Prof. Thomson, On the Dynamics of the Electric Field. 83 



On the Dynamics of the Electric Field. By Professor J. J. 

 Thomson. 



[Read 16 February 1903.] 



In Recent Researches in Electricity and Magnetism it is shown 

 that in each unit of volume of the Electric field there are 

 BF sin 6 units of mechanical momentum, B being the magnetic 

 induction, F the electric displacement and 6 the angle between 

 these vectors: the direction of the momentum is at right angles 

 both to B and F. In this paper some of the consequences of 

 this distribution of momentum are investigated : it is shown that 

 when the field is that due to a charged point e and a magnetic 

 pole m the distribution of momentum has a moment of momentum 

 round the line joining the point and the pole equal to em. This 

 system is thus a spinning top and it is shown that the mechanical 

 forces acting on a moving electrified point or magnetic pole can 

 be determined from the principle that the moment of momentum 

 of the whole system, point and pole and field, remains constant. 

 Several other cases are investigated, among others it is shown 

 that Maxwell's Vector Potential at a point P is the part of the 

 momentum of the field due to the unit electrical charge at P. 



It is shown that all the laws relating to the distribution of 

 momentum in the field follow from the view that the lines of 

 electric force carry along with them a portion of the ether through 

 which they pass, the mass of ether entangled with the tubes 

 being per unit volume proportional to the electrostatic energy 

 of the field in that unit volume ; the ether thus entangled can 

 slide along the line of electric force, but as far as motion at right 

 angles to the line is concerned, the entangled ether moves with 

 the line of force, the momentum in the electric field is the 

 momentum of the ether gripped by the lines of force. It was 

 suggested that all mechanical momentum and not merely electrical 

 momentum was really momentum of the ether ; the molecules of 

 matter contain a number of electrified bodies ('corpuscles'), and 

 the lines of force starting from these corpuscles grip a certain 

 amount of the ether, and consequently the mass of the bod}' is really 

 the mass of the ether gripped by the lines of electric force starting 

 from its corpuscles. The potential energy of the field is on this 

 view the kinetic energy of the turbulently moving ether im- 

 prisoned by the lines of force. 



6—2 



