PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 99 
Preparation of Materials for a Standard Cadmium Cell. 
1. Mercury.—The commercial mercury should be squeezed through 
wash-leathér and passed in the finely divided condition in which it 
emerges, first through dilute nitric acid (1 to 6 of water) and mercurous 
nitrate solution, and afterwards through distilled water, both liquids 
being conveniently contained in long glass tubes. The mercury is then 
to be twice distilled in vacuo. Mercury suspected of abnormal contami- 
nation should not be employed. 
2. Amalgam. Type A.—This is a 125 per cent. amalgam, and is 
intended for all commercial cells. The method of preparation is practi- 
cally identical with that used by Professor Carhart. A current is passed 
from a thick rod of pure commercial cadmium to distilled mercury, the 
intervening liquid being cadmium sulphate solution rendered slightly 
acid with a few drops of H,SO,. The kathode is weighed before deposi- 
tion takes place, and again afterwards, the percentage of cadmium in the 
amalgam being thus calculable. More than the requisite amount should 
always be deposited, and the percentage reduced to 124 by the addition 
of more mercury. The fall of potential from anode to kathode should not 
exceed 0°3 volt. To prevent the anode slime having access to the kathode 
it is desirable to surround the anode with a small porous pot, as in the 
Richards silver voltameter, or to place a small crystallising dish beneath 
it for the anode powder to settle in. Contact with the kathode is made 
with a platinum wire sealed into a glass tube so as to protect it from 
direct: contact with the cadmium sulphate solution, and a rough estimate 
of the quantity of cadmium deposited is obtained from the readings of an 
ammeter placed in the circuit. The amalgam so prepared, together with 
the mercury added to reduce the percentage of cadmium to 124, is now 
heated on a water-bath and stirred so as to ensure homogeneity, some 
cadmium sulphate solution still flooding the surface. It is then cooled 
and the acid sulphate removed, neutral cadmium sulphate solution 
taking the place of the latter, and consisting of saturated solution plus an 
equal volume of distilled water. This 124 per cent. amalgam is then 
ready for use and is entirely liquid at a temperature approximating to 
60° C. 
Type B.—This amalgam is liquid at the temperature of melting ice, 
and is intended for cells of a slightly better type than those made with 
the 125 per cent. amalgam. The cells may be used at a higher tempera- 
ture than 0° C., but they are not intended to be so used as their tempera- 
ture coefficient is about —0°043 per cent. per rise in temperature of 1° C. 
The cells are primarily intended for standardising laboratories, and their 
E.M.F. at 0° C. is equal to the E.M.F. of the cells prepared with the A 
amalgam if this latter E.M.F. is corrected to 0° C. with the temperature 
coeflicient formula of the cell. This is equivalent to saying that if an 
A cell was in a steady condition at 0° C. and nothing abnormal occurred 
its E.M.F. would be identical with that of a Bcell at 0° C. It is not wise, 
however, to use a 125 per cent. amalgam cell at low temperatures ; an 
8 per cent. amalgam may be so used, but its upward range (with a small 
temperature coefficient) is lower than that of the 12} per cent. amalgam 
cell. For commercial purposes probably the 124 per cent. amalgam will 
be of most service. 
To prepare the type B amalgam take some of that previously prepared 
and add sufficient mercury to reduce the percentage of cadmium to 3. 
H2 
