202 Electro-Magnet. 
was formed from a bar of Swede’s iron three inches square and thir- 
ty inches long. Before bending the bar into the shape of a_horse- 
shoe, it was flattened on the edges, so as to form an octagonal prism, 
having a perimeter of 103 inches. ‘The other dimensions of 
magnet, as measured before winding it with wire, are as follows :— 
perpendicular height of the exterior arch of the horse-shoe 112 inches 
—around the outside from one pole to the other 29,°; inches—n- 
ternal distance between the poles 34 inches. 
he armature or lifter is formed from a piece of iron from the 
same bar, not flattened on the edges; it is nearly 3 inches square, 
94 inches long, and weighs 23 lbs. The upper surface is made per- 
fectly flat, except about an inch in the middle where the angles are 
rounded off so as to form a groove, into which the upper part of @ 
strong iron stirrup, surrounding the armature, fits somewhat loosely. 
The weight to be supported is fastened to the lower part of the stit- 
rup, and by means of the groove is made to bear directly on the cen- 
ter of the armature. 
For the purpose of suspending the magnet, a piece of round iron 
with an eye on one end, is firmly screwed into the crown of the arch 
~~ vand is attached to the cross beam of a frame, similar to that figured 
in the last number of the Journal. 
__»The magnet is wound with 26 strands of copper bell wire, covel- 
ed with cotton thread 31 feet long; about 18 inches of the ends are 
“left projecting, so that only 28 feet actually surround the iron; the 
aggregate length of the coils is therefore 728 feet. ach strand 's 
wound on a little less than an inch; in the middle of the horse-shoe 
it forms three thicknesses of wire, and on the ends or near the poles 
it is wound so as to form six thicknesses. 
Two small galvanic batteries are soldered to the wires of the mag 
net, one on each side of the supporting frame, in such a manner as © 
cause the poles to be instantaneously reversed, by merely dippimS: 
the batteries alternately into acid. ‘To render these as compact % 
possible, they are formed of concentric copper cylinders with eylin- 
ders of zinc plates interposed and so united as to form but one gal- 
vanic pair. Each of these batteries presents to the action of the acid 
measuring both surfaces of the plate, 47 square feet—they are I 
inches high and about 5 inches in diameter. 
In experimenting with this magnet, a battery containing 5 of 2 
square foot of zinc surface was first attached to the wires; with ie 
the magnet could not be made to support more than 500 Ibs. An- 
