W. WN: Hili—Electro- Dynamometer for Large Currents. 11 
the coil to the zero point determined by the telescope, scale and 
mirror, This instrument gives good results. Measurements 
ean be made more rapidly with it than by the galvanometric 
method and without shunting. 
During the past year, the writer has been experimenting at 
the U. S. Torpedo Station with an electro-dynamometer, differ- 
ing from Trowbridge’s in the manner of observing or determin- 
ing the deflective power of the current. In its general plan, 
including the arrangement for taking the entire current to be 
measured, it follows Trowbridge’s form. : 
ig. 1 is a general view of the instrument. Figs. 2 and 3 
show the details of the suspended coil. The large tixed coils 
are made of copper ribbon 387™™ wide by 1$™™ thick. The 
turns are separated by ebonite rings and fastened together by 
brass rods and screw-nuts insulated by ebonite. The metal 
frame-work is similarly insulated from the coils. The suspen- 
sion arrangement is placed on the top of the fixed coils and in- 
sulated from them. The upper cylinder which carries the 
suspension pulley is capable of vertical motion and can be fixed 
in any position by set-screws. 
The deflecting coil (figs. 2 and 8) is made of copper ribbon 
49™™ wide by 14™ thick, fastened with insulated rivets. In 
the center of the coil and parallel with it, is a light brass rod 
or pointer. A copper rod in connection with the outer end of 
the coil has an iron or nickel-plated point, which dips in mer- 
cury contained in a double-walled metal cup, B, on the base- 
oard. A similar rod from the inner extremity of the coil, 
ends in an iron or nickel-plated cup, C, containing mercury. 
By means of a ring, the coil is hung directly under the metal 
cylinder, D, which lies centrally across the tops of the large 
el and close together. As represented in fig. 1, the large 
coils are connected “tandem.” he current would enter the 
left hand coil at the serew-post in front; from the other end of 
this coil, a thick wire leads to the metal cylinder lying across 
the large coils, making connection with the small ‘coil by its 
mercury cup, and from the mercury cup below a wire passes to 
one end of the other large coil. In order to prevent heating of 
the mereury connections, the plunger A is hollow and the cup 
B is double walled, so that a stream of cold water may be sent 
through them from the jar placed upon' the stand above the 
instrument, the necessary connections being made by means of 
small rubber tubes. 
can 
