Research on some Standards of Light. 477 



Table IV.— " Hissing " Arcs. Enclosure above 900° C. 



Electro- 

 motive force 

 in volts. 



Current in 

 amperes. 



Power in 

 watts. 



Diameter 

 of the 

 positive 

 carbon 

 in nrim. 



Current 

 density in 

 amperes per 

 sq. mm. of 

 cross-sectional 

 area of the 

 positive 

 carbon. 



Intrinsic 

 brilliancy of 

 the crater in 

 candle-power 

 per sq. mm. 1 



/y u 



id ,n 

 JLo U 





ID 



A .1 AO 



i iy 



45 -6 



42-0 



1915 



15 



0*238 



121 



60-3 



18 -6 



1122 



25 



0-038 



130 



52 -0 



55:0 



2860 



25 



0-112 



133 



41 -0 



43 -4 



1779 



25 



0-088 



137 



50 -0 



39 -3 



1965 



15 



0-222 



142 



1 Each of the figures in this column is the mean of a number of photometric 

 observations. 



If we assume that the formula :* 



£-400 = 889-6 ^b 



(t = temperature in degrees centigrade, b — intrinsic brilliancy in 

 candle power per square centimetre), holds good for carbon at these 

 high temperatures, the above change in candle power corresponds to a 

 variation of temperature of from 3866° to 4018° C. The total altera- 

 tion in absolute temperature thus works out at 4 per cent. Observa- 

 tions made on silent arcs (Tables I and II), reduced in the same 

 manner, give the extreme limits of temperature as 3935° and 4018° C, 

 or a change of 2 per cent, in the absolute temperature. 



These variations are somewhat greater than we should meet with in 

 the case of substances boiling at ordinary temperatures. It must, 

 however, be borne in mind, that even in the case of a silent arc, the 

 crater is the seat of several secondary phenomena, which under certain 

 circumstances may affect the boiling point, f The term boiling point 

 is in itself misleading, as it seems possible, if not probable, that at 

 atmospheric pressure carbon does not become liquid, but like carbon 

 dioxide, passes direct from the solid to the gaseous state. It has fre- 

 quently been stated that impurities cannot affect the temperature of 

 the crater, as all known bodies become gaseous at a lower temperature. 

 The experimental data to substantiate this are entirely wanting ; silica, 

 lime, alumina, magnesia, and other substances are solid at the tem- 



* < Phil. Trans.,' vol. 191, A, p. 515, 1898. 



f Mrs. Ayrton, "On the Hissing of the Electric Arc," ' Journ. of the Inst, of 

 Elec. Eng.,' vol. 28, p. 401, 1899 ; also Dr. J. A. Fleming's remarks during the 

 discussion of Mrs. Ayrton's paper, p. 439. 



