CHAPTER II. 



PROFESSOR KORN'S SELENIUM PROCESS EARLY 

 WORK WITH HIS ORIGINAL RECEIVER THE 

 STRING GALVANOMETER SYNCHRONISM - 

 FIRST EXPERIMENTS THE EARLY HISTORY 

 OF COMMERCIAL PHOTO -TELEGRAPHY. 



THE metal selenium is, in its crystalline state, 

 very sensitive to light. It has been utilised in 

 many instances of light telephony, from which it is 



obvious that it is sen- 

 sible of extremely rapid 

 changes in the illumi- 

 nation. 



The selenium cell, 

 FIG. 5. so-called, takes the 



form shown in Fig. 5 ; on a thin rectangular slab of 

 slate, steatite, or some other suitable material, two 

 coils of platinum wire are wound, one coil being 

 wound " inside " the other, so that no turn touches 

 another turn ; thus, in the figure, turns i, 3, 5, 7, 



9 ... belong to one coil, while turns 2, 4, 6, 8, 



10 . . . belong to the other. We now fill in the 

 spaces between the turns with selenium, so that if 

 the resistance between turns I and 2 were R, and 

 there were n turns in each of the coils, the total 



