172 HOUSTON, KENNELLY — THE PATH OF A CURKENT. [Mar. 19, 



to be regarded as pumped in at one end of a conducting wire and 

 as exuding in equal quantities at the other. The electricity does 

 indeed travel thus, whatever the travel of electricity may ultimately 

 be found to mean, but the energy does not." Poynting expresses 

 it thus : ** Formerly a current was regarded as something traveling 

 along a conductor. But the existence of induced currents and of 

 electromagnetic action at a distance from a primary circuit, from 

 which they draw their energy, has led us, under the guidance of Fara- 

 day and Maxwell, to look upon the medium surrounding the conduc- 

 tor as playing a very important part in the development of the 

 phenomena." 



I am not prepared to abandon this intermediate point of view 

 for the reasons which I shall presently state. It naturally depends 

 to a great extent on what we are to understand by an electric cur- 

 rent. The nature of electricity being unknown, this phenomenon 

 can only be dealt with as a sum of correlated effects. Some of 

 these occur in the conductor, some in the space outside of it. 



One fundamental relation was obtained when it was proved by 

 experiment that the translatory motion of a charged conductor pro- 

 duced the external effects of a current ; the effects otherwise found 

 in the conductor being then absent because the conductor moved 

 with the charge and with its external condition or electric flux. 



It might be argued that the conducting body, say a charged 

 sphere, should be considered as being mainly a discontinuity in the 

 surrounding insulating space and that the displacement of this dis- 

 continuity created the effects by disturbing the space. The experi- 

 ments of Rowland, however, have established that a rotary motion 

 of a plane conducting disc around its axis produced the same ex- 

 ternal effects as a current and in this case the conductor, considered 

 as a discontinuity in the insulating space, did not change its posi- 

 tion. Does this not- tend to show that the unknown condition 

 which moves bodily with the substance of the conductor in the one 

 case moves along the conductor in the other case, which is called 

 current proper ? 



On the other hand we find that in the case of current proper, 

 effects are produced within the conductor, and we find also that these 

 effects occur not merely at the boundary between conductor and 

 surrounding space, but throughout the cross-section of the conduc- 

 tor. This is undoubtedly true of the heating, the electrolytic 

 effects, osmotic effects, migration of the ions, actions at the 



