factor in the heating of a conductor when the electric density is
great and the frequency very high. That in the heating of conductors
by lightning discharges air is an element of great importance, is
almost as certain as an experimental fact. I may illustrate the action
of the air by the following experiment: I take a short tube which is
exhausted to a moderate degree and has a platinum wire running through
the middle from one end to the other. I pass a steady or low frequency
current through the wire, and it is heated uniformly in all parts. The
heating here is due to conduction, or frictional losses, and the gas
around the wire has--as far as we can see--no function to perform. But
now let me pass sudden discharges, or a high frequency current,
through the wire. Again the wire is heated, this time principally on
the ends and least in the middle portion; and if the frequency of the
impulses, or the rate of change, is high enough, the wire might as
well be cut in the middle as not, for practically all the heating is
due to the rarefied gas. Here the gas might only act as a conductor of
no impedance diverting the current from the wire as the impedance of
the latter is enormously increased, and merely heating the ends of the
wire by reason of their resistance to the passage of the discharge.
But it is not at all necessary that the gas in the tube should he
conducting; it might be at an extremely low pressure, still the ends
of the wire would be heated--as, however, is ascertained by
experience--only the two ends would in such, case not be electrically
connected through the gaseous medium. Now what with these frequencies
and potentials occurs in an exhausted tube occurs in the lightning
discharges at ordinary pressure. We only need remember one of the
facts arrived at in the course of these investigations, namely, that
to impulses of very high frequency the gas at ordinary pressure
behaves much in the same manner as though it were at moderately low
pressure. I think that in lightning discharges frequently wires or
conducting objects are volatilized merely because air is present and
that, were the conductor immersed in an insulating liquid, it would be
safe, for then the energy would have to spend itself somewhere else.
From the behavior of gases to sudden impulses of high potential I am
led to conclude that there can be no surer way of diverting a
lightning discharge than by affording it a passage through a volume of
gas, if such a thing can be done in a practical manner.
There are two more features upon which I think it necessary to dwell
in connection with these experiments--the "radiant state" and the
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