A NEW CURRENT WEIGHER, ETC. 465 



Committee of the British Association, and was wound by CLERK MAXWK'I.I.. LATIMKR CI.ARK also used 

 a sine galvanometer for his E.M.F. measurements, and arrived at the values 1-4573 and 1 -4562 B.A. voltt 

 at 15-5'C. with the two methods respectively. 



In 1873 F. KOHI.RAUHCH* employed the tangent galvanometer and magnetometer in determining the 

 electrochemical equivalent of silver, which he found to be 1 1363 milligrammes per coulomb. 



MAscART.t in 1882, constructed his current weigher formed of a long solenoid hung from a balance 

 arm, with its lower end in the mean plane of a large circular coil, and published the number 1*124 milli- 

 grammes as the mass of silver deposited by one coulomb. This was corrected in 1884 to 1 1 15G.J 



At the British Association Meeting in 1882 Lord RAYI.KIGH discussed the several methods of 

 measuring current absolutely which had IK-CM employed by previous experimenters, more especially those 

 used by KOHI.RAUSCH and by MASCART. He pointed out that a large part of MASCART'S long solenoid 

 was comparatively ineffective, and considered that the moving coil should be compact and situated near 

 the position of maximum effect. A further advantage would, he pointed out, be gained by duplicating the 

 fixed coil, thus making the arrangement symmetrical and doubling the force. The dimensions of (current)* 

 in the electromagnetic system lieing the same as those of force, Lord KAYLKICH showed that the constant 

 of a current weigher arranged as described above, must be a numeric, depending on the mean radii of the 

 coils as a ratio, which could be determined electrically with high precision without any linear measurements 

 whatever having to be made. 



In 1883 Lord RAYI.KIGH published the result that he had obtained with a current weigher thus 

 constructed, viz., 1-119 milligrammes of silver per coulomb. Meanwhile F. and W. KOHI.RAUSCH had 

 carried out measurements of high precision with the tangent-galvanometer and suspended-coil method, 

 obtaining the values 1-11833 and 1-11822 respectively in 1881 and 1883, although these results were 

 not published until later.f 



In a classical memoir** Lord RAYI.KIGH and Mrs. SIDGWICK showed that the number given by Lord 

 RAYI.KICH in 1883, viz., 1-119, was too high by nearly 1 in 1000, owing to inclusion of mother liquor 

 with the silver. This was due chiefly to the solution being filtered through silver acetate to secure firmer 

 deposits. With pure silver nitrate they found the equivalent to be 1 -11794, the greatest difference from 

 the mean of thirteen experiments being less than 1 part in 2500. The paper contains a full description 

 of the current weigher, the method of using it, the calculation of the force between the coils, and 

 a table of numbers for facilitating the making of these calculations by elliptic integrals. Also a very 

 careful determination of the E.M.F. of a number of CLARK cells is given. It is important to notice that 

 no measurements of length, moment of inertia, or time are necessary in determining current with a 

 current weigher made on Lord RAYLEIUH'S plan, and this constitutes one of its great advantages. 



THOMAS GRAY, in 1886,tt determined the electrochemical equivalent of silver by means of a sine 

 galvanometer of his own design, and in 1887 KoKPSELjt used an electromagnetic balance of most ingenious 

 construction, made according to VON HKLMHOLTZ'S instructions, for the same purpose. The results 

 obtained, although approximating closely to those of F. and W. KOHI.RAUSCH, and of Lord RAYLKIOH and 

 Mrs. SIDGWICK respectively, are not so trustworthy. 



* 'Pooo. Ann.,' 149, S. 170, 1873. 



t 'Jour, de Phys.,' [2], t I., p. 109, 1882. 



t ' Jour, de Phys.,' t. III., p. 283, 1884. 



'B.A. Report,' p. 445, 1882. 



|| ' Proc. Cambridge Philosophical Society, 1 vol. V., p. 50. 



IT Site, der Phys.-Med. Ges. ru Wiirzburg,' 1884 ; also WIKD. Ann.,' 27, p. 1, 1886. 

 ** 'Phil. Trans.,' 175, p. 411, 1884. 

 ft ' 1'hil. Mag.,' 22, p. 339, 1886. 

 U ' WIED. Ann.,' 31, p. 250, 1887. 

 VOL. CCVII. A. 3 O 



