Temperature of the Carbons of the .Electric Arc, <fr. 33 



is given as from 10 to 12. They differ slightly from Schleiermacher's 

 series, the ratio in his being a little lower than in ours at the same 

 temperature. 



There is a fair agreement with Rossetti's result at high tempera- 

 tures; he obtained 2'9 at about 1500 C., which is very little lower 

 than the value which would be reached by a theoretical continuation 

 of our curve (fig. 4). 



The tendency of the curve appears to be to approach, a constant 

 value of about 3, but it is impossible to dogmatise on this point, from 

 the physical limitations of the inquiry. The fact that the ratio does 

 diminish shows that the physical nature of oue or both of the surfaces 

 is different at different temperatures ; probably it is the bare platinum 

 which changes, and it is possible that the constant value of the ratio 

 is just attained at the melting-point of the metal. Experiments with 

 a molten surface would be interesting, but very difficult to carry out. 



The Balancing Experiments. 



We may now pass on to the "balance experiments," from which 

 the temperature of the carbons of the arc may be calculated. The 

 principle of the method has already been described in the earlier part 

 of the paper. 



When the arc was shining on the top receiving-surface of the radio- 

 micrometer, and the incandescent platinum on the lower, the position 

 is denoted as Position A, the reverse, by Position B. 



The angle subtended by the area of incandescent platinum, in both 

 cases, was 5 '301. 



The diameter of the hole in the screen (fig. 2), through which 

 the radiation of the arc passed, was 0'337 cm. In position B, its 

 distance from the receiving surface was 57'0 cm. ; in position A it 

 was 58'2 cm., giving angular apertures of 0'339 and 0'332 respec- 

 tively. The screen was close enough to the arc to make it certain 

 that the hole was completely filled with the brightest part of the 

 crater of the + pole. The hole was sufficiently small to form a rough 

 " pin-hole " image of the carbons, by means of which it could be seen 

 during an experiment that the brightest part of the + pole was shining 

 directly into the tube of the radio-micrometer, and so on to the 

 receiving surface. 



In position B, the ratio of the areas of platinum and carbon, as scon 

 from the receiving-surface, was 



(5-301/0-339) 2 = 245, 

 and in position A, the ratio was 



(5-301/0-332) 2 = 255. 



VOL. LVI1I. l> 



