AND MODERN PHYSICS. 1C7 



a fixed point, and his model of Saturn's rings (now in 

 the Cavendish Laboratory) to illustrate the motion of 

 the satellites in the rings. Ho had explained many of 

 the gaseous laws by means of the impact of molecules, 

 and now his fertile ingenuity was to imagine a 

 mechanical model of the state of the electro-magnetic 

 field near a system of conductors carrying currents. 



Faraday, as we have seen, looked upon electro- 

 static and magnetic induction as taking place along 

 curved lines of force. He pictures these lines as 

 ropes of molecules starting from a charged conductor, 

 or a magnet, as the case may be, and acting on other 

 bodies near. These ropes of molecules tend to 

 shorten, and at the same time to swell outwards 

 laterally. Thus the charged conductor tends to draw 

 other bodies to itself, there is a tension along the 

 lines of force, while at the same time each tube of 

 molecules pushes its neighbours aside ; a pressure at 

 right angles to the lines of force is combined with 

 this tension. Assuming for a moment this pressure 

 and tension to exist, can we devise a mechanism to 

 account for it ? Maxwell himself has likened the 

 lines of force to the fibres of a muscle. As the fibres 

 contract, causing the limb to which they are attached 

 to move, they swell outwards, and the muscle thickens. 



Again, from another point of view, we might con- 

 sider a line of force as consisting of a string of small 

 cells of some flexible material each tilled with fluid. 

 If we then suppose this series of cells caused to 

 rotate rapidly about the direction of the lino of force, 

 the cells will expand laterally and contract longi- 

 tudinally; there will again be tension along the lines 





