the hopper would fall into that portion of the bin lying to the left
of the partition. If, however, the magnet is energized from a source of
current, the magnetic particles in the falling stream are attracted
by and move toward the magnet, which is so placed with relation to
the falling material that the magnetic particles cannot be attracted
entirely to the magnet before gravity has carried them past. Hence,
their trajectory is altered, and they fall on the right-hand side of
the partition in the bin, while the non-magnetic portion of the stream
continues in a straight line and falls on the other side, thus effecting
a complete separation.

This simple but effective principle was the one employed by Edison
in his great concentrating plant already described. In practice, the
numerous hoppers, magnets, and bins were many feet in length; and they
were arranged in batteries of varied magnetic strength, in order
that the intermingled mass of crushed rock and iron ore might be
more thoroughly separated by being passed through magnetic fields of
successively increasing degrees of attracting power. Altogether there
were about four hundred and eighty of these immense magnets in the
plant, distributed in various buildings in batteries as above mentioned,
the crushed rock containing the iron ore being delivered to them by
conveyors, and the gangue and ore being taken away after separation by
two other conveyors and delivered elsewhere. The magnetic separators at
first used by Edison at this plant were of the same generality as the
ones employed some years previously in the separation of sea-shore sand,
but greatly enlarged and improved. The varied experiences gained in
the concentration of vast quantities of ore led naturally to a greater
development, and several new types and arrangements of magnetic
separators were evolved and elaborated by him from first to last, during
the progress of the work at the concentrating plant.

The magnetic separation of iron from its ore being the foundation idea
of the inventions now under discussion, a consideration of the separator
has naturally taken precedence over those of collateral but inseparable
interest. The ore-bearing rock, however, must first be ground to powder
before it can be separated; hence, we will now begin at the root of
this operation and consider the "giant rolls," which Edison devised
for breaking huge masses of rock. In his application for United States
Patent No. 672,6