74 



THE ELECTRIC TELEGRAPH. 



with the earth, it is evident that when either of the inter- 

 mediate stations interrupts the circuit, the current at the 

 stations near the ends will not be interrupted, but only 

 weakened. Kramer, unable to realise a perfectly insulated 

 line, and having to provide against emergencies, makes his 



Fig. 38. 



alarm work as well by a weakening as by a complete inter- 

 ruption of the current. 



For this purpose he short-circuits one of the coils of his 

 electro-magnets in the following manner. In front of the 

 armature a contact screw, c, is pressed upon by a metal 

 spring, b, so that when the armature is attracted towards 

 the poles of the electro-magnet, at half-way, it comes in 

 contact with b, and separates it from c. The axis on which 

 the armature turns is connected by a wire with the middle e 

 of the electro-magnet coils ; the contact screw c is in connec- 

 tion directly, by a wire, with the line L' ; and between the 

 back of the spring b, and the same line- wire L', a resistance, 

 io t equal to that of the half, m, of the electro-magnet coils, is 

 inserted. 



A current, arriving by the line L (disregarding the shunt 

 R), passes from L by m, e, m', b, c, to L'. Very little goes 

 through w, because its resistance is very great in proportion 

 to the resistance of c. The armature is thereupon attracted, 

 and as it descends, carries down the spring b with it, and 



