Analytical Study of the Alternating Current Arc. 321 



current during the phase, neither is it dependent upon any 

 variation of the mean square value of the current, because it 

 takes place even when that current is perfectly constant. It 

 appears to be due to slow changes of position of the points on 

 the carbons between which the discharge takes place. The 

 discharge is as it were seeking out new points between which 

 to take place, and it continually changes these positions as the 

 arc burns. The photometric arrangements finally adopted 

 were as follows, and are shown in outline in fig. 2 : — 



Arrangement of the Photometer and Revolving Disk. 



A represents an alternating current arc. This arc was 

 enclosed in a metal lantern in which were three openings. 

 The light from this arc passed through a lens L x in a hori- 

 zontal direction and fell upon a mirror M x placed at an angle 

 of 45°, capable of being rotated at this constant inclination 

 round a horizontal line co-linear with the axis of the revolving 

 disk, and the motor placed in front of the lens L lt The ray 

 was then reflected upwards into another mirror M 2 , and by 

 this mirror reflected at an angle very nearly equal to 45° in 

 such a manner as to pass through the slits in the revolving 

 disk D, when any one of the slits was in a position to allow 

 the ray to pass. The two mirrors M x and M 3 were rigidly 

 connected to a rocking-arm so centered that the line M. x M 2 

 could be rotated round into any required position, always 

 moving parallel to a radial line of the disk D. The disk D 

 was the disk carried on the shaft of the synchronizing motor 

 above described ; the motor, together with its associated disk, 

 was placed on the photometer bench in the required position 

 opposite to the arc-light lantern. In fig. 2 the motor itself 

 is not shown, but its position is indicated by the letter C. 

 The angular position which the rocking-arm carrying the two 

 mirrors Mj and M 2 occupied with respect to the vertical line 

 passing through the centre of the revolving disk could be 



