Usually in operating an induction coil we have set up a vibration of moderate frequency in the primary, either by means of an interrupter or break, or by the use of an alternator.  Earlier English investigators, to mention only Spottiswoode and J.E.H.  Gordon, have used a rapid break in connection with the coil.  Our knowledge and experience of to-day enables us to see clearly why these coils under the conditions of the tests did not disclose any remarkable phenomena, and why able experimenters failed to perceive many of the curious effects which have since been observed.

In the experiments such as performed this evening, we operate the coil either from a specially constructed alternator capable of giving many thousands of reversals of current per second, or, by disruptively discharging a condenser through the primary, we set up a vibration in the secondary circuit of a frequency of many hundred thousand or millions per second, if we so desire; and in using either of these means we enter a field as yet unexplored.

It is impossible to pursue an investigation in any novel line without finally making some interesting observation or learning some useful fact.  That this statement is applicable to the subject of this lecture the many curious and unexpected phenomena which we observe afford a convincing proof.  By way of illustration, take for instance the most obvious phenomena, those of the discharge of the induction coil.

Here is a coil which is operated by currents vibrating with extreme rapidity, obtained by disruptively discharging a Leyden jar.  It would not surprise a student were the lecturer to say that the secondary of this coil consists of a small length of comparatively stout wire; it would not surprise him were the lecturer to 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.]