Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/615

" The agreement of these numbers is by no means so close as is fenerally stated ; but this is no longer remarkable, for it is well nown that the electric conductivity of all pure metals alters very much with the temperature, while we have seen that as regards thermal conductivity there is but slight change with either copper or lead, though there is a large change with iron. This accords with some results of my own on the electric conductivity of iron at high temperatures (Proc. K.S.K, 1872-73, p. 32), and with the results of the repetition of these experiments by a party of my laboratory students. "Proc. E.S.E., 1875-76, p. 629. 80. The absolute values of Tail s results for the five metals of the preceding list are given in C. G. S. units in our general table. As to change of conductivity with tempera ture there is a discrepance between the results of Angstrom s and Tait s experiments. Tait finds but little difference in the conductivity of copper through the wide range of tem peratures from to 300 C., and that difference an aug mentation instead of a diminution at the high temperatures as shown in the following results (where t is measured from C.) :* Copper, Crown 076 (1 + 00040 . C 0-054 (1 +0-000552) Iro11 :...0-015 (10-001440 On the other hand, Angstrom finds for copper from ex periments described in his second paper referred to above, at mean temperatures of from 28-8 to 71 0> 5 C., results which reduced to C.G.S. are as follows : Copper -982 (1 - -0015190 Iron -199 (1- -0028740 From the admirable method of experimenting, and the care with which experiments were carried out by himself and Thalen, it is impossible for us to doubt the validity and somewhat closely approximate numerical truth of the result. On account of the discrepance from Tait s results it is desirable that Angstrom s method should be carried out for_ copper through a much wider range of temperatures. This can be done with great ease from the lowest tempera ture obtainable by freezing mixtures to temperatures up to the melting point of copper, so excellently plastic is Angstrom s method. Our proposed extension of it is to be earned out by proper thermal appliances to the end of the bar which Angstrom left to itself, appliances by which in one series of experiments it may be kept constant at - 50 or -GO C., in others left to itself to take nearly the atmospheric temperature, in others kept at high tempera tures limited only by the melting temperature of copper, if the experimenter desires to go so far. We would also suggest that the thermo-electric method first introduced by Wiedemann and Franz in their experiments on the static temperature of bars or wires heated at one end and allowed to lose heat by convection and radiation from their sides (which was rejected, not, we think, judiciously, by Angstrom) might be used with advantage instead of the mercury thermometers inserted in holes in the bar in Angstrom s apparatus; or that, if thermometers are to be used air thermometers in which the bulb of the thermometer is it self a very small hole in the bar experimented on. and the tube a fine-bore glass tube fitted to this hole, would be much preferable to the mercury thermometers hitherto employed in, we believe, all experiments except those of Wiedemann and Franz, on the conduction of heat alon" metallic bars. ,, 81 Courier s ninth chapter is entitled De la Diffusion de la Chaleur." The idea embodied in this title is the spreading of heat in a solid tending to ultimate equalization f temperature throughout it, instead of the transference of by conduction through the ^o^nlhT.s section, expr^eTl the an omt nf , f ut the uuit of heat employed is tie sTs 1 v 1 T 1 6 ;! 10 i8 ! tbe ^mperature of a cubic foot of ( 82) ^ 581 solid considered. Though Fourier makes the special sub ject of his chapter on "Diffusion" the conduction of heat through an infinite solid, we may conveniently regard as coming under the several designations " Diffusion of Heat " every case of thermal conduction in which the heat con ducted across any part of the solid has the effect of warm ing contiguous parts on one side of it, or of leaving contigu ous parts on the other side cooler, in other words, every case in which the temperature of the body through which the conduction of heat takes place is varying with time, as distinguished from what Fourier calls " Uniform Motion of Heat, " or the class of cases in which the temperature at every point of the body is constant. The experiments of Inve Teclet, Despretz, Forbes and Tait, Wiedemann and Franz, gati< were founded on the uniform conduction of heat across oftl slabs or along bores, and their determinations of relative *^ and absolute conductivities were made by comparing or by vity measuring absolutely quantities of heat that were conducted "un out of the body tested. On the other hand, it is the fora diffusion of heat that is used in the determinations of ^ thermal conductivity in absolute measure by Forbes and William Thomson from the periodic variations of under- By c ground temperature ; in those of Angstrom, from his experi- fusic ments on the spreading of periodic variations of tempera- of ll( ture through bars of iron and copper, and a series of valuable experiments a year or two later by F. Neumann, applying the same general method to bars of brass, zinc, German silver, and iron ; in experiments by F. Neumann on substances of lower conductivity (coal, cast sulphur, ice, snow, frozen earth, gritstone) formed into cubes or globes of 5 or 6 inches diameter, and heated uniformly, and then left to cool in an atmosphere of lower tempera ture, and from time to time during the cooling explored by thermo-electric junctions imbedded in them to show their internal distribution ; in similar experiments on the cooling of globes of 14cm. diameter of porphyritic trachyte by Ayrton and Perry in Japan ; and in Kirchhoff and Hansemann s recent experiments, 2 to find the thermal con ductivity of iron by the not well-chosen method of suddenly cooling one side of a cube of iron of 14 cm., and observing the temperatures by aid of thermo-electric junctions in several points of the line perpendicular to this side through its middle. 82. When the effect of heat conducted across any part Ther of a body in heating the substance on one side or leaving diffu: the substance on the other side cooler is to be reckoned" ivit >" it is convenient to measure the thermal conductivity in terms, not of the ordinary general gramme water-unit of beat, but of a special unit, the quantity required to raise unit bulk of the substance in 1. In other words, if k be the conductivity in terms of any thermal unit, and c the ther mal capacity of unit bulk of the substance, it is k/c, not merely k, that expresses the quantity of the substance on which the phenomenon chiefly depends. We therefore propose to give to k, c the name of thermal diffusivity (or - simply diffusivity when heat is understood to be the sub ject), while still using the word thermal conductivity to denote the conducting power as defined in 73, without re striction as to the thermal unit employed. It is interest ing and important to remark that "diffusivity" is essen tially to be reckoned in units of area per unit of time, that its " dimensions " are L 2 /T (see DIMENSIONS). Its regular C.G.S. reckoning is therefore in square centimetres per second. In the article DIFFUSION the relation between diffusion of heat and diffusion of matter is explained. We have added diffusion of electricity through a submarine cable, which has been shown 3 to follow the same law as the 2 Wiedemann s (late Poggendorffs) Annalen, 18SO, No ] Proc. Roy. Soc., May 1855, Wm. Thomson Oil the Theory of

the Electric Telegraph."