CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
583 
A reference to the table shows that our bottle cell No. 90 is about three parts in 
10,000 above the cell Rayleigh 1. It agrees more nearly, therefore, with Lord 
Rayleigh’s original standard. 
In comparing our result with that of other experimenters it must be remarked 
that it is given in “ volts.” To reduce it to the “legal volts ” of the Paris Congress 
it must be multiplied by the ratio 1063/1060, and it then becomes at 15° C. 1‘4382 
“ legal volt.” 
Part II. —Comparison of Various forms of Cells and of the Materials used. 
§ 13. Introduction. General Remarks . Methods of Testing. 
A normal Clark cell is one prepared according to certain definite directions, and the 
electromotive force of such a cell has been determined in Part I. above. Various 
causes may lead to a deviation from the normal value for the E.M.F. of any given 
cell, and it becomes important to consider these. We have in the course of the work 
met with at least one batch of cells which agree very closely among themselves, but 
Fig. 2. 
which differ very considerably in E.M.F. from the standard. Directions for the con¬ 
struction of a normal Clark cell have been provisionally drawn up by a sub-committee 
of the Electrical Standards Committee of the Board of Trade, and have been issued 
to various laboratories for the purpose of gaining experience and enabling the Com¬ 
mittee to issue at some future date a definite memorandum. These instructions in 
