438 



WELLS'S NATURAL PHILOSOPHY. 



What is Am- ^^^' ■^'"°°^ these, and other like phenomena, M. Ampere 

 pere's theory has propounded a theory which accounts for nearly all the 

 of magnetisa? phenomena of terrestrial magnetism. 



He supposes that all magnetic phenomena are the result of the circulation 

 of electrical currents. Every molecule of a magnet is considered to be sur- 

 rounded with an atmosphere of electricity, which is constantly circulating 

 around it, the difference between a magnet and a mere bar of iron being, that 

 the electricity which exists equally in the iron, is at rest, whereas in the mag- 

 net it is in motion. The direction of these currents ckculating in a magnet 

 is dependent upon the position in which the magnet is held. If the opposite 

 or unlike poles of two magnets be placed end to end, the electric currents of 

 each will be found running the same way, and as currents moving in the same 

 direction attract each other, the two poles will tend to come together. On 

 the contrary, if the ends of like poles be presented, the course of the currents 

 traversing each will be in opposite directions, and a repulsion will result. 



A magnetic needle tends to arrange itself 

 at right angles with a wire transmitting an 

 electric current, in order to bring the numer- 

 ous currents circulating around its particles 

 parallel with that of the wire. 



The magnetism of the earth is also explained by this theory 

 on the same principles. If we take a metal ring and warm 

 it at one point only by a spirit-lamp, no electrical effect en- 

 sues ; but if the lamp is moved an electric current runs round 

 the ring in the direction the lamp has taken. In a like man- 

 ner, currents of electricity are known to be excited and kept in motion around 

 the earth, by the sun, which heats in turn successive portions of its surface. 

 They flow round it from east to west in a direction at right angles with a line 

 joining the magnetic poles. A magnetic needle, therefore, points north and 

 south, because that position is the one in which the electric currents in it are 



parallel to those of the earth, and 

 this is the position, as has just 

 been explained, that electric cur- 

 rents tend always to assume. 



Fig. 369 represents an artificial 

 globe, surrounded by a coil of in- 

 sulated wire, surmounted by % 

 magnetic needle. The needle will 

 always point to the north pole of 

 the globe, on transmitting the bat- 

 tery current. 



The dip of the needle may be 

 also readily accounted for in the 

 same manner. At the polar re- 



Why does a 

 magnetic nee- 

 dle tend to ar- 

 range itself at 

 right angles to 

 a current ? 



How is the 

 magnetism of 

 the earth ex- 

 plained by this 

 theory ? 



