The principle of an arc lamp is that when two electrodes carrying current are separated through a small distance, an arc is struck between them. The arc lamps were used in the past for street lighting purposes but nowadays these are used when extreme brightness is required. Carbon arc lamp is most commonly used arc lamp.
Carbon arc lamp
Arc lamp is shown in Carbon arc lamp is the oldest type of lamp and is still being implied in cinema projectors and searchlights. It consists of two hard carbon rods (Electrodes). The diameter of positive electrode is double to that of negative electrode. The negative electrode is generally fixed and positive electrode is placed in adjustable holder and the process is manual or automatic. The arc consists of carbon vapours surrounded by orange red zone of burning carbon and pale green flames.
When the lamp is switched OFF, the two electrodes touch each other due to spring pressure on positive electrode. When the supply is ON a large current flows through electrodes. The temperature of carbon electrode is increased and thus the positive electrode is pulled away against its spring pressure through a small distance by coil and thus an arc is struck between electrodes. This arc is maintained by transfer of carbon particles from one electrode to other electrode. These particles travel from positive electrode to negative electrode, thus after sometime of operation positive electrode becomes hollow and negative becomes pointed. That’s why positive electrode is made double than negative electrode. In carbon arc lamp 85 % of light is given by positive electrode which produces high intensity light and only 10% by negative electrode and 5% by air. The temperature of the positive electrode is 4000°C and that of the negative electrode is about 2500°C. The luminous efficiency of such lamps is approximately 9 lumen / watt.