568 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



in use upon the continental lines of railway. The motor of the apparatus 

 consists of a battery of permanent magnets, about the poles of which 

 turns a cylinder of soft iron covered in the direction of its axis with 

 an isolated iron wire, forming a magnetizing coil. The rotation of this 

 cylinder on its axis develops induced currents alternately in opposite 

 directions. These currents thrown into the line one after the other, act 

 upon the electro-magnet of the indicator and make its armature oscillate, 

 which in turn acts on the escapement wheel carrying the indicating 

 needle. Fig. 367 represents the exterior of the complete instrument 

 which is, as we see, very simple. A is a drum or cylindrical box con- 



FIG. 365. Wlieatstoiie's letter-showing dial telegraph, 1S4U. 



taining the transmitter or manipulator, B is the indicator, M is the 

 handle which the sender turns and stops successively at the letters of 

 a dial according to the tenor of the message. The needle of the dial 

 of the indicator B follows all the movements of the handle of the 

 manipulator. 



We will briefly indicate what are the principal arrangements of 

 the mechanism of each part of the apparatus by means of Fig. 368. 



A is the rnetal disc which has the dial: twenty- six teeth on the cir- 

 cumference correspond to twenty-six divisions, and serve as stopping 

 places for the handle. On the axis o o' is fixed a toothed wheel UK, 

 which works into the pinion H. When the wheel turns through -2$ of 



