720 ELECTRIC TELEGRAPHY. 



the battery, and let the other end of the spiral be simi- 

 larly connected by a conductor with the second pole of 

 the battery, then the distant electro-magnet may be 

 magnetised at will by closing the circuit, and again de- 

 magnetised at any instant by breaking it. If now a 

 bar of iron be placed at a small distance from the poles 

 of this electro-magnet, but pulled away from it by a 

 spring of not too great strength, when the current 

 passes through the spiral, the force of the spring is 

 overcome by the attraction of the electro- magnet and 

 the piece of iron approaches it; then by making or 

 breaking contact the motion to and fro of the bar 

 of iron may be controlled at will at any distance from 

 it, and the mode of setting the bar in motion may be so 

 varied' as to constitute a preconcerted system of signals. 



Many different arrangements of the electric telegraph 

 have been devised, but the following description of a 

 Morse's Writing (or Printing) Telegraph, in its most 

 simple form, will give to the student a sufficient insight 

 into the principles applied in such an apparatus, and 

 into the general mode of its working. 



The electro-magnet e e, fig. 363, consists of two small 

 cylinders of iron, placed upon a common support of the 

 same metal, which replaces the bend of a horse-shoe 

 and forms the connection between both legs. The 

 spiral wire is not wound upon the iron itself, but upon 

 two small c bobbins ' of wood, into the hollow interior of 

 which the iron cores fit exactly. ^Near to the poles of 

 the electro-magnet is the keeper or 4 armature ' a, fixed 

 at the end of a horizontal lever, /?, movable about an 

 axis, c, which turns in two holes made in the brass 

 plates pp. At the other end of the lever there is a 



