[585] E. 8150, 1909.
The Nernst Lamp.
—The first efforts which were made for the employment of electricity in illumination endeavoured to utilize the heat produced, when a current traverses a very thin metallic filament, to raise the conductor to incandescence. Numerous efforts were made to adapt platinum to this purpose, but its melting-point was finally admitted to be too low; at length it was found possible to produce carbon filaments, and the well-known carbon lamps came into use. Numerous attempts were made to effect improvements;[586] one plan was to coat the carbon filament, after its production, with a skin of metallic conductor, and zirconium and thorium were among the metals proposed in this connection.[587] The first really important advance, however, was effected by Nernst, who took up the study of ‘conductors of the second order,’ and within a few months succeeded in adapting these to the purposes of illumination (1897-1898). The Nernst lamps gave a very intense white light with considerably less consumption of electricity than the carbon filament lamps; they enjoyed a very considerable vogue for some years, but have been almost entirely displaced by the cheaper metal filament lamps, which were occupying the attention of Auer von Welsbach at the time Nernst perfected his invention.[588]
[586] The reader is recommended to consult the Jahresberichte über die Leistungen der Chemischen Technologie of Fischer, Section ‘Beleuchtung,’ for the years 1898-1901 inclusive, from which some idea may be obtained of the innumerable proposals and suggestions, usually protected by patent, which were put forward at this time.
[587] Vide, e.g. D. R. P. 153959.
[588] Vide E. 1535, 13116 and 17580, 1898.
In his first patent,[589] Nernst proposed the use of a rod of magnesia or zirconia as filament; these oxides, which belong to his class of conductors of the second order, are non-conductors at ordinary temperatures, but their resistance decreases as the temperature rises, so that at high temperatures they will conduct electricity at the ordinary voltage. The preliminary heating was at first effected by means of a Bunsen burner, but a later patent[590] of the same year protects a method of heating by means of a platinum spiral in an auxiliary circuit, which is automatically cut out when the current in the main circuit, bearing the filament, attains its required strength. In the following year[591] it was found that filaments composed of mixtures of oxides were far more suitable than the earlier magnesia or zirconia rods; yttria, thoria, and zirconia were the chief oxides used, small quantities of ceria being occasionally introduced. With these filaments, the increase of conductivity with temperature is far more rapid than with the pure oxides; the preliminary heating required, therefore, is less and the light obtained more intense. The filaments used were in the form of rods or spirals obtained by compressing the powdered oxides.
[589] E. 19424, 1897.
[590] E. 23470, 1897.
[591] E. 6135, 1898.