THE TELECTROGRAPH 



FIG. 41. 



have a cylinder B, the same size as A, and revolv- 

 ing in synchronism, its movement being con- 

 trolled in the way already described and in 

 common use in certain 

 systems of ordinary tele- 

 graphy. 



Round B is wrapped a 

 piece of specially prepared 

 paper, containing in its 

 composition certain chemi- 

 cal substances which de- 

 compose on the passage 

 through them of an 

 electric current. The un- 

 decomposed chemicals must be colourless, the 

 decomposition products must be coloured. Hence, 

 in theory, whenever a current is sent from the send- 

 ing instrument it passes through the paper, via the 

 stylus, and a black or coloured mark is made on 

 the paper, the length of the mark, /, measured along 

 the circumference, being equal to tu, where u = 

 the velocity of the surface of the drum, and / the 

 duration of the current. When, however, the send- 

 ing and receiving instruments are connected by a 

 long distance line, instead of an ordinary resistance, 

 we find that for a current of duration t, the length 

 of the chemical mark is considerably greater 

 than tu. 



If we were to send one brief current from trans- 



