44 SCIENCE PROGRESS 
sands in the United States.1 Such an instrument, however, is a 
decided luxury, and goes far beyond the needs of the average 
laboratory investigation. For ordinary purposes it suffices to 
use a magnet such as that illustrated by the accompanying 
figure. This electromagnet is one of the ordinary pattern, and 
is supported on a wooden frame. It consists of a cylindrical 
piece of soft iron, 1 in. in diameter, bent into U-shape, and 
having vertical limbs about 6in. long and 3 in. apart. Each limb 
is provided with a bobbin, on which is wound seven layers of 
insulated wire (16-gauge), each layer having about forty turns. 
Two pole pieces, consisting of soft iron about 1} in. wide and 
4 in. thick, are slotted so as to move over screw clamps which 
fit into the ends of the limbs. By this means the pole pieces 
can be adjusted in any desired position. As already pointed 
out, this was the device adopted by Faraday to secure a magnetic 
field of strong but variable intensity. 
1 Investigation of Black Sands from Placer Mines, by Day and Richards. 
Bulletin No. 285, p. 150, United States Geological Survey, 1906. 
