December i, 1898] 



NATURE 



115 



AN AMPERE BALANCE} 



nrilE Report of the Committee on Electrical Standards for 

 ■*• 1S97 ended willi the following paragrapli : — "It thus 

 appears to be a matter of urgent importance that a redetermin- 

 ation of the electro-chemical equivalent of silver should be 

 ■made, and that the general question of the absolute measure- 

 imenl of electric currents should be investigated. . . ." This 

 work we were asked by the Committee to carry out, and a grant 

 ■of 75/. was voted in its aid. We were thus led to examine into 

 •the methods which had been employed by Lord Rayleigh, Prof. 

 Mascart and others, for determining the absolute value of a 

 current, as well as to consider some other methods which have 

 not, as far as we know, been hitherto used. 



After much consideration we decided to adopt a form of 

 apparatus which, while generally resembling the type employed 

 by some previous experimenters, possessed certain important 

 differences ; and, before expending any part of the grant of 75/., 

 to con'^truct, without expense to the British Association, the 

 following preliminary ampere balance. 



On a vertical cylinder about 17 inches high and 6 '8 inches in 

 diameter we wound two coils, about 5 inches in height, separ- 

 ated by an axial distance of 5 inches. The coils consisted each 

 of a single\2t.^^x of about 170 convolutions of wire, and were 

 wound in opposite directions. From the beam of a balance 

 there was suspended, inside this cylinder, a light bobbin about 

 4 inches in diameter, on which was wound a coil about 10 

 inches long consisting of a sitigle layer of 360 convolutions, and 

 the whole apparatus was so adjusted that when the beam of the 

 balance was horizontal the inner and outer coils were coaxial, 

 and the top and bottom of the inner suspended coil were 

 respectively in the mean planes of the outer stationary coils. 



This arrangement was adopted because with coils consisting 

 of only one layer the geometrical dimensions could be accurately 

 determined, and because the shapes of the coils lent themselves 

 to the use of the convenient formula, readily expressible in 

 elliptic integrals, for the force, F, between a uniform cylindrical 

 current .sheet and a coaxial helix, viz. : — 



F = 77/, (Mj - M,) 



where 7 is the current per unit length of the current sheet, 

 •yit the current in the helix, and Mj and Mj the coefficients of 

 mutual induction of the helix and the circular ends of the 

 current sheet. - 



The value of a particular current of about o'63 ampere having 

 been determined absolutely by means of this apparatus, the rate 

 at which it would deposit silver under specified conditions was 

 a.scerlained indirectly, by observing its silver value on a Kelvin 

 balance which had been kept screwed down in a fixed position 

 for several years past, and which had been calibrated many 

 times during that period by reference to the silver voltameter. 



The result of this preliminary investigation showed that the 

 silver value of the tri<e ampere was so nearly equal to the re- 

 puted value, viz. I '1 18 milligramme per second, as to require 

 the use of an apparatus still more perfectly constructed, and 

 therefore of a much more expensive character, to enable the 

 error, if any, in this value to be ascertained with accuracy. 



We, therefore, started on the design of the instrument, of 

 which we now submit the working drawings, and for the future 

 construction of which we would ask for a grant of 300/., in- 

 cluding the unexpended grant of 75/. voted last year.^ And we 

 anticipate that this new piece of apparatus may prove worthy of 

 constituting a national ampere balance, the counterpoise weight 

 for which will be determined purely by calculation based on the 

 dimensions of the instrument, the number of convolutions of 

 wire in the three coils, and the value of the acceleration of 

 gravity at the place where the instrument may be permanently 

 set up. In this particular it will differ entirely from the 

 " Board of Trade Ampere Standard Verified, 1894," which has 

 had its counterpoise weight adjusted so that the beam is hori- 

 zontal when a current passes through the instrument, which 

 will deposit exactly fii8 milligramme of silver per second 

 under specified conditions. In fact, the proposed ampere 

 balance and the existing ampere standard will differ exactly in 



1 Ry Prof. W. E. Ayrton, F.R S., and Prof. J Vinamu Jones, F.R.S. 

 (Read before Section A of ihe British Association, Bristol.) 



- Proceedings of the Royal Society, vol. Ixiii., "On the Calculation of 

 the Coefficient of Mutual Induction of a Circle and a Coa.xial Helix, and of 

 the Electro-magnetic Force between a Helical Current and a Uniform 

 Coaxial CircuLir Cylindrical Current Sheet." By Prof. J. V. Jones. 



