current which
destroys a fuse, a lightning discharge _can_ open a circuit over which
it passes by melting the fuse metal. But lightning may destroy a fuse
and at the same discharge destroy apparatus in series with the fuse.
There are two reasons for this: One is that lightning discharges act
very quickly and may have destroyed apparatus before heating the fuse
enough to melt it; the other reason is that when a fuse is operated
with enough current even to vaporize it, the vapor se