186 Conducting Powers of Metals at Different Ternpwatures 



length of the included portion of the wire, the nature of the con- 

 nections remaining the same, and the variations in the length of 

 the wire of experiment could be read off on a graduated scale. 



As a test of the accuracy of the method, we give the results of 

 experiments made on an iron wire of a diameter of -3223 mui at 

 the temperature of 12°*5 C. These results are cited by the 

 author to prove the law, almost axiomatic indeed, that the resist- 

 ances are proportional to the length of the wire. 



Readings of 



graduated 



scale. 



Centim etV 

 

 20 

 40 

 60 

 80 

 100 



Readings of 



index of 



rheo-stat. 



944 

 257-0 

 4196 

 5794 

 787-7 

 900-0 



Lengths of] 



standard 



wire of 



rheostat. 



•0 



1626 

 325*2 



485-0 

 6433 

 8056 



Lengths of stan- 

 dard wire equiv- 

 alent to 0-01 »* 



of the iron wire 



8-13 

 8*13 

 8-08 

 8*04 



s-06 



Second 

 column cal- 

 culated by 

 least squares 



95-9 

 256-7 

 417-6 

 578-5 

 7393 

 900-2 



Errors 



of 

 second 



column. 



-1-5 



-f0-3 

 -f-2-0 



+ 0-9 

 -1-6 

 -0-2 



Length of 

 standard 



wire equiva- 

 lent to 

 0-01"* of the 

 iron wire 

 8-043. 



The numbers in the columns following the first are to be re- 

 garded as arbitrary units of the standard wire, the unit being 

 nearly equal to 2 mm . The fifth and sixth columns we have 

 added to exhibit the degree of precision of the experiments, and 

 the best result as shown by the numbers in the fifth column is 

 that the resistance of one metre of the iron wire is equal to 804-3 

 divisions of the rheo-stat, and this is denominated the equivalent 



of 



The next inquiry considered is whether the relation of the re- 



sistance to the diameter follows the commonly received law with 

 the same degree of precision. Two iron wires unannealed, hav- 

 ing diameters of 0*7370 mm and 0-3037 nim , gave on a mean of 

 two experiments with each, the following results, the length being 

 one metre. 



Equivalent of 

 resistance. 



1st wire, 

 2d wire, 



Diameters. 



Squares of the 

 diameter. 



I Prod, of squares into 



resistances. 





85717 

 85-622 



The close correspondence of the numbers in the last column, 

 though exactly what might be expected in conductors of the 

 same metal in the same state of aggregation, is yet worthy ot 

 particular notice in the case of wires. It might be anticipated in 

 view of the violent operation of wire drawing, that the smaller 

 wire would differ so much from the larger in texture and hard- 

 ness as to cause a decided change in the conducting power, and 

 a marked deviation from the law in question on a comparison of 

 the two wires. But the result shows that in the case of iron at 

 least whatever difference of texture there may be in the two 



