ON PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 79 
solution of sodium hydrate, followed by washings with water, strong nitric 
acid, and distilled water. It is then made the anode of a silver voltameter, 
the liquid being a 15 per cent. solution of silver nitrate (an impure solu- 
tion serves) having a volume of about 350 cubic centimetres. The kathode 
should be a clean silver plate supported near the top of the solution. 
With a current of about 1 ampere the circuit should be completed for 
ten minutes at least, after which the kathode and liquid are removed from 
the bowl. The bow] is washed with water andafterwards cleaned with strong 
nitric acid; washings with distilled water, strong nitric acid, and dis- 
tilled water follow in the order named, and the bowl is dried in an electric 
oven at about a temperature of 160° C. It is removed to a desiccator 
and when thoroughly cool is weighed. A bowl of similar size and of 
approximately the same mass is convenient as a counterpoise. 
4. The Crrcutt.—The platinum bowl is placed in position in the 
intended circuit and 300 cubic centimetres of the solution of silver nitrate 
are placed in it. The anode is placed inside the filter-paper cup and the 
latter suspended by platinum wires, which are insulated from the anode 
and from the rest of the circuit. The anode and filter-paper cup are sup- 
ported so that the silver plate or disc is covered by the solution ; the 
connections to the remainder of the circuit are then made. Contact is 
made at a key and the time noted. The current is allowed to pass for an 
interval depending on the precision desired, and the time of breaking 
contact must be observed. For measurements of high precision from 7 
to 10 grams of silver should be deposited. During the passage of the 
current the voltameter should be covered over, to exclude light. 
5. Deposit of Silver.—The solution is removed from the bowl and the 
deposit rinsed with about 100 cubic centimetres of distilled water. The 
washing water is poured into a clean glass crystallising-dish and the 
operation of washing is repeated three times. The bowl is then nearly 
filled with distilled water and left for at least three hours; it is rinsed 
three times, the last of these washing waters remaining in the bowl for 
ten minutes. This should give no milkiness when added to a neutral 
solution of sodium chloride in water. The bowl is dried in an electric 
oven at a temperature of about 160° C. 
If any loose silver is observed in the solution outside of the filter- 
paper cup, or in the washing waters, these liquids must be filtered, the 
filter-paper dried, and the loose silver added to the bowl before drying the 
deposit. The bowl is cooled in a desiccator and weighed again. he gain 
in mass gives the silver deposited. 
6. Calculation. —To find the current in amperes this mass, expressed 
in grams, must be divided by the number of seconds during which the 
current has been passed and by 0001118. The result will be the time- 
average of the current, if during the interval the current has varied. 
In determining the constant of an instrument by this method the 
current should be kept as uniform as possible, and the readings of the 
instrument observed at frequent intervals of time. These observations 
give a curve.from which the reading corresponding to the mean current 
(time-average of the current) can be found. The current, as calculated 
from the voltameter results, corresponds with this reading. 
Notes on Observations.—If this specification is carefully followed the 
mass of silver deposited for the passage of one coulomb through the 
voltameter is constant within the limits of the errors of measurements of 
the highest precision, It is certainly constant to 1 part in 100,000. 
