AND MODERN PHYSICS. 195 



facts. Ho failed to answer the second. He says 

 (" Electricity and Magnetism," voL L p. 132): 



44 It must be carefully borne in mind that we have made 

 only one step in the theory of the action of the medium. We 

 have supposed it to be in a state of stress, but have not in any 

 way accounted for this stress, or explained how it is maintained. 

 ... I have not been able to make the next step, namely, to 

 account by mechanical considerations for these stresses in the 

 dielectric." 



Faraday had pointed out that the inductive action 

 between two bodies takes place along the lines of 

 force, which tend to shorten along their length and 

 to spread outwards in other directions. Maxwell 

 compares them to the fibres of a muscle, which 

 contracts and at the same time thickens when 

 exerting force. In the electric Held there is, on 

 Maxwell's theory, a tension along the lines of electric 

 force and a pressure at right angles to those lines. 

 Maxwell proved that a tension K II'"/** *r along the 

 lines of force, combined with an equal pressure in 

 perpendicular directions, would maintain the equili- 

 brium of the field, and would give rise to the observed 

 attractions or repulsions between electrified bodies. 

 Other distributions of stress might be found which 

 would lead to the same result. The one just stated 

 will always bo connected with Maxwell's name. It 

 will bo noticed that the tension along the lines of 

 force and the pressure at right angles to them are 

 each numerically equal to the potential energy stored 

 per unit of volume in the field. The value of each of 

 the three quantities is K R 2 /8?r. 



In the same way, in a magnetic field, there is 

 a state of stress, and on Maxwell's theory this, too, 

 M 2 



