go PHOTO-TELEGRAPHY 



make the balancing of the stylus and various other 

 mechanical details equally precise. 



If we refer to Fig. I (Chapter I.) we see that 

 at the sending station we have a revolving metal 

 drum A, to which the lead foil half-tone photo- 

 graph is attached. This revolves while . a stylus 

 traces a spiral path over the picture in virtue of 

 its being given a lateral motion. The style is 

 fixed to an arm (Fig. 28, Chapter III.), which 

 has screwed to it a half-nut, threaded so as to 

 fit on the shaft F, which is also threaded. Hence 

 as the shaft turns, between steel centres, the 

 half-nut travels along it, and so draws the stylus 

 along too. In the telectrograph there are 75 

 threads to the inch, so that the style moves later- 

 ally with a velocity of j^oth inch per second. 



The style is of special design, as shown diagram - 

 matically in Fig. 41, turning about pivots P and 

 being provided with a tension screw T. The point 

 of contact is a V-shaped iridium tracer I, which 

 withstands the constant sparking caused by the 

 makes and breaks of the current. The actual 

 point must be very fine, yet not fine enough to 

 scratch the lead ; the tension is regulated by the 

 screw T. 



The battery, usually 100 volts, consists of 

 secondary cells, and hence a series of currents 

 are sent into the line exactly as in the case of 

 Korn's telautograph. At the receiving end we 



