The combined discoveries of all experimenters had made it evident that certain conditions were necessary to success, regardless of the structure of the carbon filament. It was essential that the vessel containing the filament should be entirely of glass; that the current should be conveyed in and out this by means of platinum wires hermetically sealed through the glass; and that the glass globe must be as thoroughly exhausted as possible. This last requirement proved a difficult one for a time, but by improved methods it finally became possible to produce almost a perfect vacuum in the bulbs, with a corresponding increase in the efficiency of the lamps. THE TUNGSTEN LAMP For twenty years the carbon-filament lamp stood without a rival. But meanwhile the science of chemistry was making rapid strides and putting at the disposal of practical inventors many substances hitherto unknown, or not available in commercial quantities. Among these were three metals, osmium, tantalum, and tungsten, and these metals soon menaced the apparently secure position of the highly satisfactory, although expensive, Edison lamp. It will be recalled that the early experimenters had used two metals, platinum and iridium, for lamp filaments; and that these two, although unsatisfactory, were the only ones that had given even a promise of success. But in 1898 Dr. Auer von Welsbach took out patents, and in 1903 produced a lamp using an osmium filament. Its advent marked the beginning of the return to metal-filament lamps, although the lamp itself did not prove to be very satisfactory and was quickly displaced by a lamp invented by Messrs. Siemens and Halske, having a tantalum filament. On account of its ease to manufacture, its brilliant light, and relatively low consumption of power, this lamp gained great popularity at once, and for a single year was practically without a rival. Then, in 1904, patents were taken out by Just and Hanaman, Kuzel, and Welsbach, for lamps using filaments of tungsten, and the superiority of these lamps over the tantalum lamps gave them an immediate popularity never attained by either of the other metal-filament lamps. Needless to say there is good ground for this popularity, which may be explained by the simple statement that the tungsten lamp gives more light with much less consumption of power per candle power than any of its predecessors. Unlike the carbon filament, which projects in the familiar elongated horse-shoe loop, or double loop, into the exhausted bulb, the tungsten filament is wound on a frame, so that several filaments (usually eight or more) are used for producing the light in each bulb. The chief defect of this lamp is the fragility of the filament, which breaks easily when subjected to mechanical vibration. On the other hand, tungsten lamps can be used in places at a long distance from the central generating plant, where the electric current is too weak for carbon-filament lamps.