[BaARNES] RELATION OF ELECTRICAL AND MECHANICAL UNITS 73 
wire was reduced to volts by comparison on an accurate potentiometer 
with the E. M. F. of a Clark cell, the value of which being taken as 
1°4342 volts. There can be very little hesitation as to which of the 
units, ohm or Clark cell, to refer the error between the two values of 
the mechanical equivalent. As against a large number of exceedingly 
consistent measurements of the ohm we have only the one determina- 
tion of the Clark cell, which was made only indirectly. Moreover, there 
have been fairly recently two determinations of the value of the Clark 
cell, which were made by comparing the E. M. F. with that across a 
standardized resistance through which an electric current was flowing, 
the current being at the same time measured on an absolute electro- 
dynamometer. One determination was made by Kahle at the Reich- 
sanstalt, who obtained the value 1:4329 int. volts at 15° C., and the 
other was made by Carhart and Guthe at Michigan University which 
came 1°4333 int. volts. Although not agreeing particularly well they 
both show that the older value of the Clark cell obtained by Glazebrook 
and Skinner is too high. 
In view of this uncertainty and of the discrepancy in the values of 
the mechanical equivalent of heat it is interesting to calculate back in 
the equation of the electrical method for the value of the Clark cell by 
assuming the mean value of the equivalent as obtained in Reynolds and 
Moorby’s experiments. The value so obtained comes 1°43325 int. volts 
at 15° C., which is identical to the value obtained by Carhart and 
Guthe. A similar calculation by assuming Rowland’s value of the 
equivalent over the range of his experiments, 2.e., 6° and 36° C., with 
the value from the present experiments over the same range gives a 
value for the Clark cell equal to 1°4336 int. volts at 15°. This agrees 
to 3 parts in 10,000 with the value in terms of Reynolds and Moorby’s 
experiments. The agreement of the value obtained by meaning these 
two last determinations, which comes 1:4334, with the value obtained 
by meaning the two entirely independent absolute determinations which 
is seen to be 1:4331 int. volts, is so close as to be quite within the 
limits of error of the several experimental determinations upon which 
they depend, and gives a more satisfactory verification of the absolute 
values assumed for the international ohm, volt and by implication the 
ampere. 
