474 Mr. J. E. Petavel. An Experimental 



Fig. 3. — a, asbestos box ; b, lining of refractory bricks ; c, graphite crucible ; 

 d, diaphragm ; s, screen. 



SSj _ a 



figure), by aid of which the relative positions of the carbons could be 

 adjusted and the length of the arc measured. 



" Apostle " carbons were used in all the experiments. The size of 

 the positive carbon varied from 6 to 25 mm. in diameter. 



To obtain reliable results a sufficient current density must be used to 

 give a fairly large crater. The arc must also be sufficiently stable for 

 the crater to remain some considerable time without shifting its posi- 

 tion. Finally, it is desirable that the arc should be burning in a 

 normal manner, and therefore neither hissing nor roaring. 



Given these conditions, it is impossible to vary either the current, 

 the electromotive force, or the length of arc, within very wide limits. 



In the experiments recorded in Table I, the mean intrinsic brilliancy 

 is 147 candle power per square millimetre; the variations from the 

 mean amounted to 10 per cent. 



Table II gives experiments made under similar conditions, but with 

 the enclosure in which the arc was burning at a temperature of over 

 900° C. The average intrinsic brilliancy calculated from this table is 

 143, or about 3 per cent, lower than when the enclosure was at the 

 ordinary temperature. It would, however, be unwise to attach too 

 much importance to this change. The difficulty of obtaining consistent 



