OF ARTS AND SCIENCES. 303 



XXVI. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF 



HARVARD COLLEGE. 



XL — CHANGE OF ELECTRICAL RESISTANCE IN WIRES BY 



STRETCHING. 



By Geokge S. Pine. 



Presented, May 10, 1876. 



The electrical resistance of a wire of constant section and material 

 is directly proportional to the length, and inversely proportional to the 

 area of the cross-section. When the wire is stretched, its thickness or 

 cross-section, as well as its length, undergoes a change. This investiga- 

 tion was undertaken to see whether the change in resistance is directly 

 as the length, and inversely as the cross-section of the wire ; or 

 whether the copper or the iron, whichever the substance may be, is a 

 better or a worse conductor. 



Let I = original length of the wire. 



l^ = length at the end of the experiment. 



r = original radius. 



r^ = final radius. 



H = original resistance of the wire. 



7?j = tinal resistance. 



V = volume of wire, — supposed constant. 



'K z=z resistance of wire whose length equals its cross-section. 



Suppose that X does not alter. On this supposition, let us find the 

 resultant resistance. We must compare this with the observed resist- 

 ance to see whether our supposition is right ; to see whether I does or 

 does not alter. 



Supposing 1 does not vary 



* = -. i?. = A 





7rr,2 



(1) 



