his causes current to flow from the cars toward the power
stations, the return path being made up jointly of the rails, the
earth itself, actual return wires which may supplement the rails, and
also all other conducting things in the earth, these being principally
lead-covered cables and other pipes. These conditions establish
definite areas in which the currents tend to leave the cables and
pipes, _i.e._, in which the latter are positive to other things. These
positive areas usually are much smaller than the negative areas, that
is, the regions in which currents tend _to enter_ the cables form a
larger total than the regions in which the currents tend _to leave_
the cables. These facts simplify the ways in which the cables may be
protected against damage by direct currents leaving them and also they
reduce the amount, complication, and cost of applying the corrective
and preventive measures.

All electric roads do not use direct current. Certain simplifications
in the use of single-phase alternating currents in traction motors
have increased the number of roads using a system of
alternating-current power supply. Where alternating current is used,
the electrolytic conditions are different and a new problem is set,
for, as the current flows in recurrently different directions, an area
which at one instant is positive to others, is changed the next
instant into a negative area. The protective means, therefore, must be
adapted to the changed requirements.

CHAPTER XIX

PROTECTIVE MEANS

Any of the heating hazards described in the foregoing chapter may
cause currents which will damage apparatus. All devices for the
protection of apparatus from such damage, operate either to stop the
flow of the dangerous current, or to send that flow over some other
path.

Protection Against High Potentials. Lightning is the most nearly
universal hazard. All open wires are exposed to it in some degree.
Damaging currents from lightning are caused by extraordinarily high
potentials. Furthermore, a lightning discharge is oscillatory; that
is, alternating, and of very high frequency. Drops, ringers,
receivers, and other devices subject to lightning damage suffer by
having their windings burned by the discharge. The impedance these
windings offer to the high frequency of lightning oscillations is
great. The impedance of a few turns of heavy wire may be negligible to
alternating currents of ordinary frequencies because the resistan