CLARK CELL AS A STANDARD OE ELECTROMOTIVE FORCE. 
5G9 
In the following specification the term silver voltameter means the arrangement of apparatus by 
means of which an electric current is passed through a solution of nitrate of silver 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 1 ampere the following arrange¬ 
ments should be adopted. The kathode on which the silver is to be deposited should take the form of 
a platinum bowl not less than 10 cm. in diameter, and from 4 to 5 cm. in depth. 
The anode should be a plate of pure silver some 30 square cm. in area, and 2 or 3 millims. in thickness. 
This is supported horizontally in the liquid near the top of the solution by a platinum wire passed 
through holes in the plate at opposite corners. To prevent the disintegrated silver which is formed on 
the anode from falling on to the kathode, the anode should be wrapped round with pure filter paper, 
secured at the back with sealing-wax. 
The liquid should consist of a neutral solution of pure silver nitrate, containing about 15 parts by 
weight of the nitrate to 85 parts of water. 
The resistance of the voltameter changes somewhat as the current passes. To prevent these changes 
having too great an effect on the current, some resistance, besides that of the voltameter, should be 
inserted in the circuit. The total metallic resistance of the circuit should not be less than 10 ohms. 
Method of malting a Measurement. 
The platinum bowl is washed with nitric acid and distilled watei’, dried by heat, and then left to cool 
in a desiccator. When thoroughly dry it is weighed carefully. 
It is nearly filled with the solution, and connected to the rest of the circuit by being placed on a 
clean copper support to which a binding screw is attached. This copper support must be insulated. 
The anode is then immersed in the solution so as to be well covered by it and supported in that posi¬ 
tion ; the connections to the rest of the circuit are made. 
Contact is made at the key, noting the time of contact. The current is allowed to pass for not less 
than half-an-hour, and the time at which contact is broken is observed. Care must be taken that the 
clock used is keeping correct time during this interval. 
The solution is now removed from the bowl, and the deposit is washed with distilled water and left to 
soak for at least six hours. It is then rinsed successively with distilled water and absolute alcohol, and 
dried in a hot-air bath at a temperature of about ICO 9 C. After cooling in a desiccator it is weighed 
again. The gain in weight gives the silver deposited. 
To find the current in amperes, this weight, expressed in grammes, must be divided by the number of 
seconds during which the current has been passed, and by ’001118. 
The result will be the time-average of the current, if during the interval the current has varied. 
In determining by this method the constant of an instrument the current should be kept as nearly 
constant as possible, and the readings of the instrument taken at frequent observed 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 by the voltameter, corresponds to this reading. 
In our experiments the method above described, which is that adopted by Lord 
Rayleigh, was carefully adhered to. 
While our work was in progress, a paper on “ The Causes of Variation of Clark 
Standard Cells” was communicated to the British Association by Mr. J. Swinburne 
(‘Electrical Review,’ August 28th, 1891). The results of our investigations confirm 
his conclusions in many respects. 
In the first part of the paper we shall deal with the experiments on the absolute 
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MLCCCXCH. —A. 
