570 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



turning on itself and presenting alternately first one and then the other 

 side, separated by the coil, to the poles of the magnets a, that the 

 induced currents are developed which are successively thrown into the 

 line. For each revolution of the cylinder c two currents are produced 

 in opposite directions. It remains to show how these currents produce 

 in the indicator corresponding motions of the indicating needle, and 

 this may be easily understood from Fig. 369. 



It represents the receiving mechanism placed below the dial plate 

 of the indicator. M and M' are the two coils of the electro-magnet 



FIG. 368. Manipulator of Siemens' and Halske's dial telegraph. 



which is excited by the currents in opposite directions sent along the 

 line ; P and P' are the two poles of that electro-magnet. Between 

 these poles passes the branch of a fork of soft iron aW, which is 

 constantly polarized by contact with the poles of the permanent magnet 

 AA'. It follows that, according to the direction of the current sent, 

 the branch a is sometimes attracted by the pole P and repelled by P', 

 and sometimes attracted by P' and repelled by P. These oscillations, 

 twenty-six in number, when the handle of the manipulator makes a 

 complete revolution, allow at each movement the escape of one of 



