The Nernst filaments differ markedly from those of the ordinary electric glow lamp in that they are not conductors in the ordinary sense (or of the first order, as Nernst puts it) but electrolytes, the passage of the current being actually attended by an electro-chemical change in the filament.[592] The oxide is ionised; the ions of the metals travel to the cathode or negative pole, where the liberated atoms of metal instantly recombine with the oxygen of the air, whilst oxygen ions travel to the anode, from which the gas is liberated. There is thus a gradual redistribution, resulting in accumulation of oxide at the cathode with a corresponding loss at the anode, which is balanced, after some time, by diffusion, so that equilibrium is attained. In consequence of this redistribution the filament glows more brightly at the anode, where it is thinnest, than at the cathode.
[592] Vide Nernst, Zeitsch. Elektrochem. 1899, 6, 41.
Industrial Applications of Zirconium
It has already been mentioned that zirconium received considerable attention as a suitable substance for the preparation of metallic filaments during the early stages of their development. Whilst at the present time this element has been abandoned for the purpose,[593] several zirconium lamps were at one time on the market, and a brief mention of some of the work done in this direction may not be out of place.
[593] Vide Baumhauer, Zeitsch. angew. Chem. 1910, 23, 2065.
One of the general methods for the preparation of the metallic filaments may be illustrated by a patent taken out in 1902 by Sander,[594] for the preparation of filaments of zirconium, with or without addition of zirconium carbide. The metal, or a compound which on heating will yield the metal and a volatile substance which can be removed, is prepared in a finely divided condition, and made into a paste with some organic binding material; the paste is then forced through a tiny aperture, and the resulting thread is shaped and heated to a high temperature in vacuo or in an inert atmosphere. If an organic substance be used to form a paste with metallic zirconium, the final process of heating results in the formation of the carbide; the same compound is also obtained by another process protected by Sander (loc. cit.) in which the hydride of zirconium, prepared by the reduction of the oxide by means of powdered magnesium in an atmosphere of hydrogen, is mixed with a cellulose solution, and the liquid treated as in the manufacture of artificial silk, the threads obtained being then heated to remove all organic matter as far as possible.
[594] D. R. P. 133701, July, 1902.
The carbide is also probably obtained by the process of the British Thomson-Houston Company, in which advantage is taken of the fact that zirconium oxalate is a pasty gelatinous substance, which can be forced through a die to form a thread without addition of any agglutinating agent. The oxalate, precipitated by addition of ammonium oxalate to a solution of a zirconium salt, is mixed with finely divided carbon, and the threads obtained from the pasty mass heated to a very high temperature in a furnace.[595] Zirconium oxalate is also proposed as a binding material for powdered tungsten, in the preparation of filaments from that metal.[596]
[595] E. 5415, 1908.
[596] E. 10590, 1908.