120 



Prof. EL L. Callendar and Mr. H. T. Barnes. 



be set in such a manner as to keep the temperature steady for any- 

 desired length of time at any point between 5° and 30° C. 



The method of regulation adopted for these baths was similar to 

 that described by Griffiths,* but much less elaborate. The baths 

 were made of copper, and were encased in felt and wood. They 

 were heated by a stream of tap -water passing through a copper tube 

 over a regulated gas flame. The regulators were made to cut off the 

 gas so sharply that a difference of temperature of one-tenth of a 

 degree sufficed to change the gas supply from full flame to no flame. 

 With these very sensitive regulators, some trouble was experienced 

 at first, partly owing to the excessive variations of the Montreal gas 

 pressure, and partly owing to the sudden changes of the climate. 

 In the end, however, these difficulties were so successfully overcome 

 that, on the longest continuous run, extending over nearly a fort- 

 night, the temperature of the bath did not vary by so much as 

 0'02° C. throughout the whole period. 



The temperature of the tap- water averaged about 8° C. in mid- 

 winter, and seldom rose above 13° C. in summer. This generally 

 sufficed to keep the baths steadily at 15° C. even if the temperature of 

 the room was as high as 25° C. In order to obtain steady tempera- 

 tures at points below 15° C, the stream of tap-water was led through 

 a copper spiral immersed in melting snow or ice before passing over 

 the gas flame. In this manner the baths could be set to regulate- 

 steadily at temperatures as low as 5° C. 



The two baths were generally set to regulate at different tempera- 

 tures, so that by transferring a cell from one bath to the other the 

 effect of a sudden and definite change of temperature could be ob- 

 served. The time required by cells of different forms to reach their 

 steady final values could thus be determined, and the effects of 

 diffusion-lag could be readily distinguished from the immediate 

 change of E.M.F. consequent upon a change of temperature. 



§ 4. Electrical Apparatus. 



By the use of these accurately regulated water-baths, the tempera- 

 ture of the cells became so much a matter of certainty that we 

 found it desirable to make the comparisons to the hundredth of a 

 millivolt, corresponding to the hundredth of a degree Centigrade of 

 temperature. From the results of our experiments, we have reason 

 to conclude that the Clark cell, under suitable conditions, permits 

 the attainment of this order of accuracy, and is far superior to the 

 silver voltameter for accurate determinations. 



The comparisons of the cells were made by the usual Poggendorff 

 method, with a 6000-ohm galvanometer. The potentiometer used 



* < Phil. Trans.,' A, vol. 184 (1893), p. 374. 



