46 PHOTO-TELEGRAPHY 



100 and 200 volts to the selenium cells in the 

 manner already shown in the compensation 

 diagram, while current is led from P to the motor 

 and galvanometer magnets. 



It is hardly necessary to say that accumulators 



'mm 



FIG. 22. Prof. Korn at the telephone, when awaiting 

 the first picture from Paris in 1907. 



are essential in order to ensure smooth running and 

 constant current. 



Much trouble is experienced with some selenium 

 cells, while others will last without any bother for 

 many months. The " fatigue " often spoken of is 

 really a permanent lag in the cells; a definite 

 physical change appears gradually to take place in 



