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 eventually a lower potential, is required to operate the motor.  A single coil wound upon a core has a high self-induction; for this reason principally, this type of motor was chosen to perform the experiment.  Were a secondary closed coil wound upon the core, it would tend to diminish the self-induction, and then it would be necessary to employ a much higher frequency and potential.  Neither would be advisable, for a higher potential would endanger the insulation of the small primary coil, and a higher frequency would result in a materially diminished torque.

It should be remarked that when such a motor with a closed secondary is used, it is not at all easy to obtain rotation with excessive frequencies, as the secondary cuts off almost completely the lines of the primary—­and this, of course, the more, the higher the frequency—­and allows the passage of but a minute current.  In such a case, unless the secondary is closed through a condenser, it is almost essential, in order to produce rotation, to make the primary and secondary coils overlap each other more or less.

But there is an additional feature of interest about this motor, namely, it is not necessary to have even a single connection between the motor and generator, except, perhaps, through the ground:  for not only is an insulated plate capable of giving off energy into space, but it is likewise capable of deriving it from an alternating electrostatic field, though in the latter case the available energy is much smaller.  In this instance one of the motor terminals is connected to the insulated plate or body located within the alternating electrostatic field, and the other terminal preferably to the ground.