502 Mr. S. Bidwell on the Electrical Resistance 



element just above it. Thus a current of electricity, in pass- 

 ing from the lowest to the highest point of such a wire, 

 is always flowing from relatively unstretched to relatively 

 stretched portions. If, then, the wire were of copper, heat 

 would be evolved throughout its whole length ; the tempera- 

 ture of the wire would rise, and its resistance would conse- 

 quently be increased. With a current flowing from top to 

 bottom, the temperature of the wire would, fall and its resist- 

 ance diminish. So also an iron wire would be cooled and 

 and have its resistance lowered by an upward current, while 

 a downward current would heat it and increase its resistance. 

 The changes of resistance are thus, as I believe, proximately 

 due to changes of temperature. 



The resistance of the bridge-wire used in my experiments 

 was *244 ohm, and, as already mentioned, an additional resist- 

 ance of 100 ohms was placed in each of the gaps adjoining 

 the bridge-wire. Denoting the resistance of the half A of 

 the suspended wire by a, and that of B by b, we have, from 

 the first experiment with the copper wire (the result of which 

 agrees closely with the mean) : — 



For direct current, 





a i(xr+-5t)yx 



•244"' 





&~"l00 w + -431x 



•244 w 





100139 





Also 



100105 





Hence 



a+b=2-ir. 







a=l-0551792 a, * : 



> 





& = l-0548208 w . 





For 



reversed current, 







a 100" + -633 x 



•244 w 





&~100 W -P367x 



•244 w 





100154 





100090 



* Of course the resistances are not really measured to the high degree 

 of accuracy suggested by these figures j but any small error of "excess or 

 defect would be approximately the same for the two values of a (with 

 direct and reversed currents) and would not materially affect their differ- 

 ence, to which alone importance is attached. 



