330 Dynamic Theory. 



toward each other. If the currents go different directions they will 



float apart. 



From what has gone before it is not difficult to see that a magnet is 

 simply a body under the influence of electricity. According to Am- 

 pere's theory, there are currents of electricity flowing around all the ul- 

 timate molecules of the magnet, in the same direction. Those currents 

 around molecules in the interior of the magnet, neutralize each other so 

 that the final effect of all the molecular currents is the same as that of 

 a set of surface currents flowing around the magnet in such direction 

 that if the magnet be standing with its positive end toward the north, 

 the current passes toward the west on its underside, and toward the 

 east on its upper side ; in other words, in the direction of the hands of 

 the watch when it stands on edge with its back toward the north. 



FIG. 141 Bar of soft iron made 

 into a temporary magnet by a coiled 

 current. 



Sg^^^^^^!!^!I) If a bar or rod of soft iron 

 >M be wrapped with a coil of in- 



FIG. i4i. sulated wire carrying a cur- 



rent of electricity, the bar becomes an electro magnet. We have seen 

 that electric whirls about a wire produce a current in such wire. If the 

 wire be a short piece not connected at each end so as to allow a current 

 to pass, it will merely possess tension at each end and will then be a 

 temporary magnet. A soft iron bar ceases to be a magnet as soon as 

 the current in its surrounding coil ceases, but if a steel bar be thus mag- 

 netized by a coil, its magnetism remains permanent. In every magnet or 

 magnetized body the motion set up in its molecules and its surrounding 

 field appears to be centifugal in a lateral direction, and centripetal or 

 contracting in a longitudual direction. ' < Nairne had observed that me- 

 tallic wires submitted to discharges of static electricity underwent a 

 diminution in their length. M. Edmond Becquerel found that this 

 diminution was inversely proportioned to the cube of the diameter of the 

 wire." 1 After a wire has been brought to incandescence by a persis- 

 tent current, it is found to have lost 5 or 6 per cent, of its length, the 

 current having opposed the cohesion of the metal, and compressed the 

 molecules of the metal together in a longitudiual direction. ( See chap- 

 ter on muscles. ) 



The attraction which magnets possess for certain substances is an ex- 

 emplification and further evidence of the contractile tendency of the 

 force in certain directions. Bodies are attracted toward either end of 

 the magnet, so that if a magnet be bent into the shape of a horse shoe, a 

 bar may be placed so as to be attracted at one end to the positive pole 

 of the magnet and at the other end to the negative pole. Such a bar 

 completes the circuit of the magnet and is called an armature. The 

 1 Gaston Plante Storage of Electrical Energy, 243. 



