to the moving field, which means that in that experiment the magnet
may not rotate, or may even rotate in the opposite direction to the
moving disc. Here, then, is a motor (diagrammatically illustrated in
Fig. 17), comprising a coil and iron core, and a freely movable copper
disc in proximity to the latter.

[Illustration: FIG. 17.--SINGLE WIRE AND "NO-WIRE" MOTOR.]

To demonstrate a novel and interesting feature, I have, for a reason
which I will explain, selected this type of motor. When the ends of
the coil are connected to the terminals of an alternator the disc is
set in rotation. But it is not this experiment, now well known, which
I desire to perform. What I wish to show you is that this motor
rotates with _one single_ connection between it and the generator;
that is to say, one terminal of the motor is connected to one terminal
of the generator--in this case the secondary of a high-tension
induction coil--the other terminals of motor and generator being
insulated in space. To produce rotation it is generally (but not
absolutely) necessary to connect the free end of the motor coil to an
insulated body of some size. The experimenter's body is more than
sufficient. If he touches the free terminal with an object held in the
hand, a current passes through the coil and the copper disc is set in
rotation. If an exhausted tube is put in series with the coil, the
tube lights brilliantly, showing the passage of a strong current.
Instead of the experimenter's body, a small metal sheet suspended on a
cord may be used with the same result. In this case the plate acts as
a condenser in series with the coil. It counteracts the self-induction
of the latter and allows a strong current to pass. In such a
combination, the greater the self-induction of the coil the smaller
need be the plate, and this means that a lower frequency, or