from a high frequency alternator, as in the
next experiment, a systematic study of the phenomena is rendered
much more easy. In such case, in varying the strength and frequency of
the currents through the primary, we may observe five distinct forms of
discharge, which I have described in my former paper on the subject[A]
before the American Institute of Electrical Engineers, May 20, 1891.
[Footnote A: See THE ELECTRICAL WORLD, July 11, 1891.]
It would take too much time, and it would lead us too far from the
subject presented this evening, to reproduce all these forms, but it
seems to me desirable to show you one of them. It is a brush discharge,
which is interesting in more than one respect. Viewed from a near
position it resembles much a jet of gas escaping under great pressure.
We know that the phenomenon is due to the agitation of the molecules
near the terminal, and we anticipate that some heat must be developed
by the impact of the molecules against the terminal or against each
other. Indeed, we find that the brush is hot, and only a little thought
leads us to the conclusion that, could we but reach sufficiently high
frequencies, we could produce a brush which would give intense light
and heat, and which would resemble in every particular an ordinary
flame, save, perhaps, that both phenomena might not be due to the
same agent--save, perhaps, that chemical affinity might not be
electrical in its nature.
As the production of heat and light is here due to the impact of the
molecules, or atoms of air, or something else besides, and, as we can
augment the energy simply by raising the potential, we might, even
with frequencies obtained from a dynamo machine, intensify the action
to such a degree as to bring the terminal to melting heat. But with such
low frequencies we would have to deal always with something of the
nature of an electric current. If I approach a conducting object to the
brush, a thin little spark passes, yet, even with the frequencies used this
evening, the tendency to spark is not very great. So, for instance, if I
hold a metallic sphere at some distance above the terminal you may see
the whole space between the terminal and sphere illuminated by the
streams without the spark passing; and with the much higher
frequencies obtainable by the disruptive discharge of a condenser, were
it not for the sudden impulses, which are comparatively few in number,
sparking would not occur even at very small distances. However, with
incomparably higher frequencies, which we may yet find means to
produce efficiently, and provided that electric impulses of such high
frequencies could be transmitted through a conductor, the electrical
characteristics of the brush discharge would completely vanish--no
spark would pass, no shock would be felt--yet we would still have to
deal with an electric phenomenon, but in the broad, modern
interpretation of the word. In my first paper before referred to I have
pointed out the curious properties of the brush, and described the best
manner of producing it, but I have thought it worth while to endeavor
to express myself more clearly in regard to this phenomenon, because
of its absorbing interest.
When a coil is operated with currents of very high frequency, beautiful
brush effects may be produced, even if the coil be of comparatively
small dimensions. The experimenter may vary them in many ways, and,
if it were nothing else, they afford a pleasing sight. What adds to their
interest is that they may be produced with one single terminal as well
as with two--in fact, often better with one than with two.
But of all the discharge phenomena observed, the most pleasing to the
eye, and the most instructive, are those observed with a coil which is
operated by means of the disruptive discharge of a condenser. The
power of the brushes, the abundance of the sparks, when the conditions
are patiently adjusted, is often amazing. With even a very small coil, if
it be so well insulated as to stand a difference of potential of several
thousand volts per turn, the sparks may be so abundant that the whole
coil may appear a complete mass of fire.
Curiously enough the sparks, when the terminals of the coil are set at a
considerable distance, seem to dart in every possible direction as
though the terminals were perfectly independent of each other. As the
sparks would soon destroy the insulation it is necessary to prevent them.
This is best done by immersing the coil in a good liquid insulator, such
as boiled-out oil. Immersion in a liquid may be considered almost an
absolute necessity for the continued and successful working of such a
coil.
It is of
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