[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 serves as a conducting path for an instant after being formed.  This conducting path may be of high resistance and still allow currents to flow through it, because of the extremely high pressure of the lightning discharge.  A comprehensive protective system may include fuses, but it is not to be expected that they always will arrest lightning or even assist other things in arresting lightning.  They should be considered as of no value for that purpose.  Furthermore, fuses are best adapted to be a part of a general protective system when they do all that they must do in stopping abnormal currents and yet withstand lightning discharges which may pass through them.  Other things being equal, that system of protection is best in which all lightning discharges are arrested by gap arresters and in which no fuses ever are operated by lightning discharges.