A. M. Mayer—New form of Lantern-Galvanometer. 417 
render the latter more or less astatic. Also, in case the needle 
with poles reversed. 
In working with thermal currents we use a smaller needle 
and condenser which allows the spirals to approach nearer; but 
for thermal currents it is better to wind close around the needle 
under the control of the damping magnets.* The breadth of 
the coil used in this last device need not exceed ,';th of an inch, 
and its image on the screen can answer for a rough zero point. 
I will now give a few experiments in which this galvanome- 
ter has been used ; and they will serve to show its usefulness. 
Experiment 1. A coil of 24 feet in diameter, containing 40 
turns of 300 feet of ;';th inch wire, was placed, with its plane, at 
right angles to “the dip.” Its terminals were connec h 
the galvanometer whose needle was rendered astatic by means 
of the the damping magnets, I now quickly rotated the coil 
80° around an axis at right angles to the direction of dipping- 
needle. The galvanometer needle was deflected about 12° by 
the magneto-electric current induced by the earth’s magnetism. 
cp. 2. I poyed the coil, used in Exp. 1, on a wooden wheel 
provided with a commutator and rotated it around an axis at 
right angles to the dip. The galvanometer-needle went steadily 
up to a deflection of 85° and was held there as long as the coil 
revolved. 
_ «Hep. 8. The two cores of the large electro-magnet of the 
Stevens Institute of Technology were placed end to end, thus 
oe one iron bar, seven feet long and six inches in diam- 
. 1S was surrounded by its eight bobbins contaming 1n 
all 2000 feet of 4 inch copper wire; and through them was 
sunt the electricity dovdiopsd by the most advantageous com- 
bination of 60 plates of zine and carbon, 10X8 inches. 
whionh® UPPer needle of this astatic combination swings in the interior of the coil 
incloses e needle and the condenser c; the lower un- 
Am. Jour. Sct.—Turrp Serres, Von. III, No. 18.—Jons, 1872 
27 
