to the Passage of an Electric Current. 



409 



Again subtracting from the values of — those of — we shall obtain 



s s 



the residual alteration of resistance produced by the stretching-force. 



[Since making the above experiments I have found that Sir William 

 Thomson has investigated the effects of strain on iron and copper, and 

 states that he attempted to eliminate the effects of elongation and nar- 

 rowing, and had very nearly established, for iron wire at least, that the 

 augmented resistance due to tension, either temporary or permanent, is 

 a very little more than can be accounted for by the change of form. 

 (Phil. Trans. Eeb. 28, 1856, § 152.)] 



In the following Table are shown the results obtained : — 





Torsional 

 rigidity in grms. 

 per sq. cent. =r. 



Ratio of lateral con- 

 traction to longitu- 

 dinal dilatation, = a. 



Increase of pg~ 

 sistance per unit 



of resistance, 

 resulting from in- 

 crease of length 

 and diminution of 

 ds 



section, = — . 



s 



Residual increase of 

 resistance per unit of 

 resistance -resulting 

 from the stretching. 





Steel. 



(2) 



(3) 



Iron. 



a) 



(2) 



Brass. 



a) ...... 



746-5x10-6 

 782-3 x 10-6 



771-1 

 637-2 



332-5 



•269 

 •259 



•325 

 •321 



•486 



811-6 xlO- 12 

 770-1 x 10- 12 



807-6 xlO- 12 

 975-2 xlO- 12 



995-5 x 10- 12 



1098-8 x 10-i 2 | 

 1060-4 x 10-i 2 | 



1292-7x10-12 \ 

 1221-4x10-12 [ 



233-7xl0-i 2 ■ 



Mean. 

 1079-6x10-12 



1257-6x10-12 

 233-7 x IO-12 



It would appear from this last Table that the increase of resistance 

 produced by a given stretching-force is, in the case of steel, iron, and 

 brass, not to be accounted for by mere increase of length and diminu- 

 tion of section of the wire ; and the residual increase of resistance, 

 which results from subtracting from the whole observed increase that 

 due to mere increase of length and diminution of section, is greater 

 in iron than in steel, and much greater in steel than in brass. In 

 all probability this residual increase is due to the increased distance 

 between the particles of the wire along the line of flow of the 

 current ; and perhaps, if we examined the effect of strain in a direc- 

 tion perpendicular to the direction of the current, there would be a 

 diminution of resistance ; but I have experiments now in hand by which 

 I hope to show the effect of strain in a direction at right angles to that 

 of the current. In conclusion, I may mention that, in testing for the 



