370

INVENTION'S OF NIKOLA TESLA.

perature as well as if the gas were at very low pressure; in fact,exhausting the bulb is only necessary because we cannot produce,(and possibly not convey) currents of the required frequency.

Returning to the subject of electrode lamps, it is obviously ofadvantage in such a lamp to confine as much as possible the heatto the electrode by preventing the circulation of the gas in thebulb. If a very small bulb be taken, it would confine the heatbetter than a large one, but it might not be of sufficient capacityto be operated from the coil, or, if so, the glass might get toohot. A simple wav to improve in this direction is to employ aglobe of the required size, but to place a small bulb, the diameterof which is properly estimated, over the refractory button con-

Fig. 157. Pig. 158.

tamed in the globe. This arrangement is illustrated in Fig. 157.

The globe r. has in this case a large neck n, allowing the smallbulb b to slip through. Otherwise the construction is the sameas shown in Fig. 147, for example. The small bulb is conveni-ently supported upon the stem .*>•, carrying the refractory buttonin. It is separated from the aluminum tube a by several layersof mica m, in order to prevent the cracking of the neck by therapid heating of the aluminum tube upon a sudden turning onof the current. The inside bulb should be as small as possiblewhen it is desired to obtain light only by incandescence of theelectrode. If it is desired to produce phosphorescence, the bulb