INTRODUCTION. xlUi 



cell. Each laboratory has means of preserving the unit. Any discrepancies 

 between the bases of the different countries at the present time would be due 

 only to possible variations in the reference cells of the national laboratories. 

 Such discrepancies are probably less than 2 parts in 100,000. 



The figure i. 01 83 has been in use since January i, 191 1. The value used in 

 the United States before 1911, 1.019126 at 20° C or 1.0189 at 25° C, was as- 

 signed to a certain group of cells maintained as the standard of electromotive 

 force at the Bureau of Standards. The high value is partly due to the use of 

 commercial mercurous sulphate in the cells. The old and the new values, i. 01926 

 and 1. 01 83, thus apply to different groups of cells. The group of cells to which 

 the value 1.019126 was assigned before 1910 differed by 26 microvolts from the 

 mean of the international group, such that the international group to which the 

 value 1.0183 is now assigned had the value 1.019126 + 0.000026, or 1.019152, 

 in terms of the old United States basis. The difference between 1.019152 and 

 1.0183 is 0.000852. 



The electromotive force of any Weston cell as now given is therefore 0.000852 

 volt smaller than on the old United States basis, i.e., the present international 

 volt is 84 parts in 100,000 larger than the old international volt of the United 

 States. 



Upon the new international basis the Clark cell set up according to the old 

 United States legal specifications has an emf of 1.43280 international volts at 

 15° C. The Clark cell set up (with specially purified mercurous sulphate) accord- 

 ing to improved specifications used at the Bureau of Standards has an emf of 

 1.43250 international volts at 15° C or 1.42637 at 20° C. 



Portable Weston Cells. — The standard cell used in practice is the Weston 

 portable cell. It is like the Weston normal cell except that the cadmium-sulphate 

 solution at ordinary temperatures is unsaturated. As usually made, the cad- 

 mium-sulphate solution is saturated at about 4° C; at higher temperatures the 

 crystals are dissolved. Plugs of asbestos or other material hold the chemicals 

 in place. Its resistance is usually about 200 to 311 ohms. The change of emf, 

 wholly negligible in most electrical measurements, is less than 0.0000 1 volt 

 per degree C. The two legs of the cell have large and opposite temperature 

 coefficients so that care must be taken that the temperature of the cell is kept 

 uniform and the cell must be protected from draughts or large changes of tem- 

 perature. The electromotive force of a portable cell ranges from i.oi8i to 

 1.0191 international volts and must be determined by comparison with stand- 

 ards. It decreases very slightly with time, usually less than 0.000 1 volt per 

 year. 



Absolute and Semi-absolute Volt. — Since the direct determination of the 

 volt in absolute measure presents great difficulties, it is derived by Ohm's law 

 from the absolute measures of the ohm and ampere. From the absolute values 

 of these already given, 



I international volt = 1.00043 absolute volts. 

 The electromotive force of the Weston normal cell at 20° C is 1.01830 interna- 

 tional volts and i. 01874 absolute volts, A semi-absolute volt is that potential 



