452 



NATURE 



[[April 6, 1876 



of his balance is not excessive, and that in practice it will 

 safely indicate the millionth of a grain. 



One observation of the weight of sunlight is given ; it 

 was taken on December 13 ; but the sun was so obscured 

 by thin clouds and haze that it was only equal to io"2 

 candles 6 inches off. Calculating from this datum, it 

 is seen that the pressure of sunshine is 2*3 tons per square 

 mile. 



The author promises further observations with this in- 

 strument, not only in photometry and in the repulsion 

 caused by radiation, but in other branches of science in 

 which the possession of a balance of such incredible deli- 

 cacy is likely to furnish valuable results. 



SCIENCE IN GERMANY 

 {From a German Correspondejit.) 



A FEW years ago Edlund attempted to decide the 

 question whether the galvanic current is capable of 

 directly altering the volume of a conductor through which 

 it flows, or not, i.e., whether changes of volume were 

 demonstrable that were independent of the heat produced 

 in the conducting wire ? The results of his experiments 

 appeared to furnish an affirmative answer to this question. 

 More recently, Streintz published an investigation, the 

 result of which was a confirmation of Edlund's view on 

 the expanding power of the galvanic current. The ex- 

 panding action found by Edlund was from 2"8 to 6'5 per 

 cent, of the action of the heat simultaneously produced ; 

 that found by Streintz was considerably greater. In soft 

 iron it amounted to 27 per cent, of the action of the heat. 



From the fundamental importance which attaches to 

 this question, in relation to the theory of galvanism, and 

 from the difficulty of demonstrating the volume-changes 

 referred to, apart from the actions of the heat simulta- 

 neously produced, it was desirable that the subject should 

 be investigated by a method as free from error as pos- 

 sible. Such an investigation has lately been carried out 

 by Herr Exner, in Vienna. The essential points of his 

 method are as follows : — 



Two pieces, a and b, of the same wire, about equally 

 long, were suspended vertically one over the other, as 

 indicated schematically in the figure. The lower piece, 

 b, passed centrally through a glass tube, c, which was 

 quite open above, but closed below with a cork, which 

 merely gave passage to the wire b hy a. short glass tube 



(2mm. wide) inserted in it. From the lower end of the wire 

 b hung the plate d for holding weights. This was fur- 

 nished at its base with a sharp iron point, meant to act 

 on one arm of a lever which could be turned about e, 

 while the other arm bore the mirror / at right-angles to 

 its axis, and so in a vertical plane. If the image of a 

 vertical scale were observed in this mirror with a tele- 

 scope, t' e least change in length of the wires a and b 

 could iL^-cby be perceived. From the point of con- 



nection between a and b a wire was connected with the 

 battery ^. The other pole of the battery was connected 

 with the commutator //, and thus the current could be sent 

 either through the rheostat i to the suspending point of the 

 wire a and through the lalter back to the battery, or on the 

 other side to the mercury cup k, in which is dipped the 

 bent end of a short copper wire soldered to the plate, d, 

 establishing thus a conductive connection between the 

 commutator h and the wire b. Through the latter the 

 current then went back to the battery. One could thus 

 easily send the current successively through each of the 

 two wire-pieces, a and b, separately, and so observe the 

 elongation experienced by each. Since, as has been 

 said, the two pieces a and b were not exactly equal in 

 length, their elongations were also not exactly equal ; to 

 make them equal, the rheostat i was inserted, by which 

 the resistance in the circuit ghi ag was so regulated, that 

 with unchanged battery the successively observed elon- 

 gations of a and b were the same. Water was now allowed 

 to pass through the glass tube c, in order to take away as 

 much as possible of the heat produced in the wire b by 

 the galvanic current. If, now, the current passed through 

 b, only the elongation which might occur independently 

 of the heat action of the current would be observed, the 

 heat produced being removed by the flowing water.^ If, 

 however, the current passed through a, both an elongation 

 produced in a through direct action of the current, and the 

 elongation through action of heat would be observed at 

 the same time. [These experiments might of course also 

 be made with only one piece of wire, e.g. b. The second 

 piece a, serves only for making the obvervations more 

 quickly in succession.] 



It was found that the galvanic expansion expressed in 

 percentage of the heat-expansion was only about i'2 to 

 2 '2 per cent. ; and no connection was recognisable with 

 the nature of the metal employed. If it be considered 

 that these values, of course, can only be an upper limit, it 

 will follow from the smallness of the effect obtained that 

 there is no sufficient ground for the hypothesis of a 

 special expansion-power of the galvanic current. There 

 can hardly be any doubt that the slight expansion which 

 the water- inclosed wire still shows is simply and alone 

 due to the heat remaining in it. W. 



THE INTERNATIONAL METRIC COMMIS- 

 SION AT PARIS 



IN previous numbers of Nature" some information 

 has been given of the proceedings of the International 

 Metric Commission of Paris, and of the progress of their 

 work in providing new international standards of the 

 metric system. The construction of the new standard 

 metres and kilogrammes of platinum-iridium, which was 

 entrusted to the French section of the Commission, is 

 now approaching completion, and th^ir comparisons with 

 the old standards of the Archives and with each other will 

 probably be commenced early this spring. 



It has been already explained that the definitive verifi- 

 cation of the new standards was entrusted by the Com- 

 mission to a permanent committee of twelve of their 

 members, each representing one of the principal civilised 

 countries interested. For the purposes of providing the 

 committee with the necessary means of exercising their 

 duties, and of giving an authoritative international 

 character to the new standards, and to the regulations to 

 be adopted for the custody and use of the new inter- 

 national metric prototypes, a diplomatic conference was 

 held at Paris in March 1875, when a convention was 

 entered into for effecting these objects. 



Papers relating to the meeting and proceedings of this 

 diplomatic conference, drawn up by Mr. Chisholm, the 

 Warden of the Standards, who was the representative of 



' It may happen that the heat of the wire is not entirely carried off ky the 

 flowing water. 



' Vol. vii. pp. 197 and 237; vol. viii. p. 403 ; vol. x. p. 130. 



