122 Mr. 0. Heaviside on the 



apparent attraction or repulsion between them. Oppositely- 

 going currents repel when they are decreasing and attract 

 when they are increasing. Thus, send a current into a loop, 

 one wire the return to the other, both -being close together. 

 During the rise of the current it w T ill be denser on the sides 

 of the wires nearest one another than on the remote sides H 



An iron wire, through which rapid reversals are sent, should 

 afterwards be found, by reason of its magnetic retentiveness, 

 magnetized in concentric cylindrical shells, of alternately 

 positive and negative magnetization. This would only occur 

 superficially. The thickness of the layers would give infor- 

 mation regarding the amount of retardation, from which the 

 inductivity could be deduced. The case is similar to that of 

 the superficial layers of magnetization produced in a core in 

 a coil through which reversals are sent, the magnetization 

 being then, however, longitudinal instead of circular. 



The linear theory is departed from in the most extreme 

 manner, when the return-current closely envelops the wire. 

 The theory of the rise of the current in this case I have given 

 in the ' Electrician ' for May 14, 1886, and the case of the 

 return-current at any distance is considered June 11, and 

 will be followed by others. The investigation following in 

 this paper is more comprehensive, taking into account both 

 electrostatic and electromagnetic induction, working down to 

 the electromagnetic theory on the one hand, and approxima- 

 ting towards the electrostatic theory (long submarine cable) 

 on the other ; with this difference, that inertia is not so 

 wholly ignorable in the long-line case as is elastic yielding in 

 the case of a T short wire. Nor is the variation of current- 

 density wholly ignorable. 



But first as regards the transfer of energy in the electro- 

 magnetic field. This is avery important matter theoretically. 

 It is a necessity of a rationally intelligible scheme (even if it 

 be only on paper) that tne transfer of energy should be expli- 

 citly definable. It fc-the absence of this defmiteness that 

 makes the, Germar methods so repulsive to a plain man who 

 likes tq see wherG he is going and what he is doing, and hates 

 metaphysics in science. -- 



I found that I had been anticipated by Prof. Poynting in 

 the deduction of th6 transfer of energy formula appropriate 

 to Maxwell's electromagnetic scheme, in the main. It is, 

 therefore, only as having given the equation of activity in a 

 more general form, the most' general that Maxwell's scheme 

 admits of, and having deduced it in a simple manner, that I 

 can attach myself t ; o the matter. In connection with it, how- 

 ever, there is. another matter of some importance, viz. the use 



