The Nernst Light. Scientific American 80 (1899) 150, 1899-03-11
The Nernst electric light is creating great interest abroad, and the paper of James Swinburne before the Society of Arts, of London, ends with the following sentences: "I feel that I have but feebly shown forth the probable future of what seems to me to be the greatest invention in electric lighting that we have seen for many years. Still, I am sure that I have not been to sanguine." We have already referred to this lamp, and in the current SUPPLEMENT we publish Mr. Swinburne's original paper, as presented before the Society.
Prof. Nernst's Apparatus
Prof. Nernst has achieved a wonderful result by the very simple means of rendering an insulator a conductor by heating it. The knowledge that an insulator could be made to conduct electricity by heating it was known some twenty-three years ago, but apparently no one thought of the simple expedient of heating a very good insulator, then applying a current of suitable potential, and thus obtaining a brilliant light. The result is clearly of great commercial importance, aside from its being interesting from the purely scientific side.
The Nernst light appears to have very practical advantages. It can be made to run singly on a pressure of from 500 to 1,000 volts, and any lamp which can be worked with voltages higher than those possible with the present glow lamp must bring about great economy in electric supply. It does not require a thin glass bell and a vacuum, and thus we get rid of a very serious difficulty, because these bulbs are very fragile and are easily destroyed. In the ordinary incandescent lamp many of them are defective, owing, not to a fault in the filament, but to an imperfect vacuum. Of course, a lamp of the Nernst type would not need regulating machinery and no trimming would be necessary, and on this account it would appear that an ideal from of street lighting has, at last, been found. The possibilities of the carbon filament are about exhausted. There has been little improvement for a long time, and it is a remarkable thing that just when the carbon filament was failing to meet the requirements this new invention should be made, which seems to meet the case. It is very like the discovery of gutta percha at the critical period, which brought electrical cable makers out of their difficulties. As yet the Nernst lamp is in an experimental stage, and it is possible that in time some of the features which militate against its success will be modified. At present the conducting and light-emitting rod when cold is an insulator and must be heated with a match or by some electrical means. While the Nernst lamp is far from being a commercial success as yet, still it is also far from being only the impractical scheme of an inventor. The lamp is based upon sound scientific principles which appeal at once to practical electricians, who have been extraordinary quick in this instance to see the wonderful potentialities of the lamp.
The operation of Prof. Nernst's apparatus is as follows: The preliminary heating of the magnesia, A, the professor accomplishes by placing it in the focus of a reflector, C, see left figure. On the inner side of the reflector is a spiral wire of platinum, D, which when brought to incandescence by a current produces heat sufficient to render the magnesia a conductor; a current is then passed directly through the oxide by the wire, B, and that in the spiral is shut off. A complicated form of lamp is seen in right figure. Here the magnesia, A, is placed within a cylinder, C, which also encloses a platinum spiral, D. As soon as the incandescent spiral has heated the magnesia sufficiently, a current is passed through the oxide by the wire, B. Within this circuit is a coil, G, which, upon becoming magnetic, draws down the iron bar, E, thus lowering the now incandescent magnesia from within the cylinder. Upon breaking the circuit the coil loses its magnetism, and a spring, F, raises the iron bar and the magnesia to their former position.
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