360 ELECTRO-MAGNETIC INDUCTION. [SKCT. xxxr. 



equilibrium between these two forces takes place, will indi- 

 cate the intensity of the galvanic current. The galvano- 

 meter, constructed upon this principle, is employed to 

 measure the intensity of galvanic currents collected and 

 conveyed to it by wires. This instrument is rendered much 

 more sensible by neutralizing the efiects of the earth's 

 magnetism on the needle, which is accomplished by placing 

 a second magnetized needle so as to counteract the action 

 of the earth on the first a precaution requisite in all deli- 

 cate magnetical experiments. 



Electro-magnetic induction has been elegantly and use- 

 fully employed by Professor Wheatstone, as a moving power 

 in a telegraph, by which intelligence is conveyed in a time 

 quite inappreciable, since the electricity would make the 

 circuit of the globe in the tenth of a second. 



