tantalum, a filament in order to operate satisfactorily on a standard voltage must be long and thin.  This necessitates storing away a considerable length of wire in the bulb without permitting the loops to come into contact with each other.  After the filaments have been in operation for a few hundred hours they become brittle and faults develop.  When examined under a microscope, parts of the filament operated on alternating current appear to be offset.  The explanation of this defect goes deeply into crystalline structure.  The tantalum filament was quickly followed by osmium and by tungsten in this country.

The osmium filament appeared in 1905 and its invention is due to Welsbach, who had produced the marvelous gas-mantle.  Owing to its extreme brittleness, osmium was finely divided and made into a paste of organic material.  The filaments were squirted through dies and, after being formed and dried, they were heated to a high temperature.  The organic matter disappeared and the fine metallic particles were sintered.  This made a very brittle lamp, but its high efficiency served to introduce it.

In 1870 when Scheele discovered a new element, known in this country as tungsten, no one realized that it was to revolutionize artificial lighting and to alter the course of some of the byways of civilization.  This metal—­which is known as “wolfram” in Germany, and to some extent in English-speaking countries—­is one of the heaviest of elements, having a specific gravity of 19.1.  It is 50 per cent. heavier than mercury and nearly twice as heavy as lead.  It was early used in German silver to the extent of 1 or 2 per cent. to make platinoid, an alloy possessing a high resistance which varies only slightly as the temperature changes.  This made an excellent material for electrical resistors.  The melting-point of tungsten is about 5350 deg.F., which makes it desirable for filaments, but it was very brittle as prepared in the early experiments.  It unites very readily with oxygen and with carbon at high temperatures.

The first tungsten lamps appeared on the market in 1906, but these contained fragile filaments made by the squirting process.  When the squirted filament of tungsten powder and organic matter was heated in an atmosphere of steam and hydrogen to remove the binding material, a brittle filament of tungsten was obtained.  The first lamps were costly and fragile.  After years of organized research tungsten is now drawn into the finest wires, possessing a tensile strength perhaps greater than any other material.  Filaments are now made into many shapes and the greatest strides in artificial lighting have been due to scientific research on a huge scale.