236 
IOWA ACADEMY OF SCIENCES 
moment produced by the electromagnetic action between the two coils 
was balanced by an accurately measured angular twist of the suspend- 
ing phosphor-bronze wire, i. e., by a torsional moment which may be 
determined in terms of centimeters, grams, and seconds. To obtain this 
moment a cylinder of known moment of inertia w’^as vibrated when sus- 
pended from the same wire as the movable coil and the period of its tor- 
sional vibrations determined. 
Let K be the moment of inertia, T the time of vibration of the cylin- 
der, then the torsional moment for unit angle of twist of the suspension 
is given by the equation, 
j=i£ilK 
The dimensions of the two coils of the electrodynamometer were so 
chosen that the field strength inside the stationary could be expressed by 
the simple equation. 
4piN 
I=C I 
y 
w'here N is the number of turns of wire on the coil, D its diameter, 
L its length and I the current flowing through it; and the effective area 
of the movable coil by the equation 
A = pi r-n, 
where r is the average radius of the movable coil and n the number of 
turns. The torsional moment produced by the current I flowing through 
the electrodynamometer is then, with the coils at right angles to each 
other T^=ACr’; this was balanced by the torsional moment of the sus- 
pension wdien the torsion head was turned 90 degrees, or n over 2 radians. 
Solving for the current, we obtain as final formula: 
I /pi IV 
' 2N n 
As you see, only measurements of length, mass and time enter into 
the calculation and this is therefore an absolute determination of the 
current. 
This current was sent through resistance standards and the differ- 
ence of potential produced compared by means of an accurately calibrated 
potentiometer with the electromotive force of a dozen standard cells. 
The absolute value of the latter is then easily calculated from the 
current and resistance by means of Ohm’s law. 
The electrochemical equivalent of silver had been previously deter- 
mined* by direct comparison between the electromotive force of one of 
the standard cells and the potential difference at tae terminals of the 
same standard resistances w^hen a constant silver depositing current 
passed through it. 
The standard cells were kindly given to me by Prv)fessors Carhart and 
Hulett. Most of them were set up with electrolytically prepared Hg 2 S 04 
It is needless to say that in the actual measurements a great many 
difficulties were encountered; but thanks to the valuable help of my 
*Guthe, Phys. Rev. 19, p. 152, 1904. 
