Self -induction of Wires. 121 



wires. If this be correct, it is exactly what I should have 

 expected and predicted. In fact, I have already described 

 the phenomenon in this Journal ; or, rather, the phenomenon 

 I described contains . in itself the above interpretation. In 

 the ' Electrician ' for January 12, 1884" [corrected to 

 January 10, 1885, in a subsequent letter, May 7, 1886], "I 

 described how the current starts in a wire. It begins on its 

 boundary, and is propagated inward. Thus during the rise 

 of the current it is less strong at the centre than at the boun- 

 dary. As regards the manner of inward propagation, it takes 

 place according to the same laws as the propagation of mag- 

 netic force and current into cores from an enveloping coil, 

 which I have described in considerable detail in this Journal. 

 The retardation depends upon the conductivity, upon the 

 inductivity, and upon the section, under similar boundary- 

 conditions. If the conductivity be high enough, or the induc- 

 tivity, or the section be large enough, to make the central 

 current appreciably less than the boundary- current during the 

 greater part of the time of rise of the current, there will be 

 an apparent reduction in the time-constant. Go to an ex- 

 treme case — very rapid short currents, and large retardation 

 to inward transmission. Here we have the current in layers, 

 strong on the boundary, weak in the middle. Clearly then, 

 if we wish to regard the wire as a mere linear circuit, which 

 it is not, and as we can only do to a first approximation, we 

 should remove the central part of the wire — that is, increase 

 its resistance, regarded as a line, or reduce its time-constant. 

 This will happen the sooner the greater the inductivity and 

 the conductivity, as the section is continuously increased. It 

 is only thin wires that can be treated as mere lines, and even 

 they, if the speed be only great enough, must be treated as 

 solid conductors. I ought also to mention that the influence 

 of external conductors, as of the return conductor, is of im- 

 portance, sometimes of very great importance, in modifying 

 the distribution of current in the transient state. I have had 

 for years in manuscript some solutions relating to round wires, 

 and hope some day to arrive at them in the course of the 

 paper 1 am at present publishing, or, rather, not publishing, as 

 the editor has been able to afford so little space for it lately. 



" As a general assistance to those who go by old methods, 

 a rising current inducing an opposite current in itself and in 

 parallel conductors, this may be useful. Parallel currents are 

 said to attract or repel, according as the currents are together 

 or opposed, This is, however, mechanical force on the con- 

 ductors. The distribution of current is not affected by it. 

 But when currents are increasing or decreasing, there is an 



