THE TELEGRAPH 323 



namely, that a piece of soft iron becomes a magnet while a 

 current flows around it, but loses its magnetism as soon as ,/ 

 the current ceases. 



In the electric bell, the electromagnet, clapper, push button, 

 and battery are relatively near, usually all are located in 



FlG. 217. Diagram of the electric telegraph. 



the same building ; while in the telegraph the current may 

 travel miles before it reaches the electromagnet and produces 

 motion of the armature. 



The fundamental connections of the telegraph are shown 

 in Figure 217. If the key K is pressed down by an operator 

 in Philadelphia, the current from the battery (only one cell 

 is shown for simplicity) flows through the line to New York, 

 passes through the electromagnet M, and thence back to Phila- 

 delphia. As long as the key K is pressed down, the coil M 

 acts as a magnet and attracts and holds fast the armature A ; 

 but as soon as K is released, the current is broken, M loses 

 its magnetism, and the armature is pulled back by the spring 

 D. By a mechanical device, tape is drawn uniformly under 

 the light marker P attached to the armature. If K is closed 

 for but a short time, the armature is drawn down for but 

 a short interval, and the marker registers a dot on the tape. 

 If K is closed for a longer time, a short dash is made by the 

 marker, and, in general, the length of time that K is closed 

 determines the length of the marks recorded on the tape. 



