220 Mr. Dyke on the Flux of Light from the 



or approximately 



d L v^ 2 +(i/; 2 d 2 +(i/ 2 ) 2 J 

 ^ L ^-i-(i/) 2 j 



Sd 2 - 



d 2 \ 2(d 2 +(U) 



2a/ d 2 +d^d 2 +(jiy \ 



\ 2UF+(U) 2 ) 



Hence 



d 2 must be multiplied by the factor 



2{d 2 +am 



cF + d\/d 2 +(hJ) 2 



The author is indebted to Mr. W. C. Clinton, B.Sc, for 

 bringing this correction to his notice. 



A hand-regulated arc was used in the experiments, the 

 length of arc being kept constant by observing the image of 

 the arc thrown on to a screen by means of a lens. 



During each series of experiments the arc was maintained 

 at a constant length, whilst the power supplied was varied by 

 means of an adjustable resistance. 



In order to obtain a mean result ten photometric readings 

 were taken at each setting. 



Altogether eight lengths of arc were employed, viz. 

 jJg in., ^ ^ n -> 8 ni -> an d hence by 16ths to fy in. 



The table which follows gives the mean results of these 

 experiments, involving nearly four thousand observations. 



If now these results are plotted, taking watts as abscissa? 

 and mean spherical candle-power as ordinates, we obtain the 

 series of curves shown in PL II., the full-line curves 

 referring to continuous-current arcs and the dotted lines to 

 alternating-current arcs. 



It will be seen that, within the limits of experimental 

 error, the relation between mean spherical candle-power and 

 watts follows a straight-Hue law. This straight line, however, 

 does not pass through the origin, but for zero candle-power 

 there is still an outstanding amount of power amounting to 

 some 200 to 400 watts. 



The amount of this outstanding effect increases, in general, 

 with the arc-length, and is probably due to the energy dissi- 

 pated as heat by radiation, and as chemical energy in evapora- 

 tion of the carbon at the crater. 



The author hopes, at some future date, to investigate this 

 matter experimentally. 



It is found from these curves that for each arc-length the 

 point in which the efficiency curve cuts the axis of watts is 



