ELECTRIC TELEGRAPHY. 719 



mitted through telegraph wires have traversed distances 

 at the rate of over twenty-seven millions of metres, 

 or a little more than 17,000 English miles, in one 

 second. The time required by the current to traverse 

 a circuit many miles in length is therefore indefinitely 

 small. 



When a keeper has been attracted by an electro- 

 magnet, and is in contact with its two poles, the attrac? 

 tion does not cease completely when the current is 

 interrupted; when not too much weighted the keeper 

 remains adhering to the poles until it is torn off. When 

 contact is once broken in this manner by the application 

 of force, the keeper will not again be attracted unless 

 the current is transmitted anew through the spiral of 

 the electro-magnet. In order to render the attractive 

 force exerted by electro-magnets upon iron available for 

 electric telegraphy, it is most important to prevent the 

 effect of this residual attraction, and this is effected 

 by interposing some substance between the bar to be 

 attracted and the poles of the electro-magnet, which 

 prevents direct contact between them. Thus if a thin 

 sheet of paper be placed between the keeper and electro- 

 magnet in fig. 361, the keeper, after being attracted 

 while the current lasts, will immediately drop when 

 the current is interrupted. The lifting power is of 

 course diminished by the interposition of any substance 

 between the electro-magnet and the iron bar; but this 

 is of no consequence in the present application of electro- 

 magnetism. 



Let one of the poles of a battery be in metallic connec- 

 tion by means of a wire with one end of the spiral of 

 an electro-magnet erected at a considerable distance from 



