are widely used
in power circuits generally and are recommended by fire insurance
bodies. Fig. 219 illustrates an arrester having a fuse of the enclosed
type, this example being that of the H. W. Johns-Manville Company.

[Illustration Fig. 220. Bank of Enclosed Fuses]

In telephony it is frequently necessary to mount a large number of
fuses or other protective devices together in a restricted space. In
Fig. 220 a group of Western Electric tubular fuses, so mounted, is
shown. These fuses have ordinarily a carrying capacity of 6 or 7
amperes. It is not expected that this arrester will blow because 6 or
7 amperes of abnormal currents are flowing through it and the
apparatus to be protected. What is intended is that the fuse shall
withstand lightning discharges and when a foreign current passes
through it, other apparatus will increase that current enough to blow
the fuse. It will be noticed that the fuses of Fig. 220 are open at
the upper end, which is the end connected to the exposed wire of the
line The fuses are closed at the lower end, which is the end connected
to the apparatus. When the fuse blows, its discharge is somewhat
muffled by the lining of the tube, but enough explosion remains so
that the heated gases, in driving outward, tend to break the arc which
is established through the vaporized metal.

A pair of Cook tubular fuses in an individual mounting is shown in
Fig. 221. Fuses of this type are not open at one end like a gun, but
opportunity for the heated gases to escape exists at the caps. The
tubes are made of wood, of lava, or of porcelain.

Fig. 222 is another tubular fuse, the section showing the arrangement
of asbestos lining which serves the two purposes of muffling the sound
of the discharge and absorbing and cooling the resulting gases.

[Illustration: Fig. 221. Pair of Wooden Tube Fuses]

_Air-Gap vs. Fuse Arresters._ It is hoped that the student grasps
clearly the distinction between the purposes of air-gap and fuse
arresters. The air-gap arrester acts in response to high voltages,
either of lightning or of high-tension power circuits. The fuse acts in
response to a certain current value flowing through it and this minimum
current in well-designed protectors for telephone lines is not very
small. Usually it is several times larger than the maximum current
apparatus in the line can safely carry. Fuses _can_ be made so delicate
as to operate on the very smallest current which could injure appa