456 



Prof. D. E. Hughes. 



[May 27, 



as to its truth, for according to this view we should be able to reduce 

 the self-induction to a very great extent by employing thin flat sheets 

 where the outlying portions are at a comparatively great distance from 

 the central portions. This I have experimentally proved to be the 

 fact, but this experiment alone does not show if the reduction of self- 

 induction is due to a different arrangement of the current in sheets as 

 compared with solids. If the reduction is due to the greater separa- 

 tion of the streamlets, then we should be able to reduce this induction 

 in a still greater degree by employing a conductor composed of 

 numerous small copper wires through which the current is equally 

 divided, and which could be separated or brought close together as 

 desired. This proves to be also an experimental fact, for the con- 

 ductor formed of numerous strands has far less self-induction than a 

 thin sheet when its wires are separated, so that they can no longer 

 react on each other, and surpasses the thin sheet and approaches the 

 value of a circular wire when these wires are brought near together, 

 so that their mutual reactions can approximate those of the numerous 

 streamlets in a solid conductor. 



Iron shows a still greater reduction in its self-induction when in 

 the form of thin flat sheets or numerous small iron wires separated 

 from each other, with the exception that we cannot restore or approach 

 the value of a solid wire by bringing them in close proximity. The 

 reduction of the induced currents by employing thin sheets instead 

 of a wire is so great in iron that we could not account for it on the 

 mere separation of contiguous portions of the same current, but if we 

 assume that the comparatively high force in iron wires is due to the 

 induced circular magnetism, and that this almost disappears in flat 

 sheets, we account for the fact that a thin flat sheet of iron has less 

 inductive capacity than a similar sheet of copper of the same resist- 

 ance, but of different widths ; and if thin strips of copper, brass, 

 lead, and German silver are compared with similar iron strips, and 

 their resistance rendered equal by the added resistance in the bridge,, 

 no difference is found between iron and the non-magnetic metals, for 

 under these conditions their inductive capacities appear equal. 



Reactions of an Electric Current between Separate Portions of the same 



Conductor. 



In order to distinguish two distinct effects I have defined self- 

 induction as the effect produced by an electric current on its own 

 portion of the wire, and mutual induction as the effect of the reactions 

 between separate portions of the wire on each other. 



The mutual induction in iron and copper wires is very different in 

 degree, as shown by the following table : — 



