The Followers of Maxwell. 349 



In the special case of the field which surrounds a straight 

 wire carrying a continuous current, the lines of magnetic force 

 are circles round the axis of the wire, while the lines of electric 

 force are directed along the wire ; hence energy must be flowing 

 in the medium in a direction at right angles to the axis of the 

 wire. A current in any conductor may therefore be regarded 

 as consisting essentially of a convergence of electric and magnetic 

 energy from the medium upon the conductor, and its trans- 

 formation there into other forms. 



This association of a current with motions at right angles to 

 the wire in which it flows doubtless suggested to Poynting the 

 conceptions of a memoir which he published* in the following 

 year. When an electric current flowing in a straight wire is 

 gradually increased in strength from zero, the surrounding space 

 becomes filled with lines of magnetic force, which have the form 

 of circles round the axis of the wire. Poynting, adopting 

 Faraday's idea of the physical reality of lines of force, assumed 

 that these lines of force arrive at their places by moving out- 

 wards from the wire ; so that the magnetic field grows by a con- 

 tinual emission from the wire of lines of force, which enlarge 

 and spread out like the circular ripples from the place where a. 

 stone is dropped into a pond. The electromotive force which is- 

 associated with a changing magnetic field was now attributed 

 directly to the motion of the lines of force, so that wherever 

 electromotive force is produced by change in the magnetic field,, 

 or by motion of matter through the field, the electric intensity 

 is equal to the number of tubes of magnetic force intersected 

 by unit length in unit time. 



A similar conception was introduced in regard to lines of 

 electric force. It was assumed that any change in the total 



where W denotes the vector 



- (k + 4w/3) div e . e + n [curl e . e] ; 



and since the expression which is differentiated with respect to t represents the 

 sum of the kinetic and potential energies per unit volume of the solid (save for 

 terms which give only surface-integrals), it is seen that W is the analogue of the 

 Poynting vector. Cf. L. Donati, Bologna Mem. (5) vii (1899), p. 633. 

 * Phil. Trans, clxxvi (1885), p. 277. 



