MAGNETISATION BY CURRENTS. 483 



On this hypothesis a magnet is no longer a continuous substance, 

 but an assemblage of distinct molecules, giving rise to a very com- 

 plicated distribution of the magnetic force and of the potential. But 

 a material simplification results, for the force and the magnetic 

 induction are then defined in the same way, and the force satisfies 

 Laplace's equation, both inside and outside the magnet. 



Ampere's hypothesis raises, however, a difficulty of prime im- 

 portance, for it cannot be conceived that currents can permanently 

 exist without the disengagement of heat, and therefore without a 

 continued expenditure of energy. But, if the currents are supposed 

 to be localised in the molecules themselves, the constitution of which 

 is unknown, it is not impossible to assume that the resistance may be 

 zero, and that the currents exist in a form which cannot be experi- 

 mentally attacked. The assumption does not therefore necessarily 

 imply a contradiction. 



499. MAGNETISATION BY CURRENTS. Magnetisation by currents 

 was discovered by Arago in 1820. He had observed that a copper 

 wire traversed by a current attracts iron filings; every particle of 

 filing becoming a small magnet, places itself at right angles to the 

 wire the north pole on the left of the current, as in CErsted's 

 experiment. Ampere observed that the action of the current on a 

 bar of soft iron or steel could be greatly increased by coiling the wire 

 round the bar ; in this way, and particularly with soft iron, temporary 

 magnets are obtained which are called electromagnets. 



Electromagnets may acquire a far greater power than that of even 

 the best steel bars ; but their principal characteristic is that of almost 

 instantaneously gaining or losing their magnetic properties. They 

 have further this curious property, that by suitably coiling the wire 

 on the soft iron core, the magnetism may be distributed at pleasure, 

 and any number of poles, or of consequent points^ may be obtained 

 on the bar. 



The calculation of the effects of electromagnets is in general very 

 difficult, even when the currents which surround the core are equi- 

 distant and parallel. In this case the system of currents develops an 

 internal field, the strength of which is defined by the induction of a 

 uniform magnet bounded by the same surface ; the soft iron core 

 placed in this field acquires a magnetisation at each point which 

 depends on the strength of the field, and also on the magnetism de- 

 veloped by induction on the body itself. The magnetism can only be 

 uniform then in the particular cases which we have examined (385). 

 The external action is the resultant of that of the system of currents, 

 or of the equivalent uniform magnet, and of that of the soft iron core. 



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