oliN 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



paper carried by a rotating drum F, which made 20 revolutions per 

 second about a horizontal axis. This drum was enclosed in a light- 

 tight box at the back of an improvised camera. A horizontal slit S, 



Figure 5. Current-voltage curves of molybdenite rectifier, with large current. 



immediately in front of the rotating drum, shut off all luminescence in 

 the tube except that in the line of motion of the spot. 



The rotating drum was driven by a synchronous motor operating on 

 the 60-cycle alternating current mains of the laboratory. The alter- 



so CM. 



E C^) V\ ALTERNATING 



UO VOLTS 



4'HI'r 1 



Figure 6. Oscillographic apparatus. 



nating current sent through the rectifier and the deflecting magnets 

 was taken from the same supply. The synchronism of the drum with 

 the deflections of the luminescent spot was so perfect that exposures of 

 four minutes could be made, during which time the image of the spot 



