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.]

Now compare this phenomenon which you have just witnessed with the