PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 157 



APPENDIX III. 



An Ain2Jere Balance. By Professor W. E. Ayrton, F.R.S., and 

 Professor J. Viriamu Jones, F.R.S. 



The Report of the Committee on Electrical Standards for 1897 ended 

 with the following paragraph : — ' It thus appears to be a matter of urgent 

 importance that a redetermination of the electrochemical equivalent of 

 silver should be made and that the general question of the absolute 

 measurement of electric currents should be investigated. . . .' This work 

 we were asked by the Committee to carry out, and a grant of Ibl. was 

 voted in its aid. We were thus led to examine into the methods which 

 liad been employed by Lord Rayleigh, Professor 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 construct, 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, separated by an axial distance 

 of 5 inches. The coils consisted each of a single layer of about 170 convo- 

 lutions of wire and were wound in opposite directions. Fi-om 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 consist- 

 ing of a single 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 con- 

 venient formula, readily expressible in elliptic integrals, for the force, F,. 

 between a uniform cylindrical current sheet and a coaxial helix, viz. : — 



F=yy,(Mi-M,) 



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

 in the helix, and Mi 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 6 3 ampere having been 

 determined absolutely by means of this apparatus, the rate at which it 

 would deposit silver under specified conditions was ascertained 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 true ampere was so nearly equal to the reputed value, viz. 1-118 

 milligramme per second, as to require the use of an apparatus still more 



' See Proceedings of the Boyal Society, vol. 63 : 'On the Calculation of the Coeffi- 

 cient of Mutual Induction of a Circle and a Coaxial Helix, and of the Electro- 

 maprnetic Force between a Coaxial Current and a Uniform Coaxial Circular 

 Cylindrical Current Sheet.' By Professor J. V. Jones. 



