not broken at all,
but merely grounded. Each method has its advantages.

Complete Line Protection. Fig. 225 shows the entire scheme of protectors
in an exposed line and their relation to apparatus in the central-office
equipment and at the subscriber's telephone. The central-office
equipment contains heat coils, springs, and carbon arresters. At some
point between the central office and the subscriber's premises, each
wire contains a fuse. At the subscriber's premises each wire contains
other fuses and these are associated with carbon arresters. The figure
shows a central battery equipment, in which the ringer of the telephone
is in series with a condenser. A sneak-current arrester is not required
at the subscriber's station with such equipment.

Assume the line to meet an electrical hazard at the point _X_. If this
be lightning, it will discharge to ground at the central office or at
the subscriber's instrument or at both through the carbon arresters
connected to that side of the line. If it be a high potential from a
power circuit and of more than 350 volts, it will strike an arc at the
carbon arrester connected to that wire of the line in the central
office or at the subscriber's telephone or at both, if the separation
of the carbons in those arresters is .005 inch or less. If the carbon
arresters are separated by celluloid, it will burn away and allow the
carbons to come together, extinguishing the arc. If they are separated
by mica and one of the carbons is equipped with a globule of
low-melting alloy, the heat of the arc will melt this,
short-circuiting the gap and extinguishing the arc. The passage of
current to ground at the arrester, however, will be over a path
containing nothing but wire and the arrester. The resulting current,
therefore, may be very large. The voltage at the arrester having been
350 volts or more, in order to establish the arc, short-circuiting the
gap will make the current 7 amperes or more, unless the applied
voltage miraculously falls to 50 volts or less. The current through
the fuse being more than 7 amperes, it will blow promptly, opening the
line and isolating the apparatus. It will be noted that this
explanation applies to equipment at either end of the line, as the
fuse lies between the point of contact and the carbon arrester.

[Illustration: Fig. 225. Complete Line Protection]

Assume, on the other hand, that the contact is made at the point _Y_.
The central-office carbon arre