372 Prof. W. E. Ayrton and Mr. W. R. Cooper. 



which ice could be packed. As the ice melted the water was carried 

 off by a tube so that it stood at a level slightly above the bottom of 

 the inner bath. The whole was surrounded by cotton-wool in a 

 wooden case. By filling the outer bath with ice every evening it was 

 found possible to maintain the temperature of the inner bath fairly 

 constant throughout the following day, the variation rarely exceeding 

 two or three-tenths of a degree in the hottest weather. 



In order to check any variation of the standard through causes 

 other than temperature, it was frequently balanced against the P. D. 

 due to a known current of about ^ ampere passing through a known 

 resistance of about 3 ohms. The latter was made especially for the 

 purpose, of manganin wire, wound upon an ebonite frame and im- 

 mersed in paraffin oil. It was previously aged by heating for five 

 hours at 140° C, and it assumed a steady value after two or three 

 weeks, as shown by frequent comparisons with standard manganin 

 coils. The current was measured by a Kelvin centiampere balance, 

 the constant of which had frequently been checked by means of 

 a silver voltameter. By this means a variation of 0*00015 volt 

 could be detected without difficulty. 



The standards were soon found to become remarkably constant in 

 value. The E.M.F. of one cell was tested absolutely, as above de- 

 scribed, before and after every experiment, and the three cells were 

 also frequently compared during the course of an experiment. The 

 results were very consistent. 



Fig. 2. 



A stretched wire, ABD (fig. 2), 3 metres in length, was em- 

 ployed in comparing cells under test. The resistance of this being 

 known, the strength of current necessary to give a drop of potential 

 of one ten -thousandth of a volt per half centimetre length was calcu- 

 lated, and also the resistance required when traversed by this current 

 to balance one of the standards whose E.M.F. was known. The resist- 

 ance Ri in the diagram was adjusted so as to be as nearly up to this 

 value as possible, and the deficit was found in terms of a length AP of 

 the stretched wir3. For example, suppose the resistance necessary to 

 balance the E.M.F. of the standard cell E was found by calculation to 



