CHAP, xii.] ELECTRICITY AND MAGNETISM. 401 



duplex and the diplex methods. On one and the same 

 line are used two sets of instruments, one of which 

 (worked by a " polarised " relay) works only when the 

 direction of the current is changed, the other of which 

 (worked by a non-polarised relay adjusted with springs 

 to move only with a certain minimum force) works only 

 when the strength of the current is changed and is inde- 

 pendent of their direction. 



429. Submarine Telegraphy. Telegraphic com- 



x^^ o F \' i r^5s 



Fig. 164. 



munication between two countries separated by a strait 

 or ocean is carrie4-on through cables, sunk to the 



