ELECTRO-CHEMICAL EQUIVALENT OF SILVER. 
427 
In the present investigation, however, a is not measured directly, but by comparison 
with A. If we write a/A = a, so that 
da da dA 
a a A ’ 
and eliminate da/a, we have 
d l. 
f 
da dA 
2-23 —+-28-r —-28 
« A 
dB 
TP 
which is the equation by which the suitability of the proportions is to be judged. It 
will be seen that the stress is thrown upon the measurement of a, and that the errors 
of A and B enter to the extent of only about one quarter. If the proportions had 
been those described as ideal, the coefficients of dA/A and cZB/B would have been zero. 
It must not be forgotten that the error of f itself is halved in the final result, 
which thus involves the errors of A and B only after division by 8. 
If the current be i, and the number of turns in the fixed and suspended coils n, n, 
the attraction or repulsion is measured by 
/ *0 /» 
7 rim ly. 
This is expressed in absolute units. To find the value in gravitation units we must 
divide by g. If m be the observed difference of weights in air necessary to counter¬ 
poise the suspended coil when the current is reversed in the fixed coils, 
Trnn'if—\ mgX '99986, 
the last factor representing the “ correction to vacuum ” rendered necessary by the 
finite density of the brass weights. 
The value of g at Cambridge is taken to be 981'2282. Introducing this and the 
numerical values of n, n, f already given, we find 
when 
Z — jX -y/fi n, 
^=•0370484. 
The silver voltameters. 
§ 16. The arrangement adopted for the voltameters is similar to that recommended 
originally by Poggendokff. The deposits are formed upon metallic basins (usually of 
platinum) charged with a neutral 15 per cent, solution of pure silver nitrate. They 
are prepared by careful cleaning with nitric acid and distilled water with subsequent 
ignition. After complete cooling in a desiccator, they are weighed to yg- milligrm. 
m a delicate balance with trustworthy weights. The anode, by which the current 
enters the voltameter, is formed of fine silver sheet, suspended by platinum wire in 
2 i 2 
