4G0 
PKOFESSOR W. WATSOX OX A DETERMIXATIOX OF THE YALETE OF 
Rayleigh*. 
Glazebrook and SkinneuI 
KahleJ. 
Carhaet and Guthe§ . . 
Clark at 15°. ' Clark at 0°. 
Cadmium at 204 
C.G.S. 
Int. V. 
C.G.S. 
Int. V. 
C.G.S. 
Int. V. 
1-4344 
1-4324 
1-4333 
1-4344 
1-4342 
1-4328 
1-4488 
1-4492 
1-0183 
1-0186 
From the above table it will be seen that the value obtained by Kahle for the 
E.M.F. of the Clark is lower than the values obtained by other observers. This 
cannot be entirely due to a true difference in the E.M.F. of the cells employed, for 
direct comparisons have been made between the German cells and those of Glaze- 
BEOOK and Skinner and of Carhaet and Guthe. In this way it was found that 
the E.M.F. of the cells used by Glazebrook and Skinner, which were of the Board 
of Trade pattern, were O'OOOS volt higher than the German (H-form), while the 
cells used by Carhaet and Guthe (H-form) had the same E.M.F. to within 1 part 
in 10,000 as the German. The difference seems to be due to the determination of 
the E.M.F. In the case of the values exjiressed in international volts, this difference 
is most probably due to the silver voltameter used in measuring the current. Thus, 
if the silver nitrate solution is not fresh, so that the electro-chemical equivalent is 
really greater than O’OOlllB, the value obtained for the E.M.F. on the supposition 
that the electro-chemical equivalent is O'OOlllS will be too great. 
Carhart and Guthe used the same instrument for measuring the current as was 
employed by Patterson and Guthe in measuring the electro-chemical equivalent. 
It consists of a torsion electro-dynamometer in which the movable coil is wound on 
ebonite. In addition to the difficulty in measuring with the required accuracy the 
mean radius of a coil of the size employed (radius 5 centims.)|| there is the further 
objection that the coil was wound on an ebonite reel which was by no means of 
constant size. Thus Carhart and Guthe found that the shrinkage was sufficientlv 
O «/ 
great to warrant them in giving a set of measurements made only four days before 
the measurements of radii only half weight. For the above reasons the author does 
not consider that these observations are of as much weight as those of Bayleigh or 
Kahle. IIayleigh’s cells were prepared under very much the same conditions as 
those of Glazebrook and Skinner, who, as a matter of fact, took one of Rayleigh’s 
* ‘Phil. Trans.,’ p. 411, Pt. IF, 1884, and p. 781, Pt. IF, 1885. 
t ‘ Phil. Trans.,’ A 183, p. 567, 1892. 
I ‘ 'Wiecl. Ann.,’ 59, 532, 1896. 
§ ‘Phys. Eev.,’ 9, 288, 1899. 
!l To obtain an accuracy of 1 part in 10,000 in the E.M.F., the mean radius must be known to within 
0‘0005 centim. 
