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I say slender support, for it is evident that in order to confine the heat more completely to the incandescing body its support should be very thin, so as to carry away the smallest possible amount of heat by conduction. Of all the supports used I have found an ordinary incandescent lamp filament to be the best, principally because among conductors it can withstand the highest degrees of heat.
In his earliest consideration of the problem of subdividing the electric current, he had decided that the only possible solution lay in the employment of a lamp whose incandescing body should have a high resistance combined with a small radiating surface, and be capable of being used in what is called "multiple arc," so that each unit, or lamp, could be turned on or off without interfering with any other unit or lamp.
The aluminium sheet should project above the glass stem more or less one inch or so or else, if the glass be too close to the incandescing body, it may be strongly heated and become more or less conducting, whereupon it may be ruptured, or may, by its conductivity, establish a good electrical connection between the metal tube and the leading-in wire, in which case, again, most of the energy will be lost in heating the former.
An artificial compound of one kind or another has indeed been universally adopted for the purpose by all manufacturers; hence the incandescing conductors in all carbon-filament lamps of the present day are made in that way. The last few years have witnessed a remarkable advance in the manufacture of incandescent lamps in the substitution of metallic filaments for those of carbon.
Its only fault is that it is easily fusible, and, therefore, its distance from the incandescing body should be properly estimated. Usually, a thin tube, of a diameter somewhat smaller than that of the glass stem, is made of the finest aluminium sheet, and slipped on the stem.
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