But by far the greatest proportion of the increased friction is purely
the result of the change in position of the iron molecules due to the
well known action of magnetism, which causes a direct and close
_interlocking action_, so to speak, between the molecules of the two
surfaces in contact. This may be illustrated by drawing a very thin
knife blade over the poles of an ordinary electro-magnet, first with
the current on and then off.
In the model before you, the helices are fixed firmly to, and revolve
with, the axles, the connections being maintained by brushes bearing
upon contact rings at each end of the helices. If desired, however,
the axles may revolve loosely within the helices, and instead of the
latter being connected for cumulative effects, they may be arranged in
other ways so as to produce either subsequent or opposing magnetic
forces, leaving certain portions of the circuit neutral and
concentrating the lines of force wherever they maybe most desirable.
Such a disposition will prove of advantage in some cases.
The amount of current required to obtain this increased adhesion in
practice is extremely small, and may be entirely neglected when
compared to the great benefits derived. The system is very simple and
inexpensive, and the amount of traction secured is entirely within the
control of the motor man, as in the electric system.
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