AND MODERN PHYSICS. 1G9 



and olectro-inagnctic forces ariso from its motion and 

 its stresses. 



Now, Maxwell's fundamental supposition is that, 

 in a magnetic field, there is a rotation of the mole- 

 cules continually in progress about the lines of mag- 

 netic force. Consider now the caso of a uniform 

 magnetic field, whoso direction is perpendicular to 

 the paper ; we are to look upon the lines of forco 

 as parallel strings of molecules, the axes of these 

 strings being perpendicular to the paper. Each 

 string is supposed to bo rotating in the same direc- 

 tion about its axis, and the angular velocity of rota- 

 tibn is a measure of the magnetic force. In conse- 

 quence of this rotation there will be differences of 

 pressure in different directions in the medium; the 

 pressure along the axes of the strings will be less than 

 it would be if the medium were at rest, that in the 

 directions at right angles to the axes will bo greater, 

 the medium will behave as though it were under 

 tension along tho axes of the molecules under 

 pressure at right angles to them. Moreover, it can 

 be shown that the pressure and the tension are both 

 proportional to the square of the angular velocity 

 the square, that is, of tho magnetic force and 

 this result is in accordance with the consequences 

 of experiment. 



More elaborate calculation shows that this state- 

 ment is true generally. If we draw the lines of force 

 in any magnetic field, and then suppose the molecules 

 of tho medium set in rotation about these lines of 

 force as axes, with velocities which at each point are 

 proportional to the magnetic force, the distribution of 



