Properties of Selenium Blocks. 379 



light on these bridges is identically the same as the effect 

 on the bridges or! the second type. Hence, if the effect of 

 illuminating the electrode of the latter bridges is chiefly to 

 change a "contact resistance/' then ice should expect a 

 relatively big effect of light at the electrodes in the ordinary 

 ivire bridges. Adams and Day * by means of a lime light 

 and lens focussed the light on a stick of sensitive selenium, 

 and found that " the sensitiveness was different at different 

 parts of the same stick." Actual values of the change were 

 not published, and the following experiment was devised by 

 the author to obtain the variation between the electrodes. 

 Two thin wires of phosphor-bronze were fixed parallel to 

 one another at a distance of 15 mm. on a strip of glass of 

 width 17 mm. the surface of which had been roughened 

 with sand-paper. Selenium was spread over the glass 

 between the wires, and crystallized. By means of two 

 narrow slits a strip of selenium of width 2 mm. at any part 

 of the bridge could be illuminated. The bridge had an 

 initial resistance of 141,000 ohms which changed to 140,000 

 ohms on illuminating any strip of selenium between the 

 electrodes, and to 134,000 ohms on illuminating either 

 electrode. The change, therefore, was nearly three times 

 us great at the electrode as at other points in the selenium. 

 This receives some support from an experiment by Lindner 

 und Replogle f, who devised a bridge consisting of fielenium 

 spread over a thin platinum film deposited on glass and 

 scored to and fro with a fine engraving tool. To get the 

 maximum effect on illumination, the light was admitted 

 through the glass (illuminating the platinum electrodes). 

 (Similar bridges were constructed and tested by the author.) 

 Thus it appears that the action of light on bridges of both 

 types is partly to effect a change at the contact of the 

 electrodes with the selenium. 



The Crystallization of Thin Selenium Tilms. 



Estimates of the " depth of penetration " have been made 

 by Grippenberg J from a study of thin films. Films of 

 red amorphous selenium of thickness comparable with the 

 wave-length of light can be obtained by " cathode dis- 

 integration.''' In heating the red film to convert it into the 

 metallic light-sensitive state, investigators have encountered 

 the difficulty that the selenium on melting forms into 

 isolated drops giving a discontinuous film. Grippenberg + 



# Proc. Koy. Soc. 1870. p. 113. 



+ ' Electrical World," lix. p. 251 (1912). 



X Zoc. cit. 



