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IOWA ACADEMY OF SCIENCES 
end vessels, the manner in which the current is led to the mercury col- 
umn and the factor to be used for the calculation of the end correction. 
Under such conditions it is possible to reproduce mercury ohms with an 
accuracy of 1 in 50,000, i. e,, an accuracy far surpassing that with which 
the ohm can be determined in absolute measure. 
With the ampere and the volt the case is different. We cannot con- 
struct a standard of current but must make use of one of the effects of 
a current for its measurement. The best and most reliable method 
seems to be, to measure a current by means of the electrolysis of a sil- 
ver nitrate solution, the silver to be deposited upon a planinum bowl. The 
Chicago definition of the ampere states that a constant current of one 
ampere will deposit in a voltameter or coulometer, as I shall in the fol- 
lowing always call this instrument, 1.118 milligrams of silver in one 
second. 
While we cannot construct a cell whose electromotive force is exactly 
one volt, there are standard cells which have a definite and constant 
electromotive force if properly constructed. In 1893 only one of these 
was known, the Clark Standard Cell, and it was selected as a practical 
standard, its electromotive force being assumed at that time to be 1.434 
volts. 
Here we have then the definitions of the three fundamental electrical 
units and they were recommended by the international congress for 
adoption by the Governments represented. In the United States, Great 
Britain, Canada, Germany, Austria and France government action was 
taken, but in some important points these legal definitions are not quite 
in accordance with those adopted in Chicago.* * 
It is apparent that it is not necessary to define all three units, but 
the third can always, by Ohm’s law, be derived from the other two. The 
United States legalized all three, a procedure which was bound to lead 
to trouble if it should be found that one or two of them should be incor- 
rect, because it is hardly to be expected that Mother Nature will change 
Ohm’s law in spite of all congressional decisions. The first work which 
seemed to indicate that the value of the volt was incorrect was an 
investigation at the Reichsanstalt* in Germany; and at practically the 
same time t’wo researches carried out at the University of Michigan fully 
confirmed the results obtained, and also proved that the electro-chemical 
equivalent of silver depends greatly upon the specifications of the silver 
coulometer.* Soon other scientists took up the subject and the above 
mentioned results have received confirmation. Prominent among the 
investigators of the silver coulometer is Professor Richards* of Harvard, 
who designed an apparatus which we may call a porous cup coulometer, 
for which he claimed a higher degree of accuracy than could be obtained 
by the usual form. I made an extended research* comparing the vari- 
ous forms which had been used by the different experimenters on the 
subject and found Richards’ claim to be correct, and designed a form of 
*Wolft:, Bulletin, Bureau of Standards, 1, p. 39, 1904. 
=*=Kalile,Zeit. f. Instr. K. 18, pp. 229 and 267, 1898. 
*Patterson and Guthe, Pliys. Rev. 7, p. 257, 1898. 
Carliart and Guthe, Phys. Rev. 9, p. 288, 1899. 
*Richards, Collins, and Ileimrod, Proc. Am. Ac. 35, 123, 1899. 
Richards and Heimrod, Ibid. 37. p. 415, 1902. 
*Guthe, Phys. Rev. 19, p. 138, 1904. 
