46 THE ELECTRIC TELEGRAPH. 



tion of the pinion and the coils of the electro-magnet would 

 produce a current in the line in one direction, and that the 

 continued motion in the same direction another half-revolu- 

 tion, would produce a current in the contrary direction. 



This arrangement required a slight modification also of 

 the receiving apparatus ; but in principle it remained the 

 same. 



Fig. 22. 



M. Froment, of Paris, has constructed some step-by-step 

 instruments, on the system of Professor Wheatstone, in 

 which he has succeeded in simplifying the mechanism. 



Y. TELEGRAPHS NOW IN USE. 



47. Single- needle Telegraph of Wheatstone and Cooke. 

 The single -needle telegraph of Messrs. Wheatstone and 

 Cooke is a modification of the five-needle system by the 

 same inventors, described above. 



The principle of the system depends upon the construction 

 of an alphabetical code whose basis consists in two elementary 

 signals the deflections of a vertical pointer to the right and 

 to the left as in Gauss and Weber's telegraph. 



Fig. 23 represents the front view of a single-needle instru- 

 ment a mahogany case with engraved metal face, in the 



