858 H. A. Rowland—Diamagnetic Attachment to the Lantern. 
ratus which I used consisted of a horseshoe electro-magnet, made 
of an iron bar half an inch in diameter and about ten inches 
long, bent into the proper form, and surrounded with four or five 
layers of No. 16 wire. But the following apparatus will, with- 
out doubt, be found much more convenient. It can be made of 
any size, though the dimen- 
sions given will probably be 
found convenient. 
The apparatus is represented 
in fig. 1. To a straight bar of 
iron h, 7 in. long, 4 in. thick, 
and 2 in. wide, are attached two 
pieces e e of the same kind o 
iron by two set screws g g, which move in slots in the piece /. 
position of the helices. When the apparatus is to be used» 
only one kind of work it can be much simplified by doing 
2. Theory. 
_ Leta very small particle of a body whose coefficient of magnet: 
ization x is very small, and either positive or negative, be placed 
in a magnetic field of intensity R; it will then have an induc 
magnetic moment of xvR, where v is the volume of the ele- 
