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.
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