H, A, Rowland—Absolute Unit of Electrical Resistance. 488 
Adjustments and tests.—The circle, F, must be parallel to coils 
of galvanometer, G. The circle and coils of galvanometer 
were first adjusted with their planes vertical and then adjusted 
in azimuth by measurement from the end of the bar, R. to the 
sides of the circle, F. e adjustment was always within 30’, 
which would only cause an error of one part in 25 
e needle must hang in the magnetic meridian by a fiber 
without torsion, and the coils must be parallel to it. These 
adjustments were carefully made, but, as has been shown, the 
error from this source is compensated. 
The needle must hang in the center of the galvanometer 
coils and on the axis of the circle. The error from this source 
is vanishingly small. ; 
The scale must be perpendicular to the line joining the zero 
point and the galvanometer needle, it must be level and not too 
much below the galvanometer needle. All errors from this 
source are partially or entirely compensated by the method of 
experiment. ‘ 
The induction coils, L, must be horizontal, and at the same 
level as the two galvanometers, so as not to produce any mag- 
netic action on them. The error from this source is exactly 
practically zero. The insulation of 
tators was carefully tes 
Method of Kaperiment.—As has been stated before, the 
method generally used was that of the first throw of the needle, 
though the method of recoil was also used. For the successful 
use of the first method a quickly vibrating needle and the 
damping inductor are indispensable, seeing that with a slow 
moving needle we can never be certain of its being at rest. By 
this method it is not necessary to have the needle at rest at the 
zero point, but, if it vibrates in an arc of only a millimeter or 
two, we have only to wait till it comes to rest at its point of 
greatest elongation on either side of the zero point and then 
reverse the commutator. The error by this method is in the 
direction of making the throw greater in proportion of the 
The smallest throw used was 
