The flange f conveniently serves for holding the brass piece when
fastening the wire, and also for turning it in any position when it
becomes necessary to present a fresh discharging surface. Two stout
strips of hard rubber RR, with planed grooves gg (Fig. 7) to fit the
middle portion of the pieces cc, serve to clamp the latter and hold
them firmly in position by means of two bolts CC (of which only one is
shown) passing through the ends of the strips.
[Illustration: FIG. 7.--DISCHARGER WITH MULTIPLE GAPS.]
In the use of this kind of discharger I have found three principal
advantages over the ordinary form. First, the dielectric strength of a
given total width of air space is greater when a great many small air
gaps are used instead of one, which permits of working with a smaller
length of air gap, and that means smaller loss and less deterioration
of the metal; secondly by reason of splitting the arc up into smaller
arcs, the polished surfaces are made to last much longer; and,
thirdly, the apparatus affords some gauge in the experiments. I
usually set the pieces by putting between them sheets of uniform
thickness at a certain very small distance which is known from the
experiments of Sir William Thomson to require a certain electromotive
force to be bridged by the spark.
It should, of course, be remembered that the sparking distance is much
diminished as the frequency is increased. By taking any number of
spaces the experimenter has a rough idea of the electromotive force,
and he finds it easier to repeat an experiment, as he has not the
trouble of setting the knobs again and again. With this kind of
discharger I have been able to maintain an oscillating motion without
any spark being visible with the naked eye between the knobs, and they
would not show a very appreciable rise in temperature. This form of
discharge also lends itself to many arrangements of condensers and
circuits which are often very convenient and time-saving. I have used
it preferably in a disposition similar to that indicated in Fig. 2,
when the currents forming the arc are small.
I may here mention that I have also used dischargers with single or
multiple air gaps, in which the discharge surfaces were rotated with
great speed. No particular advantage was, however, gained by this
method, except in cases where the currents from the condenser were
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