204 Dr. 0. J. Lodge on the Variation of the Thermal 



o 



temperature, for copper and iron by Angstrom*; and his 

 result for iron, reduced to the C.Gr.S. scale, can be expressed 

 in the following way, 



(|) = 2U3(1 - 002874f) ' 



-Cp/ Fe 



A! 



which may be considered as equivalent to -~jn — r for a small 



range of temperature. + 



The results of Angstrom for copper are summed up in the 

 following formula, 



k 



(±\ =1-163(1--0015190, 



which may be expressed as ^-^ when t is small. 



8. The law of variation of the ordinary or calorimetric con- 

 ductivity k can, of course, be deduced from the above by mul- 

 tiplying them by the value of the product cp, with each of its 

 factors expressed as a function of the temperature — the one 

 factor from the experiments of Bede on specific heat, the other 

 from the expansion experiments of Fizeau. This Professor 

 W. Dumasf has done. Using a mean coefficient of expansion 

 between 0° and 100° C, he writes for the density of iron, 



..p= 7-7799(1 --00003684*); 



and for its specific heat, 



c=-1053(l + -0013480. 



° k 



Multiplying Angstrom's value of — for iron by the product of 



cp 



these two quantities, we obtain 



&=-1862(l--001560. 



k 

 And multiplying Forbes's numbers for — (nominally for &), 



cp 



as reduced to the C.Gr.S. system by Dr. Everett, on page 44 



of his book on Units (first edition), by the variation factor 



(1 + -001311 1), we obtain the real values of k at the different 



Centigrade temperatures t according to Forbes's experiments, 



Phil. Mag. vol. xxv. 1863. 

 T Pogg. Ann. cxxix. See also Wiillner, Exp. Physik, vol. iii. pp. 286 



& 287. 



