thermal action may produce heat enough to melt
metals, to char things which can be burned, or to cause them actually
to burn, perhaps with a fire which can spread; the chemical action may
destroy property values by changing the state of metals, as by
dissolving them from a solid state where they are needed into a state
of solution where they are not needed; the magnetic action introduces
no direct hazard. The greatest hazard to which property values are
exposed is the electro-thermal action; that is, the same useful
properties by which electric lighting and electric heating thrive may
produce heat where it is not wanted and in an amount greater than can
safely be borne.

The tendency of design is to make all apparatus capable of carrying
without overheating any current to which voltage within the telephone
system may subject it, and to provide the system so designed with
specific devices adapted to isolate it from currents originating
without. Apparatus which is designed in this way, adapted not only to
carry its own normal working currents but to carry the current which
would result if a given piece of apparatus were connected directly
across the maximum pressure within the telephone system itself, is
said to be self-protecting. Apparatus amply able to carry its maximum
working current but likely to be overheated, to be injured, or perhaps
to destroy itself and set fire to other things if subjected to the
maximum pressure within the system, is not self-protecting apparatus.

To make all electrical devices self-protecting by surrounding them
with special arrangements for warding off abnormal currents from
external sources, is not as simple as might appear. A lamp, for
example, which can bear the entire pressure of a central-office
battery, is not suitable for direct use in a line several miles long
because it would not give a practical signal in series with that line
and with the telephone set, as it is required to do. A lamp suitable
for use in series with such a line and a telephone set would burn out
by current from its own normal source if the line should become
short-circuited in or near the central office. The ballast referred to
in the chapter on "Signals" was designed for the very purpose of
providing rapidly-rising resistance to offset the tendency toward
rapidly-rising current which could burn out the lamp.

As another example, a very small direct-current electric motor can be
turned on at a snap switch and