IQO 



NA TURE 



[December 21, 189; 



' The Organogeny of Asterina gibbosa." By E. W. Mac- 

 Bride. Communicated by Adam Sedgwick, F. R.S. 



" On Copper Electrolysis in Vacuo." By William Gannon. 



"Note on the Action of Copper Sulphate and Sulphuric 

 Acid on Metallic Copper." By Prof. Arthur Schuster, F.R.S. 



Physical Society, December 8.— Prof. A. W. Riicker, 

 I-'.R.S., President, in the chair.— A paper, by Mr. James Swin- 

 burne, on a potentiometer for alternating currents, was read 

 1-".' Mr. Blakesley. After referring to the many advantages of 

 the ''potentiometer" method of measurement, the author de- 

 scribes an arrangement by which alternating pressures can be 

 measured. A quadrant electrometer with a double fishtail- 

 shaped needle suspended by a torsionless fibre is employed. 

 The electrostatic attraction exerted by an alternating pressure 

 between the needle and one pair of quadrants is balanced by the 

 force due to a steady pressure between the needle and the other 

 pair of quadrants. The magnitude of the steady pressure is 

 determined by a potentiometer and standard cell, and the 

 effective value of the alternating pressure thus deduced. For 

 measuring aliernating currents a differential electrodynamometer 

 having two fixed coils and one moving coil, and no controlling 

 spring, is used. A direct current, measured by the fall of 

 potential over a small resistance, is passed through one of the 

 fixed coils, the alternating current through the other fixed coil, 

 and the moving coil is included in both alternating and direct 

 current circuits. When the two forces balance, the currents are 

 taken a> equal. Several small inaccuracies to which the method 

 is subject aie mentioned in the paper. Prof. S. P. Thompson 

 inquired if the fishtail-shaped needle of the electrometer was 

 novel. Mr. Blakesley said the author had mentioned the needle 

 previously. He (Mr. Blakesley) thought the name "poten- 

 tiometer " was not very suitable. In effect, the so-called 

 measurement o^ pressures was a comparison of ivio powers. —The 

 Presiilent announced that Mr. Preece's note on the specific re- 

 sistance of sea-water had been temporarily withdrawn. — Prof. 

 G. M. Minchin made a communication on the calculation of the 

 coefficient of sell-induction of a circular current of given aper- 

 ture and cross-section. Instead of assuming the cross-section of 

 the wire small, and the current density constant over the section, 

 as is usually done, the author takes into account the dimensions 

 of the section and the non-uniform distribution of the current. 

 Making use of the expressions for the vector potential (G) of the 

 current given in his previous papers {Phil. Mag, April and 

 August, 1893), the author calculates the total normal flux 

 of force through a surface intersected once in the positive 

 direction by every tube of force emanating from the given cur- 

 rent. This flux, divided by the current, gives the coefficient of 

 self-induction. The surface chosen is the circular aperture of 

 the current and half of the anchor ring formed by the wire. 

 When the current density is inversely proportional to ths dis- 

 tance from the axis of the circular current, the value of the co- 

 efficient of self-induction is found to be 



'{ 



4a(L- 2)-f-2^ 



\ 4/ 16 



,j2L-fi9)}, 



where a is the radius of the central filament of the current, 



c the radius of the cross-section of the wire, and Li? = log — 



c 

 Clerk-Maxwell's approximate expression agrees with this in 

 the principal term. As an example of the closeness of the 

 approximation, the case of a current in a wire 2 millimetres 

 diameter bent to a circle of 2 centimetres mean diameter bad 

 been taken, the approximate and corrected coefficients being 

 58"866 and 59'207 absolu'e units respectively. When the cur- 

 rent m the wne is superficial, as in ca>e of alternating currents 

 of high frequency, the Coefficient is somewhat greater, being 

 given by the expression 



ir|4a(L-2)-t-2/L-f^V-^^ (4L-f ii)j. 



Incidentally it was pointed out that the function G.r where G is 

 the vector potential at a point distance x from the axis of a 

 circular current was the same as Stoke's current function in 

 hydrod>namics. Another paper, on the magnetic field of 

 a currtnt running in a cylindrical coil, was read by Prof. 

 Minchin. The cylindrical coil is regarded as a series of equal 

 circ e^ lying close together and forming a cylindrical surface. 

 Replacitig each circular current by it.s equivalent magnetic shell, 

 the prol-lcm of nnding the magnetic potential at a point resolves 



No. 1260. VOL. 49] 



itself into calculating the gravitational potential due to two 

 circular plates of attracting matter, one positive and the other 

 negative, situated respectively at opposite ends of the cylinder. 

