DOUBLE CURRENT SYSTEM 



107 



from the sending machine are of opposite direction 

 to this continuous current, arid neutralise it. Hence, 

 whenever the stylus of the transmitter sends a 

 current through the line it neutralises the con- 

 tinuous current, and so prevents the latter from 

 making an electrolytic dot. As soon as the trans- 

 mitter current has ceased, it is immediately wiped 



FIG. 50. News photograph wired from Manchester to London 

 of a railway accident at Stalybridge. 



out by the continuous current, which is of course 

 shunted on to the telephone line. 



The use of a double current system in telegraphy 

 is well known, and greatly increases the possible 

 rate of working, as it hastens the ordinarily slow 

 discharge of the line. It thus becomes possible 

 to work with finer screens in preparing the photo- 

 graphs, and to obtain a correspondingly greater 

 amount of detail. 



