46 REPORTS ON THE STATE OF SCIENCE. 
pieces attached to make connections with the tubes. The outside edge 
of each end of the tube is to be coincident with the inner surface of the 
corresponding spherical end vessel. The leads which make contact 
with the mercury are to be of thin platinum wire fused into glass. 
The point of entry of the current lead and the end of the tube are to 
be at opposite ends of a diameter of the bulb; the potential lead is to be 
midway between these two points. All the leads must be so thin that 
no error in the resistance is introduced through conduction of heat to 
the mercury. The filling of the tube with mercury for the purpose of 
the resistance measurements must be carried out under the same con- 
ditions as the filling for the determination of the mass. 
The resistance which has to be added to the resistance of the tube 
to allow for the effect of the end vessels is to be calculated by the 
formula— 
0:80 /1 ,1 
cal fay 
A ro8ge(ertp) ° a 
where 7: and r. are the radii in millimetres of the end sections of the 
bore of the tube. 
The mean of the calculated resistances of at least five tubes shall 
be taken to determine the value of the unit of resistance. 
For the purpose of the comparison of resistances with a mercury 
tube the measurements shall be made with at least three separate fillings 
of the tube. 
SPECIFICATION II. 
Specification relating to the Deposition of Silver. 
The electrolyte shall consist of a solution of from 15 to 20 parts by 
weight of silver nitrate in 100 parts of distilled water. The solution 
must only be used once, and only for so long that not more than 30 per 
cent. of the silver in the solution is deposited. 
The anode shall be of silver, and the kathode of platinum. The 
current density at the anode shall not exceed 1/5 ampere per square 
centimetre and at the kathode 1/50 ampere per square centimetre. 
Not less than 100 cubic centimetres of electrolyte shall be used in 
a voltameter. 
Care must be taken that no particles which may become mechani- 
cally detached from the anode shall reach the kathode. 
Before weighing, any traces of solution adhering to the kathode 
must be removed, and the kathode dried. 
SCHEDULE C. 
Weston Normat CEuu. 
The Weston normal cell may be conveniently employed as a standard 
of electric pressure for the measurement both of 5.m.F. and of current, 
and, when set up in accordance with the following specification, may 
be taken, provisionally,’ as having, at a femperature of 20° C., an 
E.M.F. of 1:0184 volt. 
* See duties of the Scientific Committee, Schedule D. 
