70 PHOTO-TELEGRAPHY 



The currents transmitted by this system being 

 considerably greater than those practicable with the 

 selenium apparatus a somewhat different system of 

 synchronising the two instruments is required. 

 It becomes necessary, in fact, to reverse the direc- 

 tion of the current at the moment of synchronising. 

 The drum of the receiving apparatus is revolved 

 about i per cent, faster than the transmitting drum, 

 and finishes its revolution rather before the trans - 



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mitter, as in the case of the selenium machines. 

 When the turn is completed the drum is checked by 

 a metal stop, and the galvanometer circuit auto- 

 matically thrown out, a relay circuit being switched 

 in in its place. The transmitting drum on com- 

 pleting its turn causes a fleeting contact to be made 

 in the reverse direction to that of the line current, 

 and this actuates the relay, which, being polarised, 

 is only sensitive to current in the one direction. 

 The relay closes the local circuit which removes 

 the check by means of an electro -magnet, and both 



