899 
using it for this purpose. lf the dynamometrical part be constructed 
with two astatic movable coils we may consider this part as elec- 
trically constant and invariable. But we cannot succeed in rendering 
the galvanometer part electrically and mechanically constant and 
invariable. If this part be constructed with a movable coil and a 
permanent field magnet, this last cannot be considered as electrically 
constant for more than a few hours. With a movable magnet this 
forms one inconstant factor, the magnetism of the earth being another. 
The horizontal intensity of the earth-magnetism changes every year 
more than 0.01 to 0.03 percent and restricts the utility as a standard 
instrument to that value. But the variation in the magnetism of a 
permanent field magnet or a permanent movable magnet is probably 
many times greater. Consequently the instrument can only be expected 
to be of practical use during short consecutive periods in which no 
appreciable changes in the earth magnetism occur and when no stray 
fields are present. The influence of stray fields might perhaps be coun- 
teracted by judicially enclosing the whole instrument in a seamless 
cylindrical soft iron covering. 
We have still to consider another possible use of instruments with 
a parabolic law of deflection i.e. tor detecting or even measuring 
small changes in the horizontal intensity of the magnetism of the 
earth. An astatic electrodynamometrical system should be connected 
with a coil without iron core, moving in tbe earth field only, there 
being no field magnet. In that case the constant a depends on the 
horizontal intensity of the earth field. The galvanometric part should 
possess a sensibility many times greater than the dynamometrical 
part. For the exact measurement of the current strength a PEeLLat 
or KurviN standard instrument might be used, or perhaps a reliable 
potentiometer. I do not know whether a practical method might be 
worked out in this way ; I do not even think it offers any advantage 
over the classic methods. 
With the apparatus | had made for myself, I have been able to 
demonstrate ihe facts mentioned in this paper. In my instrument I 
found that, @ and 6 having been measured as carefully as possible, 
° 
Pay 
a current /—=W— generally failed to give an exact zero position. | 
ct 
was able to prove that this was to be expected ; the difference being 
caused by two circumstances: 1. the electrodynamometer not being 
an astatic one; 2. the instruments reacting mutually. Both circum- 
stances might have been partly eliminated by a construction which 
rendered the whole instrument perfectly symmetric about a horizontal 
plane passing through the centre of the movable magnet. 
