HIGH FREQ UK NOT AND HIGH POTENTIAL CURRENTS. 156

the several types of these machines which have been constructedby me, I have found that the type illustrated in Fig. 97 causedme the least trouble in construction, as well as in maintenance,and on the whole, it has been a good experimental machine.

In operating an induction coil with very rapidly alternatingcurrents, among the lirst luminous phenomena noticed are natur-ally those presented by the high-tension discharge. As the num-ber of alternations per second is increased, or as—the numberbeing liigb—the current through the primary is varied, the dis-charge gradually changes in appearance. It would be difficult todescribe the minor changes which occur, and the conditions which

Fig. 99.

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k-4 ft.v;

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wmm.

bring them about, but one may note live distinct forms of thedischarge.

First, one may observe a weak, sensitive discharge in the formof a thin, feeble-colored thread. (Fig. 100a.) It always occurswhen, the number of alternations per second being high, the cur-rent through the primary is very small. In spite of the exces-sively small current, the rate of change is great, and the differ-ence of potential at the terminals of the secondary is thereforeconsiderable, so that the arc is established at great distances; butthe quantity of “ electricity ” set in motion is insignificant, barelysufficient to maintain a thin, threadlike arc. It is excessivelysensitive and may be made so to such a degree that the mere actof breathing near the coil will affect it, and unless it is perfectly