456 
PEOFESSOE W. WATSON ON A DETEEMINATION OF THE VALUE OF 
The-coils are so constructed that they can be turned through 180° with reference 
to the magnetometer on which they are carried. Observations 1 to 17 and 46 to 63 
were made with the coils in one position, and observations 18 to 45 with the coils 
reversed. The mean values for the difference in the two jDositions are 0'000002 
and 0'000013, which agree to nearly 1 part in 20,000. 
Since the values of H, given in column 10 of the above Table, are obtained by 
taking the electro-chemical equivalent of silver as 0'001118, they correspond to 
international units. Thus the measurements lead to the result, that the absolute 
measurements of the earth’s field made at Kew Observatory with the standard 
magnetic instruments, give a value O'OOOOOf C.G.S. unit lower than the value of this 
field, measured in international units.^ 
The mean value for the difference when the 6 centims. long magnet was used in 
the Helmholtz galvanometer is 0'000008, while the mean value obtained with the 
3 centims. needle is 0'000006. It is thus evident that the two needles give the same 
value, and hence the neglecting of the length of the needle in the expression used to 
reduce the observations is justified. 
It will be seen that the value of H olDtained with the Helmholtz galvanometer 
agrees, within the limits of errors of experiment, with the value given by the Kew 
Observatory standard instruments. These measurements therefore afford evidence of 
the accuracy of the Kew instruments, as against the values given by many instruments 
of a similar type which have been compared with those of the Observatory. Since, 
however, the accuracy of the values given by the galvanometer dej^ends on the accuracy 
with which we know the value of the electro-chemical equivalent of silver, it is 
necessary to discuss the measurements which have been made of this quantity.! 
The various values which have been obtained for the electro-chemical equivalent of 
silver by different observers are given in the following table ;— 
1. Mascart (1882). 0-0011156. ‘ J. de Phys.,’ ii, 1, 109, 1882, and ii, 3, 283, 1884. 
2. Kohlrausch, F. and W. (1884) 0-0011183. ‘ Wied. Ann.,’ 27, 1, 1886. 
3. Eayleigh (1884). 0-0011179.+ ‘Phil. Trans.,’ 411, Part ii, 1884. 
4. POTIER and Pellat (1890) . . 0-0011192. ‘ J. de Phys.,’ ii, 9, 381, 1890. 
5. Patterson and Guthe (1898) . 0-0011192. ‘Phys. Eev.,’ 7, 257, 1898. 
6. Kahle (1899). 0-0011183. ‘Wied. Ann.,’ 59, 532, 1896. 
* Owing to a mistake in the corrections originally applied to the weights used in the silver depositions, 
the numbers given in the al)stract, which appeared in the Proceedings of the Eoyal Society, vol. 69, p. 1, 
recpiire correction. January, 1902. 
t By an international ampere is meant the current which, when passed through a solution of silver 
nitrate in water prepared in accordance Avith a certain specification, deposits sih'er at the rate of 
0-001118 gramme per second. The international ohm is the resistance of a column of mercury at the 
temperature of melting ice, the mass of which is 14-4521 grammes, the cross-section constant and the length 
106-3 centims. The international volt is the E.M.F., Avhich, applied at the ends of a conductor of resist¬ 
ance one international ohm, produces a current of one international ampere. 
I The Aveights of sih-er used in obtaining this result are those after the silver had been heated to 
incipient redness. If the AA^eights after heating to 160° C. are taken, the value 0-0011181 is obtained. 
