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Page 31
In the course of the experiments above described, another and somewhat
different method of increasing the traction of railway motors has been
devised, which is more particularly adapted to electric motors for
street railways, and is intended to be used in connection with a
system of electric street railways now being developed by the author.
In this system _electro-magnetism_ provides the means whereby the
increase in tractive adhesion is produced, and this result is attained
in an entirely novel manner. Several attempts have heretofore been
made to utilize magnetism for this purpose, but apparently without
success, chiefly because of the crude and imperfect manner in which
most of these attempts have been carried out.
The present system owes its efficiency to the formation of _a complete
and constantly closed magnetic circuit_, moving with the vehicle and
completed through the two driving axles, wheels, and that portion of
the track rails lying between the two pairs of wheels, in a manner
similar to that employed in the electrical method before shown. We
have here a model of a second motor car equipped with the apparatus,
mounted on a section of track and provided with means for measuring
the amount of tractive force exerted both with and without the passage
of the current.
You will notice that each axle of the motor car is wound with a helix
of insulated wire, the helices in the present instance being divided
to permit the attachment to the axles of the motor connections. The
helices on both axles are so connected that, when energized, they
induce magnetic lines of force that flow in the same direction through
the magnetic circuit. There are, therefore, four points at which the
circuit is maintained closed by the rolling wheels, and as the
resistance to the flow of the lines of force is greatest at these
points, the magnetic saturation there is more intense, and produces
the most effective result just where it is most required. Now, when
the battery circuit is closed through the helices, it will be observed
that the torque, or pull, exerted by the motor car is fully twice that
exerted by the motor with the traction circuit open, 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 13�, 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. grade, whereas with the traction circuit open it slips down
in very short order, notwithstanding the efforts of the driving
mechanism to propel it up.
The resistance of the helices on this model is less than two ohms, and
this will scarcely be exceeded when applied to a full sized car, the
current from two or three cells of secondary batteries being probably
sufficient to energize them.
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