290 DEVELOPMENT OF HEAT. chap.
where a and 6 are coeflBcients which gradually increase with the current strength, and I is the length of the arc.
It I becomes very small (0 5 mm.), an arc can be obtained with a potential difference of only 25 to 30 volts. It is difficult, however, to keep such an arc going. A spongy elevation of carbon, transferred from the positive carbon, forms on the negative carbon ; by this loss the well-known crater-like depression is formed in the positive carbon. If the deposit on the negative carbon increases much, the two carbon points come into contact, and, on the other hand, if it falls off, the length I suddenly increases, and the arc goes out. Ordinary arc lights have a length of at least 2 mm., and generally 4 to 5 mm., and require a potential difference of 40 to 45 volts.
Arc lights produced by a potential difference of only 30 to 40 volts do not bum imiformly, and make a hissing noise.
A certain minimum current strength is also required to produce a steady arc light. Arc lights have been success- fully produced with 1 to 2 amperes and 40 to 45 volts, but a very fine and delicate regulation of the length is necessary, and on this account such small current strengths are never used in practice. To produce arcs with small current strengths a very good, hard, thin carbon rod is required. For arc lamps the current strength used lies between 4 and 25, and is most frequently 8 amperes, and the potential difference is about 42 volts, the carbon rods having a diameter of 8 to 18 mm.
According to measurements carried out at the Electrical Exhibition at Frankfort-a-M. in 1891, the maximum length / of an arc when fed with a current of i amperes is given in the following table : —
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�t amp.
�I mm.
�t amp.
�I mm.
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