Experiments with Alternate Currents of High Potential and High Frequency | Page 6

state that, in spite of this, the coil is
capable of giving any potential which the best insulation of the turns is
able to withstand: but although he may be prepared, and even be
indifferent as to the anticipated result, yet the aspect of the discharge of
the coil will surprise and interest him. Every one is familiar with the
discharge of an ordinary coil; it need not be reproduced here. But, by
way of contrast, here is a form of discharge of a coil, the primary
current of which is vibrating several hundred thousand times per
second. The discharge of an ordinary coil appears as a simple line or
band of light. The discharge of this coil appears in the form of powerful
brushes and luminous streams issuing from all points of the two
straight wires attached to the terminals of the secondary. (Fig. 1.)

[Illustration: FIG. 1.--DISCHARGE BETWEEN TWO WIRES WITH
FREQUENCIES OF A FEW HUNDRED THOUSAND PER
SECOND.]
Now compare this phenomenon which you have just witnessed with the
discharge of a Holtz or Wimshurst machine--that other interesting
appliance so dear to the experimenter. What a difference there is
between these phenomena! And yet, had I made the necessary
arrangements--which could have been made easily, were it not that they
would interfere with other experiments--I could have produced with
this coil sparks which, had I the coil hidden from your view and only
two knobs exposed, even the keenest observer among you would find it
difficult, if not impossible, to distinguish from those of an influence or
friction machine. This may be done in many ways--for instance, by
operating the induction coil which charges the condenser from an
alternating-current machine of very low frequency, and preferably
adjusting the discharge circuit so that there are no oscillations set up in
it. We then obtain in the secondary circuit, if the knobs are of the
required size and properly set, a more or less rapid succession of sparks
of great intensity and small quantity, which possess the same brilliancy,
and are accompanied by the same sharp crackling sound, as those
obtained from a friction or influence machine.
Another way is to pass through two primary circuits, having a common
secondary, two currents of a slightly different period, which produce in
the secondary circuit sparks occurring at comparatively long intervals.
But, even with the means at hand this evening, I may succeed in
imitating the spark of a Holtz machine. For this purpose I establish
between the terminals of the coil which charges the condenser a long,
unsteady arc, which is periodically interrupted by the upward current of
air produced by it. To increase the current of air I place on each side of
the arc, and close to it, a large plate of mica. The condenser charged
from this coil discharges into the primary circuit of a second coil
through a small air gap, which is necessary to produce a sudden rush of
current through the primary. The scheme of connections in the present
experiment is indicated in Fig. 2.

[Illustration: FIG. 2.--IMITATING THE SPARK OF A HOLTZ
MACHINE.]
G is an ordinarily constructed alternator, supplying the primary P of an
induction coil, the secondary S of which charges the condensers or jars
CC. The terminals of the secondary are connected to the inside coatings
of the jars, the outer coatings being connected to the ends of the
primary pp of a second induction coil. This primary pp has a small air
gap ab.
The secondary s of this coil is provided with knobs or spheres KK of
the proper size and set at a distance suitable for the experiment.
A long arc is established between the terminals AB of the first
induction coil. MM are the mica plates.
Each time the arc is broken between A and B the jars are quickly
charged and discharged through the primary pp, producing a snapping
spark between the knobs KK. Upon the arc forming between A and B
the potential falls, and the jars cannot be charged to such high potential
as to break through the air gap ab until the arc is again broken by the
draught.
In this manner sudden impulses, at long intervals, are produced in the
primary pp, which in the secondary s give a corresponding number of
impulses of great intensity. If the secondary knobs or spheres, KK, are
of the proper size, the sparks show much resemblance to those of a
Holtz machine.
But these two effects, which to the eye appear so very different, are
only two of the many discharge phenomena. We only need to change
the conditions of the test, and again we make other observations of
interest.
When, instead of operating the induction coil as in the last two
experiments, we operate it