FAULTS OF SELENIUM 



47 



the selenium with the result that they are less 



sensitive to light, and the ratio of their " light 



sensitiveness " to " dark sensitiveness " becomes 



less. They are sometimes enclosed in glass cells 



which are sealed and exhausted, but this is not 



done in the case of Professor Korn's machines. A 



cell is liable sometimes to get into a certain state 



which renders the current " intermittent/' i.e., 



continuous but always 



variable, the variation 



amounting perhaps to 



10 or 20 per cent. This 



trouble would appear to 



be due to bad contact 



somewhere, but it is in 



reality a fault of certain 



old cells. The most 



successful working of 



the selenium machines 



was obtained after 



adopting the method patented in 1908 by Professor 



Korn for keeping the cell always slightly illuminated, 



so that the inertia was overcome. If the curve given 



in Fig. 23 represent the effect of light of increasing 



intensity upon the current (or reciprocal resistance 



of the cell), then that portion of the curve shown 



would be the best to use for photo-telegraphy, where 



r* 

 the ratio - is more or less constant ; {hat part of the 



/n this portion 



1 1 1 u m ina tio n 



FIG. 23. 