3 This grant of 300/. has since been made. 



the same way as do a Lorenz apparatus and the " Board of 

 Trade Ohm Standard Verified, 1894." 



We have to express our thanks to Mr. Mather for taking 

 charge of the construction and use of the preliminary apparatus, 

 for checking all the calculations in connection with the deter- 

 mination of the electro-chemical equivalent of silver that was 

 made with it, as well as for superintending the making of the 

 working drawings of the new ampere balance. We have also 

 to thank Messrs. W. H. Derriman and W. N. Wilson, two of 

 the students of the City and Guilds Central Technical College, 

 for their cordial assistance in carrying out the work. 



NO. I 5 18, VOL. 59] 



GEOLOGY OF BIRMINGHAM. 



(~)NE of the most important geological memoirs issued of late 

 ^^^ years is the " Sketch of the Geology of the Birmingham 

 District," by Prof. Lapworth, F. R.S., with contributions by 

 Prof. W. W. Watts and Mr. W. Jerome Harrison : a companion 

 work to that on the " Geology of South Shropshire," by Profs. 

 Lapworth and Watts, issued four years ago. The present work, 

 like the one just mentioned, was prepared with special reference 

 to the areas to be visited by the Geologists' Association during 

 their long summer excursion. It is not merely a lucid summary 

 of the facts already made known ; it contains the latest results of 

 the work done by the author and his associates. The " Birming- 

 ham district" is admittedly a large one, being the region within a 

 radius of about thirty-five miles from the city. Thus we find 

 references to the Archaean or Pre-Cambrian rocks of Malvern 

 and the Abberley Hills, of the Wrekin and Lickey Hills, of the 

 Caklecote district and Charnwood Forest. It is stated that the 

 Charnwood or " Charnian Rocks" are theoretically paralleled 

 with the Lower Longniyndian and its volcanic equivalents, and 

 the Caldecote rocks, together with the Barnt Green rocks of 

 the Lickey, are grouped with the Upper Longniyndian and 

 Uriconian. 



In the Cambrian areas order is established by comparisons 

 of the quartzites of the Wrekin, Hartshill, and the Lickey. 

 The Hartzhill quartzite is shown to be composed of three 

 main divisions, the upper one containing a band o{ Hyolithus- 

 limestone, the fauna of which answers in part to the 

 Olenellus-zone of other regions. Hence this upper or 

 "Camp Hill quartzite" of Hartshill is compared with the 

 Comley or Hollybush Sandstone of the .Shropshire and 

 Malvern successions. The Stockingford shales, which overlie 

 the Hartshill beds, are divided into three groups which repre- 

 sent the Paradoxides or Menevian zone and portions of the 

 Lingula flags. Comparisons are then made between the War- 

 wickshire strata and those in the north-west of Scotland, the 

 place of the argillaceous Stockingford shales being there taken 

 by the Durness Limestone group. 



The Silurian strata (Llandovery to Ludlow), and the Carbon- 

 iferous system are fairly well known, and the leading facts are 

 pointed out. With reference to the Permian rocks it is observed 

 that, as a general rule, they follow conformably upon the 

 Upper Coal Measures of the district. The origin of the Permian 

 breccias is discussed, and the opinion of Mr. W. Wickham 

 King is quoted to the effect that they are largely torrential 

 deposits formed more or less of scree and talus, swept down in 

 flood times from the sides of steep hill-slopes near at hand. 

 The similar views of Mr. Horace T. Brown respecting these 

 strata in the country near Burton-on-Trent might have been 

 mentioned. 



.•\ useful account is given of the Triassic strata which occupy 

 so large a portion of the Birmingham district, and this is fol- 

 lowed by a brief notice of the Rhaetic beds and Lias. The 

 petrology of the Birmingham district is dealt with by Prof. 

 Watts, and the glacial drilts are described by Mr. Harrison. In 

 conclusion there is a summary of the history of geological 

 research among the rocks of the district. 



The entire work is full of valuable information and sugges- 

 tions, the stratigraphical facts being clearly stated and supported 

 by pal^eontological evidence where that is forthcoming. Hence 

 for a long time to come, this memoir, which is well illustrated 

 with sections and pictorial views, will be the standard work of 

 reference on the area of which it treats ; and the Geologists' 

 Association may be heartily congratulated on having received so 

 important an addition to its Proceedings. Of this publication it 

 constitutes the whole of part 9 of volume .\v., and it is issued 

 to the public at the price of is. dd. 



