Variations in the E.M.F. of Clark Cells with Temperature. 369 



information upon this matter than could be found in any work upon 

 the subject that the experiments described below were begun more 

 than two years ago. It also appeared desirable to ascertain to what 

 degree of accuracy temperature coefficients could be determined by 

 observing the changes in E.M.F. during a continuous change of 

 temperature. 



The tests were in all cases carried out upon cells set up in accord- 

 ance with the specifications of the Board of Trade, the greater number 

 of the cells employed in the investigation having been constructed 

 by one of the authors. Those employed in the preliminary experi- 

 ments were set up according to the specification of October, 1892, 

 but the remainder were made according to the final specification of 

 the Board of Trade. They were contained in what are known as 

 " sample tubes," approximately 1 inch in diameter, and the quantity of 

 paste and mercury employed was not large, the figures given in the 

 instructions being adhered to as far as possible. It will be readily 

 understood that many of the results obtained only hold good for this 

 class of cell, and" that there might be considerable modifications 

 introduced by any large increase or decrease in the size of the cell. 



The method in the preliminary experiments was somewhat 

 approximate. Current from a storage cell, in series with a resist- 

 ance, traversed a potentiometer wire, the resistance being such that 

 the fall of potential along the wire amounted to either one ten- 

 thousandth or one five-thousandth of a volt per centimetre length as 

 required. The E.M.F. of the storage cell was observed at intervals,, 

 so that any necessary correction might be applied, and the current 

 could be reversed at any time by means of a key. The cell to be tested 

 was joined up in opposition to the standard, and their difference in 

 E.M.F. at once determined by the method of compensation. 



The standard used in the comparison consisted of one or other of 

 three Clark cells, kept, as far as possible, at a constant temperature,, 

 ice being employed for that purpose. The cells were placed in 

 corked boiling tubes, and these were supported in a water bath. The- 

 latter was packed round with cotton wool in a wooden case, the lid 

 of which was only removed to replenish the bath with ice. Badiation 

 was thus diminished to a great extent, and as these preliminary 

 experiments were carried on during the winter it was possible to 

 maintain a comparatively low temperature in the room itself. Never- 

 theless, variations of half a degree, or more, often occurred in this 

 preliminary investigation, and the bath was considerably warmer at 

 the bottom than at the top. It was found best to pack a large 

 amount of ice into the bath, so that this difference remained fairly 

 constant. In general it amounted to about 1*5° C. As it was difficult 

 to apply a satisfactory correction, any variations in the temperature 

 of the bath had to be neglected. One of the above cells was employed 



2 d 2 



