350 Prof. J. A. Fleming and Mr. J. E, Petavel : 



was an incandescent lamp, which was worked at rather a high 

 temperature in order to diminish the difficulty of making the 

 photometric comparison by diminishing as much as possible 

 the colour-difference in the lights compared. This incan- 

 descent lamp was kept constantly standardized against another 

 standard incandescent lamp worked at a rather lower tempe- 

 rature. The means of comparison was a Sugg's Star Disk 

 Photometer. It was found that by focussing the eye to a 

 point rather nearer the eye than the images of the Star Disk 

 as seen in the two mirrors, the difficulty of discriminating 

 between a small difference in the brightness of the two images 

 in spite of a small colour-difference was to a great extent dimi- 

 nished. We abandoned as perfectly useless any comparison of 

 the two lights in terms of red and green candle-power. By 

 the employment of the reflecting wattmeter and carbon resist- 

 ance as above described, it was perfectly possible to keep the 

 power expended in the arc constant to a certain number of 

 watts throughout long periods. In the case of the alternating- 

 current arc lamp experiments, the alternating-current arc lamp 

 was turned round its vertical axis so as to take a series of 

 observations quickly in different directions, but at the same 

 angle to the horizon, and the mean of these observations was 

 taken as the effective illuminating-power in any angular 

 direction ; at the same time, the current through the arc and 

 the potential-difference of the carbons were observed. These 

 observations having been taken, involving many hundreds 

 of photometric measurements, the results were set out in a 

 series of photometric diagrams, as shown in figs. 21 to 24, which 

 delineate the respective form of the photometric curves for the 

 two arcs and for different wattages expended in the arc. These 

 photometric diagrams were then integrated, and the mean 

 spherical candle-power calculated in the usual way by means 

 of a Rousseau's diagram, and finally the results embodied in 

 one complete table and diagram, as given below (pp. 35 G, 357). 

 The results in the Table are graphically embodied in the 

 diagram in fig. 25, from which it will be seen that, taking the 

 alternating-current arc as employed, the total mean spherical 

 candle-power is always considerably less than that of a con- 

 tinuous-current arc, taking the same mean power. Lowering 

 the frequency seems to increase the efficiency of the alter- 

 nating-current arc, as one might naturally assume it would 

 do, and it is obvious that increasing the diameter of the lower 

 carbon of the continuous-current arc would diminish its total 

 mean spherical candle-power at any given wattage. This 

 table and diagram therefore, we think, settle the question 

 that for a given expenditure of power in the arc a greater 



