66 Prof. Thomson, On the Electric Theory of Gravitation. 



the one having a positive the other an equal negative charge. On 

 the usual view, lines of force only exist between A and B, the 

 lines starting from the positive charge on A and ending on the 

 negative charge at B ; the region to the left of A or the right of B 

 being free from lines of electric force. On the other view positive 

 lines of force start from A both to right and left, and also negative 

 lines of force from B. In the region between A and B we have 

 the positive lines in one direction and the negative in the opposite, 

 both giving a contribution of the same sign to the electric force ; 

 while on the left of A and the right of B the negative and the 

 positive lines run side by side in the same direction and, as far 

 as the electric force goes, neutralize each other's effect. 



The advantages of the second method are most felt when the 

 electric charges are moving and there is a magnetic as well as an 

 electrostatic field, the magnetic force at any point being regarded as 

 due to the movement of the lines of electric force at that point, and 

 proportional to the vector product of the number of lines of electric 

 force and the velocity of these lines. Thus suppose the plate A 

 is rotating while 5 is at rest, on the second view of the disposition 

 of the lines of force in the field, all the positive lines of force 

 rotate with A, thus there is motion of these lines, and therefore 

 magnetic force throughout the whole of the field and not merely 

 between the plates A and B: and we know from experiments that 

 the magnetic force does exist throughout the whole region and is 

 not localized between A and B. If we take the first view that 

 the lines of force, due to the charges on A and B, are confined to 

 the region between A and B, then the rotation of A would only 

 set these lines in motion in the space between A and B, this 

 motion would only account for the magnetic field between the 

 plates, and we should have to introduce further hypotheses to 

 account for the magnetic forces which as experiment shows exist 

 in the other regions. 



If we consider the differences which exist between the 

 properties of negative and positive electricity, the negative being 

 located on corpuscles, the positive on bodies of atomic dimensions, 

 it does not seem by any means impossible that there may be 

 some difference between the positive and the negative lines of force^ 

 at any rate the effects of such a difference seem a legitimate 

 subject for discussion. 



Let us take the case where all the lines of force are parallel 

 to the axis of x, and suppose that through unit area, at right 

 angles to x, there are P positive lines of force running in the 

 direction in which x increases, N negative lines of force in the 

 same direction ; then on the usual theory of stress in the medium, 

 in which we do not distinguish between the effects of positive 

 and negative lines, these lines increase the tension along the 



