vv 
ON PRACTIGAL STANDARDS FOR ELECTRICAL MEASUREMENTS. th 
cadmium cell was published. It is gratifying to know that the specifica- 
tion proved of value, for in 1906 fifty-one cadmium cells were submitted for 
test at the National Physical Laboratory, and all of these were prepared 
on the lines of the specification. The cells were intended for commercial 
use, and they were packed with small crystals of cadmium sulphate to be 
more portable ; also we have reason: t« believe that in some cases the 
mercurous sulphate had not been properly washed, and in other cases the 
solution of cadmium sulphate was slightly acid. Nevertheless the e.m f. 
of these cells agreed with the N.P.L. cells to about 2 parts in 10,000, the 
N.P.L. cells having the lower voltage. Standards more carefully set up 
have been submitted by two observers for comparison with the N.P.L. 
cells in accordance with the offer made in the ‘ British Association Report,’ 
1905. The cells prepared by one of these observers—Mr. Tinsley of 
Beckenham—differed from the N.P.L. cells by about 0:1 millivolt, or 
1 part in 10,000. Mr. Mather also submitted a number of cells, and 
these had approximately the same mean e.m.f. as those from Mr. Tinsley. 
The N.P.L. cells were the lower in voltage, and freshly prepared cells 
agree with old ones.! 
In May 1907 twelve Weston cadmium cells set up by Dr. Wolff at 
the National Bureau of Standards, Washington, were compared with 
a number of the cells of the National Physical Laboratory, and a mean 
difference of 3 parts in 1,000,000 was measured. Dr. Wolff’s cells were, 
we believe, set up quite independently of the N.P.L. specification, which 
makes this remarkable agreement all the more gratifying. 
APPENDIX II. 
Specification for the Practical Application of the Definition of the 
International Ampere. 
(From the National Physical Laboratory.) 
In the following specification the term silver voltameter (or coulo- 
meter) means the arrangement of apparatus by means of which an electric 
current is passed through a solution of silver nitrate in water. The silver 
voltameter measures the total electrical quantity which has passed during 
the time of the experiment, and by noting this time the time-average of 
the current, or, if the current has been kept constant, the current itself, 
can be deduced. 
In employing the silver voltameter to measure currents of about | 
ampere the following arrangements should be adopted : The kathode on 
which the silver is to be deposited should take the form of a platinum bowl 
about 10 centimetres in diameter and 7 centimetres in depth. The mass 
of the bow] is conveniently about 80 grams. 
The anode should be a plate or disc of pure silver coated with a deposit 
of electrolytic silver, the mass of the latter being about 50 per cent. 
greater than the mass.of silver to be deposited on the kathode. The plate 
or disc of silver should be of about 6 centimetres edge (or diameter) and 
3 or 4 millimetres in thickness. Its total area will thus approximate to 
60 square centimetres. The anode should be supported horizontally in 
the liquid near the top of the solution by a silver rod riveted through its 
1 In Mr. Mather’s cells electrolytic mercurons sulphate was used; in Mr. Tinsley’s 
cells the mercurous sulphate was prepared by the chemical precipitation methcd. 
