HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 159

to one alternation. This, when produced under proper condi-tions, is probably the most beautiful discharge, and when an airblast is directed against it, it presents a singular appearance.The spray of sparks, when received through the body, causessome inconvenience, whereas, when the discharge simplystreams, nothing at all is likely to be felt if large conductingobjects are held in the hands to protect them from receivingsmall burns.

If the frequency is still more increased, then the coil refusesto give any spark unless at comparatively small distances, and thefifth typical form of discharge may be observed (Fig. 103). Thetendency to stream out and dissipate is then so great that whenthe brush is produced at one terminal no sparking occurs, evenif, as I have repeatedly tried, the hand, or any conducting object,is held within the stream; and, what is more singular, the lumi-

Fig. 103 . Flg. 104 .

nous stream is not at all easily deflected by the approach of aconducting body.

At this stage the streams seemingly pass with the greatestfreedom through considerable thicknesses of insulators, and it isparticularly interesting to study their behavior. For this pur-pose it is convenient to connect to the terminals of the coil twometallic spheres which may be placed at any desired distance,Fig. 101. Spheres are preferable to plates, as the discharge canbe better observed. By inserting dielectric bodies between thespheres, beautiful discharge phenomena may be observed. Ifthe spheres be quite close and a spark be playing between them, byinterposing a thin plate of ebonite between the spheres the sparkinstantly ceases and the discharge spreads into an intensely lumi-nous circle several inches in diameter, provided the spheres are