318 ELECTRO-MAGNETIC INDUCTION. SKCT. XXXI. 



two metals soldered together. It is truly wonderful 

 that an agent, evolved by so small an instrument, and 

 diffused through a large mass of iron, should communi- 

 cate a force which seems so disproportionate. Steel 

 needles are rendered permanently magnetic by electrical 

 induction ; the effect is produced in a moment, and as 

 readily by juxtaposition as by contact ; the nature of 

 the poles depends upon the direction of the current, 

 and the intensity is proportional to the quantity of elec- 

 tricity. 



It appears that the principle and characteristic phe- 

 nomena of the electro-magnetic science are, the evolu- 

 tion of a tangential and rotatory force exerted between 

 a conducting body and a magnet ; and the transverse 

 induction of magnetism by the conducting body in such 

 .substances as are susceptible of it. 



The action of an electric current causes a deviation of 

 the compass from the plane of the magnetic meridian. 

 In proportion as the needle recedes from the meridian, 

 the intensity of the force of terrestrial magnetism in- 

 creases, while at the same time the electro-magnetic 

 force diminishes ; the number of degrees at which the 

 needle stops, showing where the equilibrium between 

 these two forces takes place, will indicate the intensity 

 of the galvanic current. The galvanometer, constructed 

 upon this principle, is employed to measure the inten- 

 sity of galvanic currents collected and conveyed to it by 

 wires. This instrument is rendered much more sensi- 

 ble by neutralizing the effects of the earth's magnetism 

 on the needle, which is accomplished by placing a sec- 

 ond magnetized needle so as to counteract the action of 

 the earth on the first a precaution requisite in all del- 

 icate magnetica} experiments. 



'Electro-magnetic induction has been elegantly and 

 usefully employed by Professor Wheatstone as a mov- 

 ing power in a telegraph, by which intelligence is con- 

 veyed in a time quite inappreciable, since the electricity 

 would make the circuit of the globe in the tenth of a 

 second. 



