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Bismuth and Calc-spar in Absolute Measure. 369 
to a distance such that the magnetic potential was negligably 
small. course, experiments were made on both sides of the 
center of the field in order to eliminate any want of symmetry, 
and the distances through which the coil moved were all care- 
fully measured with a dividing engine. 
In order to reduce the deflections of the galvanometer to 
absolute measure, an earth inductor was included in the circuit 
with the little coil and galvanometer and the deflections pro- 
duced by this were Compared with those produced by moving 
the little coil. These deflections were taken between every 
two observations with the little coil. 
The deflections due to moving the little coil, those due to 
the earth inductor and that due to pulling the coil away from 
the center are given in the following table: 
Distance 6. Distance ¢. 
ROM .00 eu ci 4407 9°655™ 6°363™ 
Earth inductor_.. 33°138°™ 33°137™ 33°162™ 
Drawing coil away from center....-------.-- 57°416™ 
In order to determine the proper quantities for substitution 
in the expression for the magnetic ieee of the field, it was 
necessary to measure, besides the deflections due to the little 
coil when moved through various distances and those due to 
_ the earth inductor. 
The mean radius of the small coil. .--.-- .--- == 3918" 
Namber of tum 2. oe ee 
Wain BEG ee ee = °18240" 
Depth of coil Oo oo acuaes te eee 
Integral area of earth inductor - --- -- Bees se = 20716-2™ 
Horizontal intensity of earth’s magnetism .--. = *1984°=* 
The remaining part of the experiment and the part that was 
attended with greatest difficulty, was to prepare little bars of 
