The construction of tungsten lamps is very interesting. Tungsten is a very brittle metal, and at first the lamps were fitted with a number of separate filaments. These were made by mixing tungsten powder with a sort of paste, and then squirting the mixture through very small apertures, so that it formed hair-like threads. Early in 1911 lamps having a filament consisting of a single continuous piece of drawn tungsten wire were produced. It had been known for some time that although tungsten was so brittle at ordinary temperatures, it became quite soft and flexible when heated to incandescence in the lamp, and that it lost this quality again as soon as it cooled down. A process was discovered by which the metal could be made permanently ductile, by mechanical treatment while in the heated state. In this process pure tungsten powder is pressed into rods and then made coherent by heating. While still hot it is hammered, and finally drawn out into fine wires through diamond dies. The wire is no thicker than a fine hair, and it varies in size from about 0·012 mm. to about 0·375 mm., according to the amount of current it is intended to take. It is mounted by winding it continuously zigzag shape round a glass carrier, which has at the top and the bottom a number of metal supports arranged in the form of a star, and insulated by a central rod of glass. One star is made of strong, stiff material, and the other consists of fine wires of some refractory metal, molybdenum being used in the Osram lamps. These supports act as springs, and keep the wire securely in its original shape, no matter in what position the lamp is used. The whole is placed in a glass bulb, which is exhausted of air and sealed up. For some purposes lamps with specially small bulbs are required, and in these the tungsten wire is made in the shape of fine spirals, instead of in straight pieces, so that it takes up much less room. In the “Axial” lamp the spiral is mounted in such a position that most of the light is sent out in one particular direction.