S60

INVENTIONS OF NIKOLA TESLA.

A few words must now be devoted to each of these phenomena.In the first place, I will consider the incandescence of a button or ofa solid in general, and dwell upon some facts which apply equallyto all these phenomena. It was pointed out before that when athin conductor, such as a lamp filament, for instance, is connectedwith one of its ends to the terminal of a transformer of hightension the filament is brought to incandescence partly by aconduction current and partly by bombardment. The shorterand thicker the filament the more important becomes the latter,and finally, reducing the filament to a mere button, all the heat-ing must practically be attributed to the bombardment. So inthe experiment before shown, the button is rendered incandescentby the rhythmical impact of freely movable small bodies in thebulb. These bodies may be the molecules of the residual gas,particles of dust or lumps torn from the electrode ; whatever theyare, it is certain that the heating of the button is essentially con-nected with the pressure of such freely movable particles, or ofatomic matter in general in the bulb. The heating is the moreintense the greater the number of impacts per second and thegreater the energy of each impact. Yet the button wouldbo heated also if it were connected to a source of a steady po-tential. In such a case electricity would be carried away fromthe button by the freely movable carriers or particles flyingabout, and the quantity of electricity thus carried away might besufficient to bring the button to incandescence by its passagethrough the latter. But the bombardment could not be of greatimportance in such case. For this reason it would require a com-paratively very great supply of energy to the button to maintainit at incandescence with a steady potential. The higher the fre-quency of the electric impulses the more economically can thebutton be maintained at incandescence. One of the chief rea-sons why this is so, is, I believe, that with impulses of very highfrequency there is less exchange of the freely movable carriersaround the electrode and this means, that in the bulb the heatedmatter is better confined to the neighborhood of the button. Ifa double bulb, as illustrated in Fig. 194 be made, comprising alarge globe b and a small one b, each containing as usual a fila-ment y mounted on a platinum wire w and w t , it is found, that ifthe filaments f f be exactly alike, it requires less energy to keepthe filament in the globe b at a certain degree of incandescence,than that in the globe b. This is due to the confinement of the