December 30, 1915] 



NATURE 



499 



the distance between the centres of two doublets, 

 le number of doublets in unit length of a chain = 

 d\ hence N/d = I, or i = ldpe. 

 Now we have seen that 



I = Nw| 



\ t' + «^ xj 



Nf/i- along the straight part of 



the curve when n is smaller. 

 Also 



^^wCXo + zJ-I) 



We determine for I, and therefore i, in terms of X^, 

 by the graphical method ; but it will bring out the 

 main points of the theory if I confine myself to inter- 

 sections along the straight part of the curve, where 



Nm^Xo + M) 



I = Nw~ 



N;;i\ 



3 



I 



RT 



RT 



3RT 



N;«2 



SK'lo 



^'^ = 1. 



Thus o-,the specific conductivity of the metal, 



Or if we take a temperature Tq, such that 

 I •= ^ -^0^0 



.1 X.T^pe 

 ~k T-To 



\ dfoeTn . 

 k T- 1„ 



Thus the conductivity becomes infinite, or the resist- 

 ance vanishes when T = Tj. When we take the 

 accurate solution instead of the approximate one, we 

 find that the conductivity, though very great, is not 

 infinite; the curve i bends round and meets the 

 straight line, and when the intersection takes place 

 in the curved portion the current no longer follows 

 Ohm's law. We thus see that in the neighbourhood 

 of a certain temperature the resistance diminishes 

 with great rapidity, a point brought into great pro- 

 minence by Kamerlingh Onnes. 



If we compare the external force with the force fel, 

 due to the atoms, we see from these equations that 

 feI/X„ = T„(T-T„); so that near the critical tempera- 

 ture the force due to these atoms is enormously 

 greater than the external force, so that the chains are 

 held mainly by the forces between the atoms, and 

 we can easily see by the graphical method of solution 

 that when the temperature is below the critical tem- 

 perature we can withdraw the electric force and still 

 leave the greater part of the chains intact. 



Super-Conductivity. 

 Let us now consider what happens when the tem- 

 perature is diminished; the slope of the line (i) con- 

 tinually decreases and the intersection of this line 

 with the curve gets further and further away from 

 the origin ; when the intersection comes on a part of 

 the curve at an appreciable distance from the tan- 

 gent at the origin. Ohm's law will no longer hold. 

 Suppose that the slope of the line (i) has fallen so 

 that, as in Fig. 2, it is less than that of the tangent 

 at the origin to the curve I = NMF (x). and after the 

 application of a force X^ suppose the force is gradu- 

 ally removed, the value of I corresponding to the 

 diminished force will be got by drawing parallels to 

 PQ continually getting nearer to the origin, and its 

 value when the force has been entirelv removed bv 

 drawing a parallel through the origin itself. We 

 see from the figure that in this case the line through 

 the origin will intersect the curve again at S, showing 

 that I retains the finite value SN after the electric 

 NO. 2409, VOL. 96] 



force has disappeared. From the point of view of 

 this paper, however, the part played by the electric 

 force /in metallic conduction is to polarise the metal, 

 i.e., to form chains : when once these are formed the 

 electricity is transmitted along them by the forces 

 exerted by the atoms on the electrons in their neigh- 

 bours. Thus if the polarisation remains after the 

 electric force is removed the current will remain too, 

 just as it did in Kamerlingh Onnes' experiment with 

 the lead ring. The argument is similar to that by 

 which Weiss explained the existence of permanent 

 magnetism below a critical temperature. 



The remarkable results obtained by Kamerlingh 

 Onnes only occur at the temperature of liquid helium ; 

 at the temperature of liquid hydrogen the metals show 

 no sign of super-conductivity, the discovery of which 

 is thus a result of being able to lower the lowest avail- 

 able temperature a few degrees ; it is a very striking 

 instance of the truth of Browning's line : 



The little more and how much it is. 



UNIVERSITY AND EDUCATIONAL 



INTELLIGENCE. 



Cambridge. — The General Board of Studies, in its 

 annual report to the Senate, reviews the work done 

 and progress made in the experimental departments of 

 the University during the academic year, 19 14-15. A 

 large proportion of the scientific staff and some forty 

 of the laboratory assistants are absent on military 

 service ; this, together with the enlistment of most of 

 the students of military age, has led to a diminution 

 of the usual output of original experimental investiga- 

 tion. A large amount of experimental work has been 

 done gratuitously for a number of Government depart- 

 ments in connection with problems arising out of the 

 war, and certain of the laboratories have been able to 

 offer such facilities to professors of Belgian universi- 

 ties as have enabled them to continue the instruction 

 of their pupils in this country. 



The School of Forestry has been greatly assisted by 

 Lord Cowdray, who has offered ample facilities for 

 the practical study of forestry in his woodlands, which 

 cover some seven square miles of countrv in Sussex. 

 The research hospital is being used for tUe treatment 

 of wounded officers under the control of the .\rmy 

 Council, and some 300 patients have been already 

 received. The pathological museum has been enriched 

 by a collection of specimens illustrating the effect of 

 gunshot and other wounds on the bones and soft 

 tissues, the material having been provided by the 

 ist Eastern General Hospital. 



The annual meetings of the Geographical Associa- 

 tion will be held on January 6 and 7, at LTniversity 

 College, London. On the morning of January 6, the 

 presidential address will be delivered by Mr! H. J. 

 Mackinder, and afterwards a discussion on "The First 

 Steps in Geography Teaching " will be opened by Miss 

 E. G. R. Taylor. In the afternoon Dr. Marion N'ew- 

 bitrin will lecture on "The Geographical Study of 

 Rivers," and the lecture will be fellowed by a discus- 

 sion on "The Use of Home-made .Apparatus," which 

 will be opened by Mr. E. J. Orford. On the morning 

 of January 7 a joint meeting of the Geographical and 

 Historical .Associations will be held, to discuss "The 

 Relations of Geographical and Historical Teaching in 

 Schools." Mr. H. J. Mackinder and Prof. Ramsay 

 Muir will open the discussion. 



Practically the whole of our engineering colleges 

 have now overcome the earTv difficulties and opposi- 

 tion which attended the starting of making munitions 

 and otherwise rendering assistance in the present 



