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, i in. in diameter, bent into U-shape, and 

 having vertical limbs about 6 in. long and 3 in. apart. Each limb 



ZZ^5 



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 i£ in. wide and 

 ^ 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. 



