and, by
increasing the battery current until the saturation point of the iron
is reached, the tractive force is _increased nearly 200 per cent._, as
shown by the dynamometer. A large portion of this resistance to the
slipping or skidding of the driving wheels is undoubtedly due to
direct magnetic attraction between the wheels and track, this
attraction depending upon the degree of magnetic saturation and the
relative mass of metal involved.

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. It will be seen
that the car here will not, with the traction circuit open, propel
itself up hill when one end of the track is raised more than 5 inches
above the table; but with the circuit energized it will readily ascend
the track as you now see it, with one end about 131/2, inches above the
other in a length of three feet, _or the equivalent of a 40 per cent.
grade_; and this could be increased still further if the motor had
power enough to propel itself against the force of gravity on a
steeper incline. As you will notice, the motor adheres very firmly to
the track and requires a considerable push to force it down this 40
per cent