RECENT ADVANCES IN TELEGRAPHY. 



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communicates to the iron core magnetic properties, and enables it to 

 attract another piece of iron or steel called its armature ; but, when 

 the current ceases, the magnetism ceases also, and a spring too weak 

 to neutralize it draws back the armature. It is shown in section at 

 31, in Fig. 6. When the armature and the lever carrying it are dis- 

 carded, and instead of them a jointed tongue of steel, as at P31, is 

 inserted between the poles of the magnet, it will be unaffected by the 

 current except when a change occurs in its direction. It is then called 

 a polarized magnet. Its use will be explained a little further on. One 



Fig. 6. 



of the keys, K, in the diagram, is provided with a spring, which is 

 in contact with the metal of the key when this latter is in its normal 

 position, and maintains across the key a circuit including a portion of 

 the battery b' . But when the key is depressed the spring comes in 

 contact with a screw, to which another circuit is connected, apply- 

 ing the full strength of the battery to the line. The circuit across the 

 key is never broken, because the spring remains in contact with the 

 arm of the key until it begins to press against the screw. This key 

 woi-ks the magnet 31, which has its retractile spring so adjusted as 

 to be overcome only by the full intensity of the current when the key 

 is down. The other key, K', is for changing the direction of the cur- 

 rent, and working the polarized magnet, P 31. Its construction is such 

 that, when not in use, one pole of the battery, the positive, for exam- 

 ple, is in connection with the line, and the negative with the earth, 

 necessitating the passage of the current through the line in the first 

 place ; but when the key is touched the negative pole is connected 

 "to line" and the positive to earth, reversing the direction of the 

 current, These reversals of direction operate, as has been said, the 

 polarized magnet P M. 



To revert to the illustration we made use of in describing the du- 

 plex, let the reader picture to himself a water-course in which both 



