Research on some Standards of Light. 475 



Table I. — " Silent " Arcs. Enclosure at about 20° C. 



Electro- 

 motive 

 | force in 

 volts. 



Current- 

 in 



amperes. 



Power in 

 watts. 







Diameter 

 of the 

 positive 

 carbon 

 in mm. 







Current 

 density in 

 amperes 

 per sq. mm. 

 of cross- 

 sectional 

 area of 

 positive 

 carbon. 



Length 

 of arc 

 in mm. 



Intrinsic 



brilliancy 

 of the 



crater in 

 candle- 

 power 

 per 



sq. mm.* 



— 



65 '0 



8 -1 



526 



8 



"161 



4 -8 



136 



64-5 



7-0 



452 



8 



0-139 





141 



72 -0 



10-8 



742 



8 



0-205 



60 



143 



i 70-3 



73 



513 



6 



0-258 





147 



61 7 



6-0 



370 



6 



0-212 



5-i 



154 



62-0 



9 1 



564 



6 



0-322 



6'0 



160 



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

 observations. 



Table II.—" Silent " Arcs. Enclosure above 900° C. 



1 



Electro- 

 motive 



force in 

 volts. 



Current 

 in 



amperes. 



Power in 

 watts. 



Diameter 

 of the 



positive 

 carbon 

 in mm. 



Current 

 density in 

 amperes 

 per sq. mm. 

 of cross- 

 sectional 

 area of 

 positive 

 carbon. 



Length 

 of arc 

 in mm. 



Intrinsic 



brilliancy , 

 of the 



crater in 

 candle- 

 power 

 per 



sq. mm. 1 



83 '0 



11 -o 



913 



8 



0-219 





136 



57 -0 



10-9 



621 



15 



0-061 





142 



61 



8-6 



525 



8 



0-171 



6 V 



152 



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

 observations. 



results is considerable, and a 3 per cent, variation is well within the 

 experimental error. 



The conclusions we have reached may be summed up as follows : — 



1. The intrinsic brilliancy of the crater of a silent arc is about 147 

 candle power per square millimetre. 



2. Even when the most favourable conditions are selected, and the 

 intensity of current and the length of the arc are maintained constant, 

 it is difficult to obtain consistent results, variations of over 5 per cent, 

 being by no means unfrequent. The crater of the arc does not, there- 

 fore, possess the qualities required of a standard. 



