136 



THE ELECTRIC TELEGRAPH. 



work always cleaner than those given by making and 

 breaking the same current. 



The polarised relay above described is also profitably 

 employed on lines worked only with galvanic currents, with 

 which it is found to be far more delicate than the relays with 

 springs. It is, however, necessary to give the armature c 

 (Fig. 73) a bias on the side D', which is done by advancing 

 the soft iron continuation of the pole N' of the electro-magnet 

 a little nearer to the armature than N, by which, when no 

 current passes, the tongue is held against the insulated 

 contact, and the distances may be so finely adjusted that a 

 very weak current suffices to move it. 



84. The Magneto- Induction Key. Instead of the Morse 

 key, induction coil, and local battery, Siemens and Halske 

 use also an instrument arranged in the form of a key, by 



Fig. 75. 



which a coil of wire, wound on a soft iron armature, is 

 oscillated between the poles of a permanent magnet, and 

 develops alternate currents for working the polarised relay. 



The magneto-induction key is shown in Fig. 75, in per- 

 spective, s and N are two rows of permanent bar-magnets ; 

 the upper ones with their north ends, and the lower ones 



