573 -THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v 



line-wire is generally too weak to produce hence the necessity 

 of employing at the receiving station a local battery and a relay. 

 A relay is a supplementary apparatus for strengthening the current 



FIG. 374. Froment's relav. 



in the line-wire, when it is sufficient to transmit signals, but not 

 sufficient to produce the permanent mark on the paper. We can 

 see what part a relay performs by following in Fig. 375 the course 



Earth 



N Fio. 875. The Morse telegraphic apparatus, with relay. 



taken by the current which reaches the receiving station from the 

 line-wire. This current which enters the manipulator at c, enters the 

 relay R' at a and passes to an electro-magnet which is polarized by 



