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Thermodynamics

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Carbon Arc

The first arc lamp was devised by Sir Humphry Davy in about 1812. It consists of two rods of charcoal, worked by a battery of about 2000 cells. The present form of carbon arc lamp consists of two rods of carbon which can be separated by a distance of 3”/16 to 1”/4, after bringing them in contact. The carbon rods are connected to a DC potential difference of about 40 to 45 volts and the current (of about 12 amperes) is adjusted with the help of a rheostat. Initially, the rods are kept in contact and then separated. A brilliant arc passes between the rods and the arc itself provides the conducting path between the rods. The volatilized carbon particles shoot off from the +ve electrode and fall on the –ve electrode can be seen. About 85% of the illumination is due to the +ve electrode (anode) and about 15% is due to the –ve electrode (cathode). As most of the light is due to the +ve electrode, the positive carbon rod is placed in a horizontal position. The temperatures of the +ve and –ve carbons are about 4000˚C and 2500˚C respectively. The consumption of the +ve carbon is about twice that of the negative one and hence the +ve carbons are twice as thick as the negative carbons. The candle power of the arc is about 1000 and the efficiency is 1.25 candles per watt.

Ayrton gave the following equation to calculate the potential differences necessary maintain the arc.

where A, B, C and D are constants, L is the length of the arc in millimeters and I is the strength of the current in amperes. For an arc of length 5 mm and drawing the current of 10 amperes the potential difference is required is about 56 volts.

The carbon arcs can be run on A.C. potentials also. In this case, both the carbons arc of the same thickness and both become pointed. These work about 30 volts and the efficiency is about 1.12 candles per watt.

The carbon arc spectrum can be studied with the light emitted from the arc. To study the spectrum of any volatile metal, the metal is introduced in the arc, and the light emitted from the arc gives the characteristic emission lines of the arc. Copper arc, Iron arc etc. also can be produced in a similar way and the corresponding spectra can be studied.

The carbon arcs are mostly employed in cinema projection equipment, search-lights, light-houses etc. With the invention of more efficient and powerful lighting systems of the recent times, the use of carbon arc is more and more restricted though they are still used for the projection of motion pictures in theatres.