rves 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.

Mica Fuse:--A convenient and widely used form of fuse is that shown in
Fig. 216. A mica slip has metal terminals at its ends and a fuse wire
joins these terminals. The fuse is inserted in the circuit by clamping
the terminals under screws or sliding them between clips as in Figs.
217 and 218. Advantages of this method of fuse mounting for protecting
circuits needing small currents are that the fuse wire can be seen, the
fuses are readily replaced when blown, and their mountings may be made
compact. As elements of a comprehensive protective system, however, the
ordinary types of mica-slip fuses are objectionable because too short,
and because they have no means of their own for extinguishing an arc
which may follow the blowing of the fuses. As protectors for use in
distributing low potential currents from central-office power plants
they are admirable. By simple means, they may be made to announce
audibly or visibly that they have operated.

[Illustration: Fig. 216. Mica Slip Fuse]

[Illustration: Fig. 217. Postal Type Mica Fuse]

[Illustration: Fig. 218. Western Union Type Mica Fuse]

Enclosed Fuses:--If a fuse wire within an insulating tube be made to
connect metal caps on that tube and the space around the tube be
filled with a non-conducting powder, the gases of the vaporized fuse
metal will be absorbed more quickly than when formed without such
imbedding in a powder. The filling of such a tubular fuse also muffles
the explosion which occurs when the fuse is vaporized.

[Illustration: Fig. 219. Pair of Enclosed Fuses]

Fuses of the enclosed type, with or without filling,