ABSTRACT
In the conventional bulb the tungsten fi lament is heated to incandescence by an electric current. In a vacuum the temperature is about 2300 °C. Tungsten is a heavy metallic element and has the symbol W; its atomic number is 74; and its atomic weight 2.85. The pure metal is steel grey to tin white in colour. Its physical properties include the highest melting point of all metals: 3410 °C. Pure tungsten is easily forged, spun, drawn and extruded, whereas in an impure state it is brittle and can be fabricated only with diffi culty. Tungsten oxidizes in air, especially at higher temperatures, but it is resistant to corrosion and is only slightly attacked by most mineral acids. Tungsten or its alloys are therefore ideal for use as fi laments for electric light bulbs. The fi lament is normally wound into a ‘spiralled spiral’ to allow a suitable length of thin wire in a small space and to provide some mechanical strength. Figure 11.2 shows a typical bulb fi lament.
