OF ZINC, CADMIUM, LEAD, COPPER, AND LITHIUM 43 



DETERMINATION OF THE TEMPERATURE COEFFICIENTS OF CELLS 



CONTAINING ZINC AMALGAMS. 



The previously described measurements had all been made with the 

 potentiometer in its original less complete condition. For further more 

 accurate experiments, the potentiometer was modified with a view of 

 eliminating all thermoelectric currents, as has been already described." 

 The air in the case containing the potentiometer was stirred by means of 

 a fan, worked by an electric motor outside the case, so that the tempera- 

 ture inside was sensibly uniform. It was also arranged that all final 

 adjustments on the potentiometer could be made from outside the case, 

 thus avoiding all danger of thermal effects due to heating by the warmth 

 of the operator. Moreover, the connections with the cell were made so 

 as to avoid thermoelectric effects ; contact was made between copper and 

 mercury well under the surface of the thermostat and inside the cell, so 

 that all unequal heating of junctions of dissimilar metals was avoided. 



A series of test experiments with this improved potentiometer showed 

 that although thermal currents had been completely eliminated, so that the 

 potentials could be read to even less than o.oooooi volt, nevertheless, the 

 amalgam cells themselves were not constant to this same degree. It was 

 thought that the irregularities might be due to the formation of a basic 

 salt by the action between the amalgam and water. To test this, in one 

 case the electrolyte was made slightly acid (about 0.02 N. with H 2 SO 4 ) , 

 but no effect was observable, and hence in the final experiments neutral 

 electrolyte was again used. 



In all the measurements thus far recorded the potential at 29.96 was 

 first measured, and then the potential at o, and finally the readings 

 repeated at 29.96 and at o again. It was always found that the reading 

 at 29.96 remained throughout constant to about 0.000005 volt, while the 

 value at o showed much greater change, sometimes even as much as 

 0.000030 volt. If the cell at 29.96 was shaken, the values were only 

 temporarily altered, while at o this treatment caused a more permanent 

 change, which did not completely vanish even if the cell was heated to 

 29.96, and cooled to o again. 



Being unable to eliminate the difficulty, we sought to arrange the experi- 

 ments in such a way as to minimize its influence. In the final set of read- 

 ings to be recorded, the amalgams and electrolyte were cooled before using, 

 and were put into the cell as cold as possible. The readings at o were 

 first taken and then the readings at the higher temperature. Finally the 

 cell was cooled to o and measured again. The first readings at o and 

 the readings at 30 were constant, even if the cell was shaken, but the 

 second series at o showed after a time the former irregularities. 



38 This monograph, pp. 17-20. 



