CONTRIBUTIONS TO SCIENCE. 517 



it to move always in the direction in which it is urged by 

 the force acting on it, it will, of course, be repelled by the 

 surface, and will move away along some path straight or 

 curved, and will continue to move indefinitely, the force 

 diminishing as it proceeds, unless it meet with a negatively 

 electrified surface, which will attract it, and coming into 

 contact with this surface its career will terminate. The path 

 traced out by such a small electrified body constitutes 

 Faraday's line of force, which is therefore a line whose direc- 

 tion at any point is that of the resultant force at that point. 

 Such lines of force always proceed from positively electrified 

 surfaces, and terminate upon negatively electrified surfaces ; 

 or, failing this, they must proceed to infinity. Lines of force 

 proceeding from a positively electrified body placed in a room, 

 unless there be other negatively charged bodies in the neigh- 

 bourhood, will in general terminate upon the walls, floor, and 

 ceiling of the room, or upon objects in the room in electrical 

 communication with these. Faraday thus conceived the 

 whole of the space in which electrical force acts to be tra- 

 versed by lines of force which indicate at every point the 

 direction of the resultant force at that point. But Faraday 

 went further than this : he conceived the notion of causing 

 the lines of force to represent also the intensity of the force 

 at every point, so that when the force is great the lines 

 might be close together, and far apart when the force is 

 small ; and since the force in the neighbourhood of a small 

 charged body is proportional to the charge, he endeavoured 

 to accomplish this object by drawing from every positively 

 electrified surface a number of lines of force proportional to 

 its charge, and causing a similar number of lines of force to 

 terminate in every negatively electrified surface. In a paper 

 entitled " On Faraday's Lines of Force," read before the 

 Cambridge Philosophical Society on December 10th, 1855, 

 and February llth, 1856, Maxwell showed that if a system 

 of lines could be drawn according to Faraday's method, then, 

 in virtue of the law of electrical action being that of the 

 inverse square of the distance, the number of lines of force 

 passing through a unit area of any surface, drawn perpen- 



