1886.] On the Self-induction of an Electric Current. 455 



-of experiments with wires of the same length and diameter, but of 

 different resistance. These showed a marked difference provided the 

 current was increased in proportion to its diameter or conductivity, 

 but no difference could be found when the current was kept a con- 

 stant, and the interior differences in resistance of the wires were 

 compensated by an external added resistance ; but if we take wires of 

 different metals — all of the same length and resistance, but of different 

 diameters — there is a marked difference due to the mutual reactions 

 «of the current in its own wire, being less in wire of large diameter 

 than in small wires. These effects have been fully explained in my 

 late paper, and have since been completely verified by the method 

 used at present. I showed by the use of my late method a critical 

 point in the rise and fall of the induced currents ; this is not shown 

 by the present method of compensating by external resistance, but the 

 rapid decrease in the electromotive force of the induced currents, as 

 indicated by the induction balance, is well shown by the present 

 method. The following table shows the observed force of the extra 

 currents for wires of the same length (1 metre), but of different 

 diameters : — - 



Iron . . 

 Copper 



mm. 

 0'25 



mm. 

 0-50 



mm. 

 1-00 



mm. 

 2-00 



mm. 

 3-00 



mm. 

 4-00 



mm. 

 5-00 



mm. 

 6*00 



mm. 

 7-00 



mm. 

 8-00 



mm. 

 9-00 



mm. 

 10-00 



760 

 129 



621 

 113 



530 

 100 



360 

 89 



269 

 82 



220 

 78 



190 

 75 



171 

 73 



152 

 72 



138 

 71 '5 



128 

 712 



124 

 71 



The fall in force is now even more rapid than shown by my late 

 method, and it will be seen that iron is peculiarly sensitive to an 

 increase in diameter, having nearly six times the force of copper in 

 wires of 0*25 mm., and not twice the force of copper in wires of 

 10 mm. section. 



Reactions of an Electric Current in its own Portion of Conductor. 



The phenomenon of a constant decrease in the electromotive force 

 of self-induction, as measured by the induction balance, with each 

 increase of the sectional area of the conductor, is well shown by the 

 present method, and an experimental investigation of its cause has 

 shown that we should not consider a current in a wire as a single 

 element reacting solely upon exterior wires, but that the current 

 .acts precisely as would an infinite number of independent stream- 

 lets of current reacting upon each other in the interior of its own 

 wire similarly to their known effects upon exterior wires. My experi- 

 ments demonstrate this to a degree that leaves no doubt on my mind 



