CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
585 
Shake up the paste and introduce it without contact with the upper part of the walls of the test tube, 
filling the tube above the mercury to a depth of rather more than 2 cm. 
Then insert the cork and zinc rod, passing the glass tube through the hole prepared for it. Pash 
the cork gently down until its lower surface is nearly in contact with the liquid. The air will thus be 
nearly all expelled, and the cell should be left in this condition for at least 24 hours before sealing, 
whieh should be done as follows. 
Melt some marine glue until it is fluid enough to pour by its own weight, and pour it into the tes 
tube above the cork, using sufficient to cover completely the zinc and soldering. The glass tube should 
project above the top of the marine glue. 
The cell thus set up may be mounted in any desirable manner. It is convenient to arrange the 
mounting so that the cell may be immersed in a water bath up to the level of, say, the upper surface of 
the cork. Its temperature can then be determined more accurately than is possible when the cell 
is in air. 
It is clearly desirable to determine whether these instructions are sufficient to 
enable different makers to produce cells having the same E.M.F., and to investigate 
in what points a slight departure from the instructions may be made without 
materially affecting the E.M.F. of the cell. 
The following paper contains an attempt to answer the questions :— 
(1.) How far cells made with different samples of materials have the same E.M.F. ? , 
(2.) How far batches of cells from different makers agree with our standard ? 
(3.) What are the chief sources of variation in a Clark cell? 
We have examined over 100 cells, of which we shall now give the details. 
The values have generally been given in terms of the cell No. 1, constructed by Lord 
Rayleigh over eight years ago. 
The constancy of this cell has been ascertained by comparison with other standards, 
and by the fact that it was our ultimate standard of reference for the absolute value 
of the E.M.F. in the investigation already described. The result of this agrees 
almost exactly with Lord Rayleigh’s. 
The method of comparison already described in Part I. has been employed in all 
the later comparisons. Two Leclanche cells are allowed to work through two 
resistance boxes, with a total resistance of 10,000 ohms, in circuit. One of the 
Clarks, usually the bottle cell, is connected through a galvanometer and a high 
resistance to two points on this circuit, and the resistances adjusted until the 
potential difference between these points just balances the E.M.F. of the Clark. 
This Clark is then connected in turn in opposition with the other cells of which the 
E.M.F. is required, and the difference between the electromotive forces of the two 
determined in terms of the fall of potential along the Leclanche circuit. 
We found that this fall of potential changed very slightly throughout our experi¬ 
ments. The fall corresponding to 1 ohm is very approximately '00025 volt, and we 
have expressed the differences between the various cells examined in terms of this 
unit. We will collect, in a series of Tables, the results obtained, together with 
descriptions of the method of construction. 
MDCCCXCII.—A. 4 F 
