

Properties of Selenium Blocks. 377 



reached by the light. To account for the sensitiveness of 

 bridge " B " often observed, the depth of selenium affected 

 would have to be at least 2*5 mm., assuming the selenium 

 was changed to a good conductor on illumination. This did 

 not seem to be at all probable, but the following experiment 

 was devised to test the possibility of the effect of light being 

 transmitted to the dark portions of the selenium : — A block 

 of selenium was crystallized with a sheet of fine copper 

 gauze imbedded in it parallel to one face of the block and 

 at a distance of 1 mm. from the surface. The opposite face 

 of the block was pressed against a copper electrode. No 

 change could be detected in the resistance between the 

 imbedded gauze electrode and the copper plate electrode 

 when the selenium surface nearest the gauze was illuminated. 

 Hence it appears that the resistance change on illumination 

 occurs at the illuminated electrode. The change may be 

 either that of a high resistance crust over the selenium 

 block, or a ''contact resistance " ; other properties of selenium 

 blocks, which will now be discussed, point to the existence 

 of the latter. 



Effect of Filing the Surfaces. 



A cube was taken and the resistance between two faces 

 measured, using copper gauze electrodes. One of these 

 faces was rubbed carefully with a fine plane file. The 

 author was surprised to find an increase of resistance from 

 520,000 to 840,000 ohms. After the second filing the 

 resistance had increased to 1,300,000 ohms. On continuing 

 the filing the resistance first increased slightly, and then 

 remained fairly constant. This is a typical result of the filing 

 of several surfaces; a marked increase was always observed 

 at the start. The light-sensitiveness remained fairly con- 

 stant during the filing, and by continuing the filing of the 

 surface of a cube, the selenium was found to be conducting 

 and sensitive to light throughout the whole block. This 

 was found to be the case for Giltay " cells " by Brown *. 



The fact that filing the surface produces such a sudden 

 and marked effect on the resistance between the electrodes, 

 suggests that a large part of the resistance exists at the 

 electrodes. 



Resistance dependent on nature of electrodes, and surface in 

 contact with selenium during crystallization. 



The fact that the apparent resistance of selenium in the 

 dark depends on the nature of the electrodes was first 



* Loc. cit. 



