Cooling on the Electrical Resistance of Iron. 83 



at the time by means of a formula ultimately obtained for the 

 resistance of the wire at any temperature*. This formula 

 was calculated from the results of a number of very careful 

 observations at 17° C, 60° G., and 100° C, and after the 

 heating and cooling process had ceased to affect the resistance. 

 The formula thus obtained was : — 



B^=R (1 + -005131 * + -00000815 O; 



whilst the formula deducible from Matthiessen's results for 

 pure iron annealed in hydrogen is 



R^= R (1 + -005425 1 + -0000083 t*). 



The specific resistance at 0° C. (R . T ) of the author's iron 

 was 9808 eletromagnetic units, whilst that of Matthiessen's 

 pure iron (R . M ) is given on the authority of the late Prof. 

 Jenkin as 9718 f . It seems not improbable that this last value 

 is from 4 to 5 per cent, too high ; for it follows from Mat- 

 thiessen's researches that the resistances at any temperature of 

 a pure metal and its alloy should be in the inverse ratio of 

 the rates of increase of resistance at that temperature, so that 



B^ should equal [jggg, 



which, if the author's result be assumed to be correct, would 

 make R . M equal to 9277. 



Unfortunately, Matthiessen did not determine the absolute 

 resistance of iron and many of the other pure metals exa- 

 mined by him, and it appears from Prof. Jenkin's own state- 

 mentj that the calculated results for these metals cannot be 

 depended on for any great degree of accuracy. The author, 

 therefore, ventures to express the hope that the B. A. Elec- 

 trical Standards Committee may be induced to determine the 

 absolute resistances and the temperature-coefficients of those 

 of the pure metals which are in ordinary use. 



* Similarly for the higher temperature of 100° C. 



t The number actually given by Prof. Jenkin lias been multiplied by 

 •9889 (the value of the B.A. unit in terms of the legal ohm). 



% See note on p. 250 of Prof. Jenkin's book on ' Electricity and Mag- 

 netism. ' 



G2 



