



* 





Conducting Powers of Metals at Different Temperatures. 187 



wires has very little influence on their conducting power, and it 

 has been shown* that bending a brass or iron wire, or even an- 

 nealing the latter at a red heat, causes also but a slight change in 

 their conducting power. That the annealing does produce a 

 change, however, is shown by the following table, in which we 

 copy the first and last three columns only. 



Metals. 



Conducting powers 

 at 55° Fah. 



Ratio of conducting power 



of annealed to that of un- 



annealed metal. 



Unnnnealed. 



93-448 

 89-084 

 64-385 

 24-574 

 24164 

 13-656 

 13977 

 12-124 

 8-245 



Annealed. 





100-000 



91-439 

 65-458 



• * 



• • 



• • 



12-246 



• • 



1-0701 

 10264 

 1-0166 



• • 



• • 



• • 



• • 



10101 



• • 



1-0130 



• • 





Pure gold, 



Cadmium, _ 





Tin, 



Palladium, 





Lead, 



Platinum, 



8-042 8147 



Mercury, (temp. 57°'2 Fah.). , 



! 1-8017 



• ■ 



wire 



Of the metals named in the above table, we see that silver «« 

 alone has its conducting power altered in any considerable degree 

 by annealing. 



The next part of the investigation relates to the conducting 

 powers of the metals at different temperatures. The wire of ex- 

 periment having had its equivalent of resistance at the atmos- 

 pheric temperature previously determined, was coiled and intro- 

 duced along with the bulb of a thermometer into a bath of oil. 

 Two thick connecting pieces of copper, whose resistance to con- 

 duction could be neglected in comparison with that of the wire, 

 were connected with the extremities of the latter in the bath. 

 By means of this arrangement immersed in a water bath the in- 

 crease of the equivalent of resistance in the wire due to heating 

 it from the temperature of the air to the boiling point could be 

 measured with accuracy. To ascertain whether the increase of 

 resistance is proportional to the elevation of temperature, the 

 heated bath was allowed to cool gradually and observations were 

 repeated at intervals. The results of experiments on a wire of 

 iron are given in the following table. The second column of 

 temperatures was calculated from the two extreme temperatures on 

 the basis of the law just stated, i. e., that the increase of resis- 



# ■ £ \ i dm * M ~mW * / 



3980-23-7 



tance for 1° Cent, is constantly 970.8 _ 150 



45205. This 



quantity we will call 



dr 

 ~dt 



See this Journal, Bd series, vol. i, p. 239. 



