Bunsen and Roscoe's Pendulum Actinometer 



461 



fresh portions can be brought beneath the slit, against 

 which the paper is pressed, thus preventing any possibility of 

 shadows. 



It will be seen that the band represents two shutters moving 

 in opposite directions. Fig. 3 shows the relative positions of 

 the apertures in the upper and lower surface, with regard to 

 the sensitized paper. The three slits are shown side by side, 

 although of course superimposed in reality. 



Fig. 3. 



1© 



© 



©' 



1 



i 



c 



1 



b 







1 



a\ 



1 



a 



b 



\a 



< 





(1) represents the state of affairs just before an exposure, 

 the arrow b showing the position and direction of the aperture 

 in the upper portion of the band, c that of the aperture in the 

 lower portion, and a the slit beneath which the sensitized 

 paper is exposed. At this instant the top edge of c and the 

 bottom edge of b are just beginning to overlap — that is, a is 

 just about to be uncovered. (2) represents the conditions under 

 w T hich the entire length of a is exposed, c having ascended and 

 b having descended a length equal to that of a : the bottom 

 edge of c will now begin to close a from below. In (3) a is 

 completely covered by the upward motion of c. 



It will be thus seen that the opening process is entirely 

 brought about by the downward movement of b, the closing 

 process being solely due to the upward movement of c. 



If sufficient darkening of the paper has not been attained 

 by one exposure, two or more are allowed to take place, 

 otherwise the movement is stopped after the first revolution. 



In order to rotate the shutter a motor giving a uniform 

 and comparatively high rate of speed is required. That first 

 employed consisted of a tin drum to which a syphon was 

 attached, and this was floated on water, as shown in fig. 4, 

 and was connected, by means of a cord passing over a pulley, 

 to a wheel working on the spindle of one of the rollers, so 

 that on syphoning off the water the drum sank at a uniform 

 rate, causing the wheel to rotate. 



Although this arrangement answered the purpose, yet it 

 was cumbersome and troublesome to work, and has therefore 



