October 9, 1890] 



NATURE 



577 



athode were found to far exceed in number those collected 

 the anode. 



To give even an outline of all the voluminous and multitudin- 

 ^ contributions to the Section would occupy many pages, and 

 uld require that the writer should have received the training 

 a Succi or a Jacques before undertaking the task. 

 M. Du Bois read a paper on refraction and dispersion in cer- 

 tain metals. Kundt's method of observation with very thin 

 electrolytic metal biprisms was used in this investigation. The 

 dispersion was determined with all possible care, using four 

 kinds of light defined by spectral lines. It was found that light, 

 on passing from iron, cobalt, and nickel into air, begins by fol- 

 lowing Snell's law for small angles of emission, the refractive 

 index being mathematically defined as the limit of the ratio of 

 sines when the angle of incidence approaches the limit zero. 

 The dispersion in the case of each of the three metals mentioned 

 was found to be anomalous. 



Sir William Thomson, F.R.S., in a paper on an illustration 

 of contact electricity presented by the multicellular voltmeter, 

 called attention to the modification of the force between the 

 aluminium needles and the brass cells of the instrument arising 

 from the "contact electricity" difference between polished brass 

 and polished aluminium. In the instrument as at present made, 

 the observed difference of potential on reversal amounts to as 

 much as \ volt. Thus the use of the multicellular electrometer 

 gives a new and very interesting direct proof of Volta's contact 

 electricity. 



Lord Rayleigh, Sec. R.S., read a paper on defective colour- 

 vision, in which he pointed out that the existence of a defect is 

 probably most easily detected, in the first instance, by Holm- 

 gren's wool test ; but this method does not decide whether the 

 vision is truly dichromic. For this purpose. Maxwell's colour- 

 disks may be used. Lord Rayleigh found, in the case of some 

 colour-blind persons he was examining, that it looked as though 

 the third colour-sensation, presumably red, was defective, but 

 not absolutely missing. When a large amount of white was 

 present, matches could be made, in spite of considerable differ- 

 ence in the red component. But when red light was nearly 

 isolated, its distinctive character became apparent. This view 

 was confirmed by experiments with the colour-box. 



Mr. T- Swinburne, in a paper dealing with the question of the 

 production of high vacua, called attention to the great superiority 

 of the Geissler over the Sprengel form of mercury pump. 



Profs. Barr and W. Stroud, in a paper on the use of the 

 lantern in class-room work, described a simple and convenient 

 form of lantern for horizontal and vertical projection, and exhi- 

 bited an apparatus for the preparation of lantern-slides in large 

 numbers from books, periodicals, &c. 



Mr. W. N. Shaw read a paper on the general theory of ventila- 

 tion, with some applications, in which general laws of ventila- 

 tion are established similar to Kirchhoff's laws relating to the 

 distribution of currents in a network of conductors. 



On Friday, September 5, there was a discussion on electrical 

 units, opened by Mr. Glazebrook with a paper on recent deter- 

 minations of the absolute resistance of mercury, in which he 

 carefully compared and criticised the different methods employed 

 by various observers. The best determinations of the ohm 

 showed that it was very nearly indeed equal to the resistance of 

 a column of mercury 106 "3 cm. long and I square millimetre 

 cross-section at o" C. Mr. Glazebrook strongly advocated the 

 adoption of the number 106 "3 instead of 106 ; and Sir William 

 Thomson, Prof. Rowland, Prof Barker, and Mr. Preece ex- 

 pressed their concurrence in the desirability of the change. 



Principal J. V. Jones followed with a paper entitled "Sug- 

 gestions towards a Determination of the Ohm," in which he 

 described the results of experiments undertaken at University 

 College, Cardiff, in the spring of the present year. These 

 experiments gave the ohm equal to the resistance of a column of 

 mercury 106 307 cm. long and I sq. mm. sectional area. The 

 method adopted was a modification of that due to Lorenz, in 

 which a metallic disk is made to rotate in the mean plane of a 

 coaxial standard coil. Wires touching the centre and circum- 

 ference of the disk are led to the ends of the resistance to be 

 measured, and the same current is passed through this resistance 

 and the standard coil. 



