THE EAETH’S MAGNETIC FIELD IN INTERNATIONAL UNITS. 
45!) 
the 1000 a.nd the 300-ohm standard coils, also a single cadmium cell. The other 
circuit included two of the 100-ohm standard coils, the 1-ohm standard coil and tlie 
sub-divided ohm, which had been calibrated at the Reichsanstalt. This circuit also 
included two Clark cells placed in series with three cadmium cells, the Clark’s and 
cadmiums being placed so as to oppose each other. 
When jDerforming an ex23eriment the resistance of the main circuit was adjusted 
till no current passed in the circuit containing the single cadmium cell, so that the 
current was known in terms of the E.M.F. of a cadmium cell. Then the switch was 
moved over and the plugs in the sub-divided ohm were adjusted till there was no 
current in the circuit containing the two Clark’s and three cadmiums ; when the 
E.M.F. of the combination was equal to the product of the resistance included in this 
potentiometer ch’cuit into the current flowing in the main circuit, which was itself 
known from the previous balancing. By one or two successive approximations it was 
possible to arrange so that there was balance for both j3ositions of the switch, or, at 
any rate, that the position of balance lay between two-tenths of an ohm in the circuit 
containing the two Clark’s and three cadmiums, when the resistance for exact balance 
was obtained by galvanometer deflections. This arrangement was found very 
convenient, and by simply changing one connection, so as to make one of the 
secondary circuits include only the sub-divided ohm, it could be used for comparing 
the cells of the two kinds among themselves. In this w*ay it was found that the ratio 
of the Clark’s at 0° to the old cadmiums at 20° was 1‘4227, and the ratio to the new 
cadmiums was 1’4224. 
From these results it follows that if the E.M.F. of the old cadmium cells is UOIOO!), 
as found from the silver depositions, then the E.M.F. of the new cadmium cells at 20° 
is 1'01926, and that of the Clark cells at 0° is 1'4498, the E.M.F. being in each case 
expressed in international volts. Taking the mean of the old and new cadmiums, the 
E.M.F. of this ty^^e of cell is 1‘01917. 
When considering the values for the E.M.F. of these two tyjDes of cells -we liave to 
distinguish two classes of determinations, viz., tliose in which the E.M.F. has been 
detei'inined directly in absolute measure and those in which the current used to 
measure the E.M.F. has itself been measured by means of the silver voltameter. The 
results obtained by the first class will be exjDressed in C.G.S. units, while those in the 
second class wull be in international volts. The following table, as far as the author 
Is aware, contains all the measurements with any pretensions to accuracy whicli have 
been made up to the present time. The class to which the various measurements 
belong is indicated by the column in which the result obtained Is set down. Thus all 
the results given in terms of international volts were obtained by the use of the silver 
voltameter. 
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