1908 ] 
Monazite and Monazite Mining 
79 
These iron bars do not become permanent magnets but form elec- 
tro-magnets as long as the current flows around them. They 
can be given a greater and more constant strength than can 
be given to the permanent steel magnets and for this reason, in 
nearly all of the magnetic processes, electro-magnets are used 
instead of the field magnets. 
The magnetism of these electromagnets can be varied and dif- 
ferent intensities obtained ranging from indefinitely weak to a 
certain maximum strength. It is also possible to control the 
intensity of any magnetic field so that minerals that are strongly 
attracted may be separated from minerals that require a magnetic 
field of much higher intensity. This intensity of the magnetic 
field depends: 
1. On the size of the magnet. 
2. On the shape of the magnet. 
3. On the distance between the magnet and the body to be 
attracted. 
4. On the number of amperes turns in the magnet coil, that is, 
the product of the amperes or current flowing in the coil times the 
number of turns around the core. 
There are many substances that are attracted by electro-mag- 
nets that are not influenced apparently at all by the strongest steel 
magnet and for this reason, formerly many substances which were 
considered non -magnetic, have been proved to be magnetic when 
subjected to the intense magnetic field obtained in an electromag- 
netic separator. All substances are of course either attracted or 
repelled by magnets and the former are called para-magnetic and 
the latter dia-magnetic. The latter class is the most numerous, 
but since the introduction of electromagnets, the former class, 
which up to this time had been considered extremely small, has been 
largely increased . The paramagnetic substances are the metals iron , 
nickel, cobalt, manganese, chromium, cerium, palladium, plat- 
inum, osmium and many of their salts and compounds. The degree 
of attraction of these varies very widely and, as an illustration 
between a strong and weak magnetic substance, it has been esti- 
mated that if the attraction of steel be taken at 100,000, then 
magnetite would be 65,000, siderite 120, hematite, 93 to 43, lim- 
