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



In 1890 PELLAT and POTIER* employed an electrodynamometer balance in silver deposit work, which 

 had a short cylindrical coil secured with its axis vertical to one arm of a balance ; this arm projected along 

 the axis of a long horizontal solenoid fixed symmetrically with respect to the moving coil. The torque 

 between the coils was balanced by weights, the magnitude of which gave 1-1192 as the mass of silver 

 deposited per coulomb. 



With a view to simplifying the use of RAYLEIGH'S current weigher, HEYDWEiLi.ERt in 1891 modified 

 the arrangement by placing the coils with their common axis horizontal, the moving coil being carried 

 directly below the centre of the balance beam. Nearly the whole of the force was balanced by weights 

 on the horizontal arm, and the rest determined from the slight displacement of the coil from the vertical 

 position. 



To determine the E.M.F. of CLARK cells in 1896 KAHLE| used a HELMHOLTZ electrodynamometer 

 balance of novel construction, in which the moving coil and balance beam were supported by, and so that 

 they rolled on, thin metal strips which served also as leads. Rectangular coils of many turns embraced the 

 balance case in planes perpendicular to the length of the beam. The constants of these coils, as well as of 

 the suspended one, were determined by comparison of their magnetic effect with that of a large rectangle 

 of copper band stretched round a strong metal frame, the dimensions of which could be accurately 

 measured. The experiments gave the result 1-4322 at 15C. 



In 1897 the late Professor J. VIRIAMU JONES, in collaboration with one of the authors (W. E. A.), 

 devised a current weigher in which the forces could be calculated with great exactness by a formula 

 developed by the former, and a preliminary instrument was constructed with single layers of wire in 

 screw grooves, and described at the British Association Meeting in 1898.|| 



Messrs. PATTERSON and GuTHE,5I working under Professor CARHART, employed a torsion electro- 

 dynamometer with fixed coils on wood and suspended coil on vulcanite, and made determinations of silver 

 deposit (1-1192 milligrammes per coulomb) which they believed accurate to 1 part in 5000. In the 

 following year (1889) CARHART and GUTHE** measured the E.M.F. of CLARK cells with the same 

 instrument, obtaining the value 1-4333 at 15 C., and in 1902 CALLENDARfT published the result (1-4334 

 at 15 C.) got by R. 0. KING with an electrodynamometer of the British Association pattern employed in 

 his (CALLENDAR'S) researches on " Continuous Electric Calorimetry." 



Further determinations of the electrochemical equivalent of silver with PELLAT'S electrodynamometer 

 balance were made in 1903 by PELLAT and LEDUC,JJ who obtained 1-1195 milligrammes per coulomb. In 

 the same year VAN DlJK and KUNST carried out a very careful research in a new laboratory free from 

 iron and vibration, using two tangent galvanometers, magnetometer and variometer, and from the mean of 

 twenty-four closely accordant determinations of the electrochemical equivalent of silver deduced the value 

 1 -11818. This they believed to be accurate to 1 part in 10,000. 



Professors CARHART and PATTERSON || || described, at the meeting of the Electrical Congress at St. Louis 



* 'Jour, de Phys.,' t. VI., p. 175, and t. IX., p. 381, 1890. 



t ' WIED. Ann.,' 44, p. 533, 1891. 



J 'WlED. Ann.,' 59, p. 532, 1896. 



8 'Roy. Soc. Proc.,' vol. 63, p. 204, 1898. 



|| ' B.A. Report,' Bristol, 1898, p. 157 ; also ' Jour. Inst. Elec. Eng.,' vol. 35, p. 12, 1905. 



f 'Phys. Rev.,' VII., p. 257, 1898. 

 ** 'Phys. Rev.,' IX., p. 288, 1899. 

 tt 'Phil. Trans.,' A., 199, p. 81, 1902. 

 it 'Comp. Rend.,' 136, p. 1649, 1903. 



' Verel. van de Gewone Vergadering der Wis- en Natuurkundige Afdeeling,' Dec., 1903. 

 III! 'Jour. Inst. Elec. Eng.,' vol. 34, p. 185, 1905. 



