The incandescent lamp was invented in 1879 and the patents were granted to Thomas A. Edison. There were, however, a number of electrical men who were working on the idea at this time who deserve a great deal of credit for developing the lamp. The incandescent lamp, which is used chiefly for house lighting, consists of a glass bulb from which the air has been exhausted by pumps and chemical processes--in which there is a thin filament of tungsten metal wound on what is called an arbor (as shown in Fig. 4). This filament opposes high resistance to the passage of the current of electricity, and, consequently, is heated to incandescence when a current passes through it. The removal of the air from the bulb prevents the tungsten metal from burning up, as it would do if oxygen were present. The filaments of the first lamps were made of vegetable fibre. The next development was the cellulose process, which is still used in carbon and metallized lamps, although a number of processes are used now which improve the filament considerably. The discovery that tungsten metal could be used in incandescent lamps was made in 1906. The first tungsten lamp manufactured in America was made in 1907. [Illustration: THE DEVELOPMENT OF INCANDESCENT LAMPS Edison’s first lamp with a filament of bamboo fibre.] [Illustration: The carbon lamp--the oldest form of incandescent lamp.] [Illustration: Standard Mazda lamp--the highest development of the incandescent lamp.] [Illustration: The Tantalum lamp developed just before the Mazda lamp.] [Illustration: Improved Mazda lamp for lighting large areas--the most efficient lamp ever made.] The filaments of the first tungsten lamps were composed of two or three short pieces of wire. In 1910, however, a lamp with a continuous tungsten filament was invented which increased the strength of the lamp wonderfully. Mazda is a trade name given to all metal filament lamps made by the prominent American lamp manufacturers. The reason that the Mazda lamp is so much more efficient than the carbon filament lamp is because the tungsten filament can be burned at a much higher temperature than the present carbon filament, without seriously blackening the bulb. How Does an Arc Light Burn? In the arc light a current of electricity is made to leap across from the tip of one rod of carbon to the tip of another that is held a short distance from the first. In passing across the current does not follow a straight path, but makes a curve, or arc, whence comes the name “arc light.”