.

[Illustration: Fig. 230. Western Electric Station Arrester]

[Illustration: Fig. 231. Cook Arrester for Magneto Stations]

_Ribbon Fuses_. A point of interest with relation to tubular fuses is
that in some of the best types of such fuses, the resistance material
is not in the form of a round wire but in the form of a flat ribbon.
This arrangement disposes the necessary amount of fusible metal in a
form to give the greatest amount of surface, while a round wire offers
the least surface for a given weight of metal--a circle encloses its
area with less periphery than any other figure. The reason for giving
the fuse the largest possible surface area is to decrease the
likelihood of the fuse being ruptured by lightning. The fact that such
fuses do withstand lightning discharges much more thoroughly than
round fuses of the same rating is an interesting proof of the
oscillating nature of lightning discharges, for the density of the
current of those discharges is greater on and near the surface of the
conductor than within the metal and, therefore, flattening the fuse
increases its carrying capacity for high-frequency currents, without
appreciably changing its carrying capacity for direct currents. The
reason its capacity for direct currents is increased at all by
flattening it, is that the surface for the radiation of heat is
increased. However, when enclosed in a tube, radiation of heat is
limited, so that for direct currents the carrying capacity of fuses
varies closely with the area of cross-section.

City-Exchange Requirements. The foregoing has set down the
requirements of good practice in an average city-exchange system.
Nothing short of the general arrangement shown in Fig. 225 meets the
usual assortment of hazards of such an exchange. It is good modern
practice to distribute lines by means of cables, supplemented in part
by short insulated drop wires twisted in pairs. Absence of bare wires
reduces electrical hazards enormously. Nevertheless, hazards remain.

Though no less than the spirit of this plan of protection should be
followed, additional hazards may exist, which may require additional
elements of protection. At the end of a cable, either aerial or
underground, long open wires may extend into the open country as rural
or long-distance circuits. If these be longer than a mile or two, in
most regions they will be subjected to lightning discharges. These may
be subjected to high-potential contacts as well.