INTERACTIONS OF MOVING ELECTRONS 205 



called a magnet. The reason why two magnets attract 

 or repel is to be found in the interactions of these 

 oriented loops. 



The discovery of electricity in motion by Galvani 

 and that of a source by Volta was followed in 1820 by 

 the important discovery of Oersted, a Danish investi- 

 gator. His experiments showed that a magnet was 

 deflected from the magnetic meridian by a conductor 

 carrying a current when the latter was placed over 

 the needle and parallel to it. The direction of deflec- 

 tion was reversed when that of the current in the con- 

 ductor was reversed. This phenomenon came to be 

 explained and stated in terms of force by saying that 

 the current exerts a magnetic force. About the con- 

 ductor there is said to be a magnetic field, that is, a 

 region in which a magnetic force may act. 



The intensity of this magnetic field at any point was 

 quantitatively expressed in terms of the force which 

 it would exert upon a so-called "unit magnetic pole" 

 placed at the point. The direction of the field was 

 taken as that in which a " north-seeking " pole would 

 move under the action of this force. The forces 

 exerted between magnets had already been investi- 

 gated by Coulomb, who obtained a relation identical 

 in form with that for electrical charges, namely, 



(1) 



where m is the pole strength and v is a constant of 

 proportionality. Taking /* equal to unity for a vacuum 

 (or air), the equation defines unit pole as one which 

 will exert upon a like pole a force of 1 dyne at a dis- 



