CREW AND BASQUIN. — LUMINOSITY IN THE ELECTRIC ARC. 341 



machine being shunted through a bank of incandescent lamps. So that 

 the arrangement of the complete circuit was as in Figure 3. 



Figure 3. 



A^ 



Still a third source of sparking lies in the slight inaccuracy of dimen- 

 sions in the interrupter rings as they come from the mechanician. While 

 the spark which thus remains is larger than we should like, it has not at 

 all injured the sharp line of contact between the brass and the slate por- 

 tions of the interrupter ; and its only effect on the arc, so far as we are 

 able to observe, is slightly to delay the break of the current, by furnish- 

 ing, at the point of interruption, a bridge of metallic vapor. 



So far as the method of experiment is concerned, it only remains to 

 state how the intervals during which the current was off were employed 

 to observe the arc. For this purpose a steel disk 71 cm. in diameter was 

 placed on the end of the same shaft which carried the armature and the 

 interrupters. Near the periphery of this disk were cut four openings, as 

 indicated in Figure 4. 



Figure 4. 



These openings were provided with shutters, so that their angular 

 width could be varied from 0° to 26°. Behind this disk (and at the 

 same distance from the shaft as the centre of one of these openings) was 

 placed the arc. The line of vision was parallel to the axis of the shaft. 

 The steel disk was so adjusted, in azimuth, on the shaft that the light of 

 the arc was excluded from the eye until the brush was 9^° on the slate 

 sector. In like manner, the light was cutoff from the eye of the observer 

 9^° before either brush had passed off the slate sector of the interrupter. 

 The effective diameter of this steel disk was 59 cm. The disk was driven 



