THE SELENIUM CELL 23 



resistance of the " cell " would be approximately 

 TD 

 -, assuming the number of turns very great and 



their distance apart equal. The dimensions of the 



FIG. 6. 



cells made by Giltay, of Delft; Holland, are about 

 6-3 x 2*8 cm., and their resistance varies between 

 very wide limits ; thus one may have a cell whose 

 resistance is only 20,000 ohms, or one of 

 R = 250,000 11. It seems to be a general rule that 

 the greater the resistance of the cell the smaller its 

 inertia, but it is by no means always the case. 



The selenium has to be kept at a definite high 

 temperature until it assumes the crystalline, slate - 

 coloured form, when it becomes electrically con- 

 ductive and sensitive to light. The light is appa- 

 rently absorbed and made to do wok in lessening 

 the resistance ; the physical change is not relaxed 



