W. N. Hill—Electro-Dynamometer for Large Currents. 15 
suspension arrangement is supported on the large coils and 
lacks steadiness. It should be placed upon a distinct standard, 
A support for the deflecting cecil when not in use, should be 
added and an arrangement for centering it quic 
Theory of the Instrument.—The expression for the strength of 
current is very simple. The weight found is that required to 
balance the deflective force and is observed at zero, so that the 
earth’s and local attractions are avoided, nor does the torsion 
of the suspension enter. Let 
5S = strength of current in i os he 
w» = weight used, in milligram 
¢ = length of weight-arm or distance from point where weight 
acts, to center of syst 
G = constant of large fe ae 
g = constant of small coil. 
C = constant of instrument or length of magnetic arm. 
By the theory of the electro-dynamometer, the force acting 
to deflect is represented by the expression ch xgX8, 
which 2 ee is the constant of the large coils or G, and g the 
constant of deflecting coil. This force acts with the arm C, 
and is Palanced by the weight acting with the arm Hence 
oe 
B's OGy 
The coils being large, G and g are readily ascertained from 
measurement. / is a known distance. O is the constant of the 
instrument and must be specially determined. With the instru- 
Ment in question, C was found by running the same currents 
through it and through Trowbri ge’s SoS aan, the con- 
stant of which was accurately known. ©, J, G, and g being 
known, it is evident that from weight found, the current may 
be obtained with little calculation. Or, a table m may be drawn 
up from which the values desired can be obtained by inspection. 
ith this instrument, which has many turns in the fix 
and movable coils, the deflections are powerful, requiring 
Weights to balance them large enough to give sufficient sensi- 
tiveness. 
The Asap table shows the weights required for currents 
from 21 to 80 rs, and from 91 to 100 webers, with my 
Rice ied as —* 
21 70 | 91 1092 ~ 1215 = *25 
22 06 | 2 9 07 | 92 11°16 24197 12°41 26 
7 06/28 103 ‘07 | 93 11°40 24/98 1266 ‘25 
