HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 1.95

which should be well insulated. (Teissier tubes may also belighted across the points of the bent bar ; in this case, of course,it is better to employ smaller capacities. I hare found. it prac-ticable to light up in this manner a lamp, and even a (Teissiertube, shunted by a short, heavy block of metal, and this resultseems at first very curious. In fact, the thicker the copper barin Fig. 128, the better it is for the success of the experiments, asthey appear more striking. 'When lamps with long slender fila-ments are used it will be often noted that the filaments are fromtime to time violently vibrated, the vibration being smallest atthe nodal points. This vibration seems to be due to an electro-static action between the filament and the glass of the bulb.

In some of the above experiments it is preferable to use speciallamps having a straight filament as shown in Fig. 129. Whensuch a lam}) is used a still more curious phenomenon than those

Fir. 129 .

described may be observed. The lamp maybe placed across thecopper bar and lighted, and by using somewhat larger capacities,or, in other words, smaller frequencies or smaller impulsive im-pedances, the filament may be brought to any desired degree ofincandescence. But when the impedance is increased, a point isreached when comparatively little current passes through thecarbon, and most of it through the rarefied gas; or perhaps itmay be more correct to state that the current divides nearlyevenly through both, in spite of the enormous difference in theresistance, and this would be true unless the gas and the filamentbehave differently. It is then noted that the whole bulb is bril-liantly illuminated, and the ends of the leading-in wires becomeincandescent and often, throw off sparks in consequence of theviolent bombardment, but the carbon filament remains dark.This is illustrated in Fig. 129. Instead of the filament a single