234 



ELECTRO-TELEGRAPHY 



springs, studs, and cylinders, two multipliers, .and two magnetised needles, with their 

 external indexes, and two telegraph wires. One battery, however, is sufficient. One 



807 



ar more of either or both kinds of deflection of either or both needles, according to 

 the code engraved on tho dial, constitutes tho alphabet. This instrument is very 

 extensively employed ; messages are sent by it with extreme rapidity. 



Another property pos- 

 sessed by a wire convoying 

 a current is that of con- 

 verting soft iron, for the 

 time, into a magnet. The 

 attractive power, whieh 

 can thus be given to, and 

 withdrawn from, tho soft 

 iron at pleasure, is turned 

 to useful account, either 

 in producing direct mc- 

 action, or in 

 the detents of 

 a clock movement. Hero 

 also the effect of the 

 solitary wire i.s inappre- 

 ciable, and many convo- 

 lutions around tho iron 

 !'-cessary in order to 

 obtain a useful result. 



Tin! simplest applica- 

 tion of this principle is 

 shown in jig. 807. Here are two 1 tilled \\\\\\ cotton-covered copper-win- in 



one length. They are hollow, and a U-shapud bar of iron passes through them. 



chanical 



