10 W. N. Mill—LHlectro-Dynamometer for Large Currents. 
Art. IL—An senile ayaa for hate Large Our- 
rents; by WALTER N. Hr11, 8.B. (Harvard), Chemist, U.S. 
Torpedo Beason! Newport, B. L 
THE use of electric machines of large size, for the generation 
of currents of great strength, has become extensive and prom- 
ises to increase materially. In connection with this, the best 
mode of measuring the currents obtained is a matter of much 
importance, as well as one of some difficulty. 
Probably at the present time, the method by the use of the 
galvanometer—heavily shunted—and that involving the deter- 
mination of the heat developed in the circuit are the most used, 
ut they are objectionable Hom their inconvenience, complex- 
ity and liability to many err 
The method employing the’ electro-dynamometer is to be 
preferred for many reasons‘ and it has also the advantage of 
being applicable to to-and-fro currents, as well as to those in 
one direction. 
eber’s form of the electro- dynamometer is an instrument 
which, as Clerk- Maxwell says, “is probably the best fitted for 
absolute measurements.” In this , one coil is suspended within 
another, the suspension being a fine wire through which the 
current is led to the suspended coil. It is therefore only suit- 
able for the direct measurement of very small currents. If 
currents of greater strength are pris the suspending wire 
is heated and elongated. It is consequently necessary in meas- 
uring powerful eon with Weber’s electro-dynamometer to 
use very large shunts. 
As has been pointed out by Trowbridge, it is very desirable 
to avoid the use of shunts, since the entire resistance of the 
circuit is of the same order of magnitude asthe shunt. In gen- 
eral, a method depending upon the measurement of a very small 
proportional part of the whole current is objectionable, since 
very great a is necessary and errors of observation are 
exa aggerated. 
whridge has designed an electro-dynamometer through 
which aie currents may be transmitted and directly measured. 
Proc. Am. Acad. Arts and Sci., Oct. 9, 1878). It consists 
essentially oe two large fixed coils made from copper bands, be- 
tween which is suspended, from a torsion head, a small coil— 
the whole so arranged that by means of mercury connections 
the entire current will traverse the smaller coil, as well as the 
larger ones. small mirror is attached to the deflecting coil 
and by means of a cnleacaie and scale, the deflections may be 
read. In practice, however, Trowbridge found that the better 
mode of observation was to bring back, by the torsion head, 
