464 PROFESSOR W. E. AYRTON, MR. T. MATHER AND MR. F. K. SMITH : 



HISTORICAL NOTES ON THE ABSOLUTE MEASUREMENT OF CURRENT. 



A CURRENT can be measured absolutely in the electromagnetic system of units either by moans of the 

 action of the current on a magnet, or of the current on a current. The former method has the 

 disadvantage that at least two independent measurements are necessary. For example, in using an 

 electro-magnetic balance, the strength of the magnet acted on by the electric circuit has to be determined, 

 as well as the force exerted on the magnet by the circuit. In galvanometers, either of the sine or tangent 

 type, the magnetic field produced by the electric circuit is compared with the earth's horizontal field, the 

 strength of which is determined independently. Further, as the strength of artificial magnets cannot be 

 regarded as truly constant, and the earth's field is subject to diurnal and secular variations, this class of 

 measurement is not ideal. 



In the electrodynamic class of measurement the mutual action between two or more coils carrying 

 current takes the form of a torque, as in electrodynamometers, or a direct force, as in current weighers. 

 In electrodynamometers the torque may be measured with a bifilar suspension, the torsion of a wire or 

 spring, or by means of a gravity balance. Current weigher measurements are almost always made by 

 direct comparison with gravity, which is believed to be constant, and is known to a higher degree of 

 accuracy than the strengths of any magnet or magnetic field that has yet been measured. 



Shortly after the absolute system of units was devised by GAUSS and WEBER in 1832, A. BKCQUEUEL* 

 weighed the attraction between a coil and a magnet ; and two years later LENZ and JACOBI t used and 

 modified BECQUEREL'S balance by arranging a coil and magnet at each end of the beam. In 1840 

 W. WEBER determined the electrochemical equivalent of water, using the tangent galvanometer as his 

 instrument for measuring current; and in 1843 similar measurements were made by BUNSEN and by 

 CASSELMANN, followed in 1851 by JOULE. 



Meanwhile W. WEBER| had, in 1846, invented his two forms of electrodynamometer, one with the 

 suspended coil inside, and the other with this coil outside the fixed coil, and he measured the torque with 

 bifilar and unifilar suspensions. 



The first current weigher appears to have been constructed by CAZIN in 1863. This consisted of two 

 rectangular coils with their planes horizontal, one hanging from the beam of a balance directly alx>ve the 

 other, which was supported on an adjustable table. The instrument was used for determining the electro- 

 chemical equivalent of water. 



In 1864 JOULE |i made a current weigher having three circular flat coils wound with copper strip, one 

 being suspended from a balance, so that its mean plane, which was horizontal, was midway between those 

 of the other two fixed coils. This instrument had the correction to its principal constant determined by 

 comparison with a standard tangent galvanometer, and was employed in JOULE'S electrical determination 

 of the mechanical equivalent of heat. Its object was to enable a constant current to be maintained through 

 the calorimeter, independent of variations in the earth's magnetic field. 



LATIMER CLARK,H in determining the E.M.F. of his standard cell in 1872, used a bifilar olectro- 

 dynamometer with circular fixed and moving coils, each arranged in the Helmholtz fashion. The fixed 

 coils were of large size relative to the suspended ones, a fact which considerably simplified the calculation 

 of the torque per unit current. The instrument had been constructed for the Electrical Standards 



* 'Comp. Rend.,' vol. V., p. 35, 1837. 



t ' POGG. Ann.,' XL VII., p. 227, 1839. 



J 'Electrody. Mess.,' Vol. I., p. 16, 1846. 



' Ann. de Chim.,' [4], Vol. I., p. 257. 



|| ' B.A. Report,' 1864. 



f Roy. Soc. Proc.,' May 30, 1872 ; also ' Phil. Trans.,' 1874, Part I. 



