354

INVENTIONS OF NIKOLA TESLA

nomena. I noted that small differences in the density of the airproduced a considerable difference in the degree of incandescenceof the wires, and I thought that, since in a tube, through whicha luminous discharge is passed, the gas is generally not of uni-form density, a very thin wire contained in the tube might berendered incandescent at certain places of smaller density of thegas, while it would' remain dark at the places of greater density,where the convection would be greater and the bombardment lessintense. Accordingly a tube t was prepared, as illustrated in Fig.187, which contained through the middle a very line platinum wirew. The tube was exhausted to a moderate degree and it was foundthat when it was attached to the terminal of a high-frequency coilthe platinum wire w would indeed, become incandescent in patches,as illustrated in Fig. 187. Later a number of these tubes with oneor more wires were prepared, each showing this result. The ef-fect was best noted when the striated discharge occurred in thetube, but was also produced when the striae were not visible,showing that, even then, the gas in the tube was not of uniformdensity. The position of the striae was generally such, that therarefactions corresponded to the places of incandescence or greaterbrightness on the wire w. But in a few instances it was noted, thatthe bright spots on the wire were covered by the dense parts ofthe striated discharge as indicated by l in Fig. 187, though the effectwas barely perceptible. This was explained in a plausible wayby assuming that the convection was not widely different in thedense and rarefied places, and that the bombardment was greateron the dense places of the striated discharge. It is, in fact, oftenobserved in bulbs, that under certain conditions a thin wire isbrought to higher incandescence when the air is not too highlyrarefied. This is the case when the potential of the coil is nothigh enough for the vacuum, but the result may. be attributed tomany different causes. In all cases this curious phenomenon ofincandescence disappears when the tube, or rather the wire,acquires throughout a uniform temperature.

Disregarding now the modifying effect of convection there arethen two distinct causes which determine the incandescence of awire or filament with varying currents, that is, conduction cur-rent and bombardment. With steady currents we have to dealonly with the former of these two causes, and the heating effectis a minimum, since the resistance is least to steady flow. Whenthe current is a varying one the resistance is greater, and hence