CREW AND BASQUIN. — LUMINOSITY IN THE ELECTRIC ARC. 345 



from a position just over the shaft to one on the right, one sees the rise 

 of the current. In like manner, motion of the arc and head from a 

 position just over the axis to one on the left will give the observer a 

 chance to see the decay of the current. 



Occulting Screen. 



la 



X Arc. 



Figure 6. 



For the purpose of photographing the arc in various stages of its rise 

 and decay, we rigidly connected the arc and camera to a sliding table 

 top ; for each 5 mm. displacement of this table top the phase at which 

 the arc is first seen or last seen is changed by Toiou second. On 

 this table the camera was so arranged as to rotate about a vertical axis 



^^ 





jj.p S^^iich. . 



i Y I 



X Arc. 



Figure 7. 



passing through the optical centre of the lens. In this manner were 

 photographed, on one plate, several series showing the arc in successive 

 phases, differing by one or two ten-thousandths of a second. 



I. The Intermittent Arc. 



A double pole switch convenient to the observer enabled one to oper- 

 ate the arc either with the ordinary alternate current or by the alternate 



