THE EARTH’S .MAGNETIC FIELD IN INTERNATIONAL UNITS. 
4(31 
cells as a standard in their determinations. Hence we may apply the difterence found 
between Kahle’s cells and those of Glazebrook and Skinner to PtAYLEiGii’s cells. 
Thus, reduced to the German type of cell, the values obtained by these two observers 
for the E.M.F. of a Clark cell at 15° are :—- 
C.G.S. 
Internationiil volts. 
Rayleigh . . . 
1-4339 
1-4339 
Kahle. 
1-4324 
1-4328 
Difference .... 
0-0015 
0-0011 
The divergence between the two results is still greater than one would exjiect, taking 
into consideration the precautions taken in the exjieriments. 
The only experimenter who has made accurate measurements of the E.M.F. of the 
cadmium cell is Kahle. 
Taking the mean of Rayleigh’s and Kahle’s values for the E.M.F. of the Clark 
cell as being the E.M.F. of the South Kensington Clark’s, and Kahle’s value for 
the E.M.F. of the cadmium cell as corresponding to the mean of all the cadmium 
cells set up at South Kensington, we can calculate what is the E.M.F. of the old 
cadmiums as deduced from these various data. The results, together with that 
deduced from the silver depositions, are given in the following table. 
E.M.F. of the Old Cadmium Cells at 20°. 
International volts. 
From silver depositions. 
1-01909 
From Kahle’s value tor Cd. cells .... 
1-01855 
From the Clark cells. 
1-01898 
Mean. 
1-01888 
Hence, giving ecjual weight to each of these methods of obtaining the E.M.F. of the 
cadmium cells used in the magnetic experiments, we get a value 0’00021 volt lower 
than that used in the reductions already considered. Making this change we find 
that the difference between the values of H, as given by the coil and by the 
observatoiy instruments, is — 0'00002 C.G.S. unit. 
Using all the available data, we may therefore say that the number given by the 
Kew Observatory standard magnetic instruments for the horizontal component of the 
