a 
PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 39 
a function of the fineness of the sulphate.) Evidence of remarkably stable 
cells set up with purchased mercurous sulphate is afforded by six cadmium 
cells made at Bushy House in April 1902: these have been in constant 
use, and in the case of two of them have frequently been short-circuited 
through 100 ohms. One of these cells is taken as a standard in the com- 
parisons. By reference to a seventh cell made up in June 1904 witha 
paste made from sulphate identical with that employed for the previous 
ones it is thought probable that the whole six cells have fallen -07 milli- 
volt since their manufacture, Table IV. gives the result of the com- 
parisons. 
TABLE IV. 
Date of Observation | 16>17 16>18 | 16>19 16>20 | 16>21 
May 5, 1902 : | “00000 “00000 ‘00000 ‘00000 ‘00000 
Sep. 12, ,, A | + 1 0 + 1 0 
Feb. 25, 1903 ag 0 0 0 0 0 
Dec. 14, _ ,, A ae ee: 0; | — 0; 0 0 + 0; 
Feb. 6, 1904 0 0 | 0 | 0 | 0 
July 9, ,, : oi a8 OF | + 0, | -- 1; | = 0; | 0 
At the present time the E.M.F. of a cadmium cell set up with a paste 
made from fuming sulphuric acid and mercury is less than that of these 
cells by 0°21 millivolt. 
With respect to the other elements of standard cells it is proposed to. 
investigate the cadmium and zinc amalgams, and the solutions of the 
sulphates of these metals, in a manner very similar to that employed for 
the pastes. Much valuable information has fortunately accumulated 
respecting the influence of impurities in these, so that probably the task 
is a light one. Jt is interesting to note that neither of the amalgams 
should have its surface exposed to the atmosphere for any length of time ; 
it is preferably covered with water or a solution of the sulphate of the 
metal. This prevents oxidation. In addition to these investigations the 
effects of acidity and basicity are to be determined, together with observa- 
tions in connection with lag, polarisation, temperature coefficients, ce. 
Remarks on the Rayleigh H Form and the Board of Trade Tube 
Form of Standard Cells. 
In 1892 Dr. Kahle, in a paper read before the British Association at 
Edinburgh, described some researches made by him on the Clark cell. 
Comparisons of H cells set up by him with tube cells set up by Dr. Glaze- 
brook at Cambridge led to the assignment of an E.M.F. to the H cell, 
four ten-thousandths of a volt less than that of the Board of Trade form. 
This difference in value has been often quoted, and is at the present time 
accepted as a fact. In view of the discrepancies produced by the paste, 
together with theoretical considerations, it was thought desirable to 
investigate this difference if such should exist. For this purpose the 
H and tube forms of cell were combined. In addition to the usual ter- 
minals to the H cell, a zinc rod was inserted in the limb containing the 
paste, so that the arrangement of the elements in this limb was in accord- 
ance with the specification of the Board of Trade pattern. The sole 
difference in the elements of the H and tube cells was therefore the sub- 
stitution of the zinc rod for zinc amalgam. In some cases the zinc rod 
