Vrof. Thomson, On the Electric Theory of Gravitation. 65 



On the Electric Theory of Gravitation. By Sir J. J. Thomson, 

 M.A., F.R.S., Cavendish Professor of Experimental Physics. 



[Read 9 November 1908.] 



The view that gravitational attraction is due to a slight excess 

 of the attraction between unlike charges of electricity over the 

 repulsion between like charges is a very old one, indeed it seems 

 to have been regarded by some writers as almost a part of the one 

 fluid theory of electricity. The theory is an interesting one 

 because it involves the existence of effects which do not seem to 

 be hopelessly too small to be tested by experiment : some of these, 

 relating to the possible influence of the velocity of the attract- 

 ing bodies on the gravitational attraction between them, have 

 recently been considered by Lorentz. At the end of this paper 

 I shall indicate another result of the theory which is of so special 

 a character, that if it were established by experiment, it would 

 follow almost as a matter of course that gravity must be due to 

 something very closely connected with electrical action. 



Before discussing this effect I will consider the theory from 

 the point of view of stresses in a medium between the attracting 

 bodies. In one form of electrical theory we suppose that the 

 stress in the ether is altered by the passage through it of lines of 

 electric force, in such a way that the tension along the lines of 

 force and the pressure at right angles to them is increased by an 

 amount proportional to the square of the density of the lines of 

 force. "When we endeavour to use lines of electric force to describe 

 the state of the magnetic field or of fields through which electrical 

 waves are passing, I think there are considerable advantages in 

 regarding the lines of electric force from a somewhat different 

 point of view from that usually adopted in electrostatics. In 

 that subject it is usual to take as the lines of force due to say a 

 positive charge e at ^, and a negative charge — e at 5, as the 

 lines whose directions are the resultants of the radial forces 

 e/AP^ and —ejBP^, radiating from A and B respectively. We 

 might however regard the lines of force as consisting of two sets, 

 one set being straight lines radiating from A, the other straight 

 lines radiating from B. We might, that is, regard the component 

 fields of which the actual field is made up as having an actual 

 physical existence, and suppose that it is the effect they produce 

 and not their structure which is modified when several of them 

 exist simultaneously in the same field. We can illustrate the 

 difference between the two methods by considering the case of 

 two parallel vertical planes A and B, A being on the left of B, 



VOL. XV. PT. I. 5 