The features of special interest in the method employed were : — 

 (a) The employment of a long trough for holding the mercury ; 

 and, instead of measuring the distance between the electrodes, 

 one electrode is kept fixed, while measurement is made of the 

 distance moved through by the other between two positions of 



NO. 1093, "^'01- 42] 



equilibrium of the galvanometer corresponding to two different 

 rates of rotation of the disk. The latter measurement it is easy 

 to make with accuracy, for the movable electrode may be rigidly 

 attached to the movable headstock of a Whitworth measuring- 

 machine placed parallel to the length of the trough ; and the 

 two equilibrium positions may be taken near the middle of the 

 trough, so as to avoid danger of curvature in the equipotential 

 surfaces passing through the electrode in its two positions. A 

 new difficulty is, however, now encountered, viz. the deter- 

 mination of the section of the mercury column. The capillary 

 depression at the sides of the trough would make it a most 

 serious task to determine the section by direct measurements to 

 the required degree of accuracy. This difficulty is overcome by 

 a further differential method, viz. by making observations witti 

 the mercury at two different heights in the trough. The sides of 

 the trough in that part of it traversed by the movable electrode are 

 assumed plane, parallel, and vertical. The trough used in the ex- 

 periments was cut in paraffin wax contained in a strong casting of 

 iron with its sides strengthened by outside ribs. The channel was 

 43 '5 inches long, by i "5 inches broad, by 3 inches deep. Paraffin 

 was found, however, not to be perfectly satisfactory, and Prof. 

 Jones expressed the opinion that a trough of worked glass or 

 scraped marble would have been preferable. The position of 

 the mercury surface in the trough was determined electrically by 

 using a pointed steel spherometer screw. The screw may be 

 moved downwards until an electric circuit comprising the screw 

 and the mercury is completed. (j8) The employment of a 

 brush of special forin to secure good electrical contact at the 

 periphery of the rotating disk. The brush consisted of a single 

 wire perforated by a channel through which a constant flow of 

 mercury might be maintained from a cistern of adjustable height. 

 (7) In connection with the measurements necessary to enable 

 the calculation of the coefficient of mutual induction to be per- 

 formed. Prof. Jones employs a coil consisting of only one layer 

 of wire, the advantage of which is that every part is visible, and 

 that nothing is done to alter the position of the wire after 

 measurements have been made. If a coil consist of many layers, 

 it is not quite easy to say where, after measurement, the lower 

 layers go to under the pressure of the superincumbent ones. 



In conclusion, the main suggestions offered for consideration, 

 were : — 



(i) That the time is ripe for a new determination of the ohm 

 that shall be final for the practical purposes of the electrical 

 engineer. 



(2) That such a determination may be made by the method of 

 Lorenz, the specific resistance of mercury being obtained directly 

 in absolute measure by the differential method described. 



(3) That the standard coil should consist of a single layer of 

 wire, the coefficient of mutual induction being calculated by the 

 formula given in the paper. 



Sir William Thomson, in a paper on alternate currents in^ 

 parallel conductors of homogeneous or heterogeneous substance,, 

 pointed out that when the period of alternation is large in com- 

 parison with 400 times the square of the greatest thickness or 

 diameter of any of the conductors, multiplied by its magnetic 

 permeability and divided by its electric resistivity, the current 

 intensity is distributed through each conductor inversely as the 

 electric resistivity ; the phase of alternation of the current is 

 the same as the phase of the electromotive force ; and the current 

 across every infinitesimal area of the cross-section is calculated, 

 according to the electromotive force at each instant, by simple 

 application of Ohm's law. Further, that when the period is 

 very small compared with 400 times the square of the smallest 

 thickness or diameter of any of the conductors, multiplied by its^ 

 magnetic permeability and divided by its electric resistivity, the 

 current is confined to an exceedingly thin surface-stratum of the 

 conductors. The thickness of this stratum is directly as the 

 square root of the quotient of resistivity, divided by magnetic 

 permeability, of the substance in different parts of the surface. 

 The dependence of the total quantity of electricity carried on 

 extent of surface justifies Snow Harris, and proves that those who 

 condemned him out of Ohm's law were wrong, in respect to his 

 advising tubes or broad plates for lightning conductors, but does 

 not justify him in bringing them down in the interior of a ship 

 (even through the powder magazine) instead of across the deck 

 and down its sides, or from the masts along the rigging and 

 down the sides into the water. 



Sir William Thomson read a paper on anti-effective copper in 

 parallel conductors, or in coiled conductors for alternate currents. 

 It is known that by making the conductors of a circuit too thick 



