NDRED 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 th