 The magnetic potential due to one plate is then deduced in 

 terms of elliptic integrals of the first, second, and third kinds. 

 The President had pointed out that the expressions given in 

 the printed proof of the paper, only applied when the perpen- 

 dicular from the point to the plate fell within the circle ; the 

 author had therefore modified the formula so as to be true 

 generally. From this formula the equipotential curves can be 

 constructed. The same system of curves serve for the plate 

 at the other end of the cylinder by changing the signs of the 

 numerals representing the potentials and giving the curves a 

 motion of translation equal to the length of the cylinder in the 

 direction of its axis. The equipotential curves for the coil can 

 then be deduced by drawing through the points of intersection 

 of the two sets of curves whose numerical values have a constant 

 sum. In determining the curves the author had to calculate 

 tables of elliptic integrals of the third kind, and these he hoped 

 to complete before the paper was published. In reply to a 

 question on the first paper, which had been brought before him 

 by Prof. Perry, the author said that as the diameter of the wire 

 diminished indefinitely, both the self-induction and resistance 

 became infinite, but the ratio L/R became zero. It was inter- 

 esting to examine what relation betwen the aperture and cross- 

 section gave minimum impedance. If the ordinary expression 

 for it be taken the problem was impossible, but the corrected 

 form admitted of a solution. Prof. Perry hoped the work Prof. 

 Minchin had done so well for circles and cylinders would be 

 extended to cylindrical coils of rectangular cross-section. It 

 was most important to be able to find the shape of the field 

 produced by such coils. Prof. S. P. Thompson inquired if 

 there was any way of deducing the expression for the magnetic 

 force at a point other than that given in the paper on the 

 magnetic field of a circular current {Phil. Mag. April, 1893). 

 In reply Prof. Minchin explained how the formula followed at 

 once from the fundamental theorem that magnetic force is the 

 curl of the vector potential. This was based on Laplace's 

 expression for the force between a magnetic pole and an 

 element of current which had been proved experimentally. 



Zoological Society, December 5. — W. T. Blanford, 

 F.R.S., Vice-President, in the chair. — The secretary read a 

 report on the additions that had been made to the Society's 

 menagerie during the month of November. Among these 

 special attention was called to a Cunning Bassaris {Bassaris 

 asttila), obtained by purchase, to two Jerboas presented by Capt. 

 R. A. Ogilby, and to a fine adult female of the Caucasian Wild 

 Goat {Capra caucasica), presented by H. H. P. Deasy. — Prof. 

 G. B. Howes exhibited and made remarks on some specimens of 

 abnormal Marsipobranch Fishes. These were two heads of the 

 Lamprey with the first pair of gills only imperfectly developed, 

 and a Hag {Myxine glutinosa) with a supernumerary gill on one 

 side. — Mr. F. E. Beddard, F. R. S. , gave an account of the 

 general geographical distribution of Earthworms^ as treated of 

 in a work on the subject which he had in preparation. Mr. 

 Beddard recognised sixty-nine genera of this order, divided into 

 six families ; and after some preliminary remarks on the artificial 

 introduction of earthworms into districts colonised from Europe, 

 called attention to a series of tables in which the genera found 

 in the six generally recognised regions of the earth's surface were 

 shown. In addition to these six regions Mr. Beddard was dis- 

 posed to recognise, in the case of earthworms, the existence of 

 an Antarctic region, to embrace New Zealand and most of the 

 Antarctic Islands. — A communication was read from Mr. C. J. 

 Gahan, containing an account of a collection of Coleopiera sent 

 by Mr. H. H. Johnston, C. B., from British Central Africa. 

 Amongst these were examples of eight species new to science. 

 — A communication was read from Capt. F. W. Hutton, 



F. R.S., containing a report on a collection of Petrels from 

 the Kermadec Islands. Amongst them was an example of a 

 new species proposed to be called CEstrelata leucophrys. — Mr, 



G. A. Boulenger gave an account of Vipera rcnardi, a newly 

 recognised European Viper from Southern Russia and I urkestan. 



Entomological Society, December 6. — Henry John Elwe.s 

 President, in the chair. — Mr. W. F. Kirby exhibiied, for Dr. 

 Livelt, specimens of a moth taken at Wells, which Dr. Livett 

 considered to be varieties of Dasycampa rubiginea, but which 

 many entomologists present thought were varieties of Cerastts 

 vaccina. Mr. Kirby stated that specimens similar in appearance 



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