Electromotive Force of different Forms of the Clark Cell. 125 



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Fia. 1. — Curves showing Diffusion-Lag of two B.O.T. Clark Cells. 



We have also tested several of the Muirhead portable cells in a 

 special water-jacketed air bath, the temperature of which was regu- 

 lated by a thermostat. We find that for comparatively rapid changes 

 of temperature, such as 10° C. in two hours, they have a temperature- 

 coefficient of 0*00050, on the average, between 10° and 30° C. They 

 have also a slow diffusion-lag similar to other B.O.T. cells. It must 

 be remembered, however, that these cells, owing to their form, are 

 not intended for the most accurate laboratory tests, and that they 

 are quite sufficiently constant for the purposes for which they are 

 generally used. 



It is sufficiently evident from the examples above given that a 

 Clark cell of the B.O.T. form containing clear solution cannot be 

 said to have a definite temperature coefficient. The change of E.M.F.. 

 is seen to depend on the previous history of the cell, on the rate of 

 change of temperature, and on the quantity of solution and relative 

 size and position of the zinc rod. The temperature-coefficient at 

 15° C. may have any value, from 0'00040 to the value 0-00078, 

 which is generally taken. If the latter value of the coefficient is 

 regularly used, errors which are relatively considerable in accurate 

 work may readily be made, even if the rate of change of tempera- 

 ture is only 4° or 5° a day. There is no doubt that, provided suffi- 



