In an arc lamp a current flows between two electrodes which are drawn apart resulting an arc being struck in between them. The arc maintains the current and becomes very efficient source of light.
Carbon arc, flame arc and magnetite arc the forms of arc lamps of which the carbon arc lamp is most usual form.
Carbon arc lamps have electrodes of hard carbon placed and to end and connected to a d.c. source. When the current flows through them the ends of carbon rods become incandescent due to high resistance. If they are pulled slightly apart about 2-3 cm distance an arc will be formed between two carbon rods producing white light. The arc consists of carbon vapour surrounded by an orange red zero of burning carbon and pale green flames. The arc is maintained by the transfer of carbon particles from positive carbon rod to negative one. It is necessary to maintain the carbons at a constant distance apart otherwise there will be decrease in illumination due to burning of positive carbon. The luminous efficiency of carbon arc lamp is about 9 lumens per watt.
Cinema projectors and search lights.
About 45 Volts.
Arc lamp can be used on a.c. supply at its working voltage of about 55 volts.
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