ce 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.

_Air-Gap Arrester_. The tendency of a winding to oppose lightning
discharges and the ease with which such discharge may strike across
insulating gaps, points the way to protection against them. Such
devices consist of two conductors separated by an air space or other
insulator and are variously known as lightning arresters, spark gaps,
open-space cutouts, or air-gap arresters. The conductors between which
the gap exists may be both of metal, may be one of metal and one of
carbon, or both of carbon. One combination consists of carbon and
mercury, a liquid metal. The space between the conductors may be
filled with either air or solid matter, or it may be a vacuum.
Speaking generally, the conductors are separated by some insulator.
Two conductors separated by an insulator form a condenser. The
insulator of an open-space arrester often is called the dielectric.

[Illustration Fig. 203. Saw Tooth Arrester]

Discharge Across Gaps:--Electrical discharges across a given distance
occur at lower potentials if the discharge be between points than if
between smooth surfaces. Arresters, therefore, are provided with
points. Fig. 203 shows a device known as a "saw-tooth" arrester
because of its metal plates being provided with teeth. Such an
arrester brings a ground