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 resistance of the wire is low, its inductance small, and the frequency finite.  On the other hand, the impedance of such a coil to a lightning discharge is much higher, due to the very high frequency of the discharge.

Were it not for the extremely high pressure of lightning discharges, their high frequency of oscillation would enable ordinary coils to be self-protecting against them.  But a discharge of electricity can take place through the air or other insulating medium if its pressure be high enough.  A pressure of 70,000 volts can strike across a gap in air of one inch, and lower pressures can strike across smaller distances.  When lightning encounters an impedance, the discharge seldom takes place through the entire winding, as an ordinary current would flow, usually striking across whatever short paths may exist.  Very often these paths are across the insulation between the outer turns of a coil.  It is not unusual for a lightning discharge to plow its way across the outer layer of a wound spool, melting the copper of the turns as it goes.  Often the discharge will take place from inner turns directly to the core of the magnet.  This is more likely when the core is grounded.