HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 161

air is increased, and at enormous pressure it would be negligible,unless the frequency would increase correspondingly.

It will be often observed in these experiments that when thespheres are beyond the striking distance, the approach of a glassplate, for instance, may induce the spark to jump between thespheres. This occurs when the capacity of the spheres is some-what below the critical value which gives the greatest differenceof potential at the terminals of the coil. By approaching a di-electric, the specific inductive capacity of the space between thespheres is increased, producing the same effect as if the capacityof the spheres were increased. The potential at the terminalsmay then rise so high that the air space is cracked. The experi-ment is best performed with dense glass or mica.

Another interesting observation is that a plate of insulatingmaterial, when the discharge is passing through it, is stronglyattracted by either of the spheres, that is by the nearer one, thisbeing obviously due to the smaller mechanical effect of the bom-bardment on that side, and perhaps also to the greater electrifica-tion.

From the behavior of the dielectrics in these experiments, wemay conclude that the best insulator for these rapidly alternatingcurrents would be the one possessing the smallest specific induc-tive capacity and at the same time one capable of withstandingthe greatest differences of potential; and thus two diametricallyopposite ways of securing the required insulation are indicated,namely, to use either a perfect vacuum or a gas under great press-ure ; but the former would be preferable. Unfortunately neitherof these two ways is easily carried out in practice.

It is especially interesting to note the behavior of an exces-sively high vacuum in these experiments. If a test tube, providedwith external electrodes and exhausted to the highest possibledegree, be connected to the terminals of the coil, Fig. 105, theelectrodes of the tube are instantly brought to a high temperatureand the glass at eacli end of the tube is rendered intensely phos-phorescent, but the middle appears comparatively dark, and for awhile remains cool.

When the frequency is so high that the discharge shown inFig. 103 is observed, considerable dissipation no doubt occurs inthe coil. Nevertheless the coil may be worked for a long time,as the heating is gradual.

In spite of the fact that the difference of potential may be