560 T. Crook — A Form of Permanent Magnet, 



VI. — A Simple Form; of Permanent Magnet suitable for the 



Separation of Weakly Magnetic Minerals. 



By T. Crook, F.G.S. 



rpHE separation of the weakly magnetic minerals of crushed rocks, 

 J_ sands, etc., is best effected by using an electromagnet, which, 

 if suitably constructed, is, for any given size, much stronger than 

 a permanent magnet.' The electromagnet exerts no attractive force 

 when the current is shut off, and for this reason the process of separation 

 is made easy, as the grains which have been attracted to the poles 

 drop when the current is switched off. 



An ordinary permanent magnet of considerable strength has to 

 be rather large ; it requires re-magnetising at intervals ; and to 

 detach grains which have been attracted, it is necessary to strike them 

 sharply with some object such as a suitably folded piece of paper or 

 cardboard. Hence for speed, efficiency, and convenience in use, the 

 electromagnet easily supersedes the permanent magnet. However, 

 if an electric current is not available, as happens to be the case with 

 the prospector, it becomes a decided advantage to have some handy 

 form of permanent magnet capable of yielding an intense magnetic 

 field. An electromagnet put into action by a hand dynamo has been 

 tried, but such an apparatus makes a heavy load, and the dynamo 

 readily becomes impaired. Indeed, under any conditions the use of 

 a hand dynamo in making mineral separations is clumsy and inefficient, 

 and involves unnecessary expense. 



Permanent magnet with adjustable poles for securing a magnetic field of high but 

 variable intensity. 



It is therefore of interest to note that a simple permanent magnet 

 of handy size can be made use of quite effectively in examining 

 a small quantity of sand or crushed rock. It is convenient to have 

 the magnet made U-shaped, the limbs being about 6 inches long. The 



' For an account of the iise of the electromagnet in this way see "The use of the 

 Electromagnet in Petrography," Science Progress, No. .5, July, 1907. 



