154 



NA TURE 



[June i8, 1896 



Owiny to the oft'er of Sir George Baden-Powell to take 

 an observing party to Novaya Zemlya in his yacht, it is 

 quite possible that that station may be occupied by 

 English observers as well as by the expeditions sent by 

 the St. Petersburg Academy of Sciences and the Kasan 

 Society of Naturalists. 



An expedition from Harvard College Observatory will 

 accompany Prof Todd to Japan, with the special object 

 of obtaining photographs of the progress of the eclipse 

 vith a large prismatic camera. 



Mr. Crommelin has communicated to the Journal of 

 the British .\stronomical Association a reduced map of 



the stars and planets near the sun at the time of totality 

 (F'g. 2), and a diagram showing the relation of the sun's 

 axis and equator, and the path of the moon's centre to the 

 horizon at \'ads6 (Fig. 3). These, by the courtesy of Mr. 

 Crommelin, we are enabled to reproduce. 



According to Prof Mohn, the weather chances in Lap- 

 land are vastly superior to tliose in Japan. We are glad 

 also to learn that many lovers of nature are taking advan- 

 tage of the opportunity of seeing one of the grandest of 

 natural phenomena under the favourable conditions 

 aftbrded by modern travel, not forgetting that the eclipse 

 takes place in a region of beautiful scenery and in the 

 holiday season of the year. 



THE ELECTRICAL RESISTANCE OF ALLOYS. 

 'T'HE recent researches of Profs. Dewar and Fleming 

 -*■ upon the electrical resistance of metals at low 

 temperatures have brought into strong relief the difter- 

 •ence between the behaviour of pure metals and of alloys. 

 In the former case the resistance shows every sign of 

 tending to disappear altogether as the absolute zero of 

 temperature is approached, but in the case of alloys this 

 ■condition of things is widely departed from, even when 

 the admixture consists only of a slight impurity. 



-Some years ago it occurred to me that the apparent 

 resistance of an alloy might be partly made up of thermo- 

 electric effects, and as a rough illustration 1 calculated 

 the case of a conductor composed of two metals arranged 

 in alternate lamina? perpendicular to the direction of the 



NO. 1390, VOL. 54] 



current. Although a good many difficulties remain un- 

 touched, I think that the calculation may perhaps sug- 

 gest something to those engaged upon the subject. At 

 any rate it affords ci priori ground for the supposition that 

 an important distinction may exist between the resistances 

 of pure and alloyed metals. 



The general character of the effect is easily explained. 

 According to the discovery of Peltier, when an electric 

 current flows from one metal to another there is develop- 

 ment or absorption of heat at the junction. The tempera- 

 ture disturbance thus arising increases until the conduc- 

 tion of heat through the laminie balances the Peltier 

 effects at; the junctions, and it gives rise to a thermo- 

 electromotive force opposing the passage of the current. 

 Inasmuch as the difference of temperature at the alternate 

 junctions is itself proportional to the current, so is also 

 the reverse electromotive force thereby called into play. 

 Now a reverse electromotive force proportional to current 

 is indistinguishable experimentally from a rcj/j/rt/ztY/ so 

 that the combination of laminated conductors exhibits a 

 false resistance, having (so far as is known) nothing in 

 common with the real resistance of the metals. 



If t' be the thermo-electric force of the couple for one 

 degree difference of temperature of the junctions ; /, / 

 the actual temperatures ; then the electromotive force for 

 one couple is e(t — {). If we suppose that there are n 

 similar couples per unit of length perpendicular to the 

 lamination, the whole reverse electromotive force per unit 

 of length is nc(t — f). Again, if C be the current corre- 

 sponding to unit of cross-section, the development of heat 

 per second at each alternate junction is per unit of area 

 273 X ^ X C, the actual temperature being in the neigh- 

 bourhood of zero Cent. This is measured in ergs, and is 

 to be equated to the heat conducted per second towards 

 the cold junctions on the two sides. If /!■, k' be the con- 

 ductivities for heat of the two metals, / and /' the corre- 

 sponding thicknesses, the heat conducted per second is 

 (t - t')\kll + k'ir\ ; 



or if /(/ + /') =/, l'j{l+l') = g, / + /' = il>!, the con- 

 ducted heat is 



n(t - i'){kjp + k'l<i\. 



In this expression/ + ^ = i, the symbols/ and q denot- 

 ing the proportional amounts by volume in which the 

 two metals are associated. Thus when a stationary state 

 is reached, 



273 X ,■ X C = «(/ - t')\klp\ k'lq\. 



This determines [t — /') when C is given ; and the whole 

 back electromotive force per unit of thickness is ;-C, 

 where 



,.= 273 X c- 

 /■// + k'j,i 

 This is the expression for the false resistance per unit 

 of thickness, which, it should specially be noted, is inde- 

 pendent of //, the number of couples. The number ot 

 couples which co-operate is indeed increased by finer 

 lamination, but the efficiency of each is decreased in the 

 same proportion by the readier conduction of heat between 

 the junctions. It is scarcely necessary to point out that 

 the false resistance is called into play only by currents 

 which flow across the lamin;e. 



In my original calculation the metals chosen for illus- 

 tration were iron and copper. In this case (Everett's 

 C.G.S. system of units, p. 192) e = 1600. The con- 

 ductivities are to be measured in ergs. For iron, 

 X: = '164 X 4'2 X 10"; for copper, /■' = rii x 42 x 10". 

 Thus, if the metals are in equal volumes {p = q = \), 

 _ 2 X 273 X 1600- _ ,, 

 4'2 X lo" X I '27 



This is the thermo-electric addition to the true specific 

 resistance, and is about \\ per cent, of that of copper. 

 Such an addition may seem small ; but it should be re- 



