The metal tungsten, from which the lamp filament is made, was discovered in 1871. It is not found in the metallic state but occurs as tungstate of iron and manganese and as calcium tungstate. Up to 1906 it was known only in laboratories and on account of its rarity the price was very high. As greater bodies of ore were found and the process of extraction became better known, the price soon dropped to a point permitting its use for lamp filaments in a commercial scale.
Pure tungsten is hard enough to scratch glass. Its fusing point is higher than any other known metal; under ordinary conditions it is almost impossible to melt it and this property gives its value as an incandescent filament. One of the laws that affect the lighting properties of incandescent lamps is: “the higher the temperature of the glowing filament, the greater will be the amount of light furnished for a given amount of current consumed.” The high melting point permits the tungsten filament to be used at a higher temperature than any other known material. Tungsten is not ductile, and in ordinary form cannot be drawn into wire. Because of this fact, the filaments of the first lamps were made by the “paste” process, which consisted of mixing the powdered metal with a binding material, in the form of gums, until the mass acquired a consistency in which it might be squirted through a minute orifice in a diamond dye. The resulting thread was dried, after which it was heated, and finally placed in an atmosphere of gases which attacked the binding material without affecting the metal. When heated by electricity in this condition, the particles of metal fused together to form a filament of tungsten. While the “paste” filaments were never satisfactory in general use, their efficiency as a light-producing agent inspired a greater diligence in the search for a more durable form.
Although tungsten in ordinary condition is not at all ductile, methods were soon found for making tungsten wire and the wire-filament lamps are now those of general use. One process of producing the drawn wire is that of filling a molten mass of a ductile metal with powdered tungsten after which wire is drawn from the mixture in the usual way. The enclosing metal is then removed by chemical means or volatilized by heat.
Of the difficulties encountered in the use of metal-filament lamps that of the low resistance offered by the wire was overcome by using filaments very small in cross-section and of as great length as could be conveniently handled. The long tungsten filament requires a method of support very different from the carbon lamp. The characteristic form of tungsten lamps is shown in Fig. 217, in which the various parts of the lamp are named.
Fig. 217.—An Edison Mazda lamp and its parts.
The filament of an incandescent lamp is heated because of the current which passes through it. The electric pressure furnished by the voltage, forces current through the filament in as great an amount as the resistance will permit. A 16-candlepower carbon lamp attached to a 110-volt circuit requires practically ½ ampere of current to render the filament incandescent; the filament resistance must, therefore, allow the passage of ½ ampere. With a given size of filament, its length must be such as will produce the desired resistance. A greater length of this filament would give more resistance and a correspondingly less amount of current would give a dim light because of its lower temperature. Likewise, a shorter filament would allow more current to pass and a brighter light would result. When the size and length of filament is once found that will permit the right amount of current to pass, if the voltage is kept constant, the filaments will always burn with the same brightness. This is in accordance with Ohm’s law which as stated in a formula is
E = RC
that is E, the electromotive force in volts, is always equal to the product of the resistance R, in ohms, and the current C, in amperes.
In the incandescent lamp, if the electromotive force is 110 volts and the current is ½ ampere, the resistance will be 220 ohms and as expressed by the law