166 On the Conducting- Power of Copper and Iron for Heat . 

 experiments, and get 



^=72*96 (1-0-00214 0, 



whence 



£=61-63 (1-0-00214*). 



The inequality in the value both of a and of a in these deter- 

 minations may be satisfactorily explained by assuming a differ- 

 ence in the chemical nature of the bars used in the two sets of 

 experiments. And here it should be remarked that the great 

 copper bar used in the experiments described in this paper was 

 obtained last year direct from A vesta, and consisted of Fahlun 

 copper ; whilst the smaller copper bars taken for the former in- 

 vestigations were procured many years ago here in Upsala, and 

 whence they came I am unable to say. From analyses made 

 many years ago, it is moreover known that Fahlun copper con- 

 tains iron. 



§ 3. The values of the coefficients for the heat-conducting 

 power of copper and iron deduced from the foregoing observations 

 are smaller than the coefficients of those metals for electricity 

 have been usually found. Since, however, the latest researches 

 of Matthiessen* upon copper prove that this coefficient varies 

 considerably (from 8 to 42 per cent, for 100° C. according to 

 the purity and peculiar character of the metal), it is impossible 

 to say at present whether or not the coefficients which have been 

 obtained hold for electricity. This question can only be answered 

 by a direct determination of the electrical conducting-power of 

 both metals. In order to be rigidly accurate, it is further neces- 

 sary to make these determinations with bars in precisely the same 

 state as they were in when a determination of the heat-coefficient 

 was made, since the act of drawing out the metal into wire might 

 easily bring about an alteration in the conducting-power. In 

 satisfying these conditions, a difficulty is encountered which is 

 by no means small, and which relates to the thickness of the 

 bars. A preliminary experiment in which the large copper and 

 iron bars formed parts of a so-called Wheatstone's balance gave 

 no certain result, inasmuch as the least alteration in the contact 

 between the bars and the other parts of the balance exercised a 

 great influence upon the result. I hope, however, to overcome 

 these difficulties. 



§ 4. This investigation has shown 



1. The practicability and sufficient accuracy of the method, 



* See Matthiessen's paper in the Philosophical Magazine, Sept. 1861. 

 The quotation above is not quite correct. The 8 and 42 refer to the varia- 

 tion in electrical resistance and not to conducting-power. — J. A. W. 



