nd this is just what all well-designed protective devices are
intended to do. The general problem which these heating hazards
present with relation to telephone apparatus and circuits is: _To
cause all parts of the telephone system to be made so as to carry
successfully all currents which may flow in them because of any
internal or external pressure, or to supplement them by devices which
will stop or divert currents which could overheat them._

Electrolytic hazards depend not on the heating effects of currents but
on their chemical effects. The same natural law which enables primary
and secondary batteries to be useful provides a hazard which menaces
telephone-cable sheaths and other conductors. When a current leaves a
metal in contact with an electrolyte, the metal tends to dissolve into
the electrolyte. In the processes of electroplating and electrotyping,
current enters the bath at the anode, passes from the anode through
the solution to the cathode, removing metal from the former and
depositing it upon the latter. In a primary battery using zinc as the
positive element and the negative terminal, current is caused to pass,
within the cell, from the zinc to the negative element and zinc is
dissolved. Following the same law, any pipe buried in the earth may
serve to carry current from one region to another. As single-trolley
traction systems with positive trolley wires constantly are sending
large currents through the earth toward their power stations, such a
pipe may be of positive potential with relation to moist earth at some
point in its length. Current leaving it at such a point may cause its
metal to dissolve enough to destroy the usefulness of the pipe for its
intended purpose.

Lead-sheathed telephone cables in the earth are particularly exposed
to such damage by electrolysis. The reasons are that such cables often
are long, have a good conductor as the sheath-metal, and that metal
dissolves readily in the presence of most aqueous solutions when
electrolytic differences of potential exist. The length of the cables
enables them to connect between points of considerable difference of
potential. It is lack of this length which prevents electrolytic
damage to masses of structural metal in the earth.

Electrical power is supplied to single-trolley railroads principally
in the form of direct current. Usually all the trolley wires of a city
are so connected to the generating units as to be positive to the
rails. T