The length of the arc at first increases rapidly with the current strength, then more slowly. The carbon used is of such a size that there is about 01 ampere per square milli- metre of the section.
Uppenbom (J) determined a for an arc between ' carbon rods of 12 mm. diameter to be 38 volts, 325 for the positive, and 5*5 for the negative pole; for h he found about 1 volt per millimetre. From this it can be understood that the greater heat development takes place at the positive pole, which radiates 85 per cent, of the whole light emitted. Nevertheless, according to measurements by Violle (4) the carbon cannot be heated above 3500^ at the ordinary pressure, for at this temperature it volatilises without previous fusion. The glowing gases in the arc are heated to a greater extent, their temperature being estimated by Eosetti (5) at about 4800^
Of the good conducting substances so far investigated, carbon resists the heat best, with the exception of some oxides used in the Auer-, Jablochkoff-, and Nernst-lamps; carbon may be heated to 3000° without appreciably gasifying, at a somewhat higher temperature it becomes soft, and may be welded.
The arc light may be interrupted for a short time, about 0*1 second, without losing its conductivity; consequently the arc may be produced by an alternating current, which is to be preferred in electrochemical practice when we are concerned with the production of heat. In this case, of course, the carbons are equally heated, and become equally
corroded.
The possibility of concentrating the heat in a small space has led to the adoption of electrical heating methods for the production of high temperatures, and by the aid of these certain reactions can be brought about which only take place when the temperature is very high.
Influence of Temperature on Chemical Reactions. — As has been stated in previous chapters, the temperature exerts a double influence on chemical reactions. On the
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