This is
because a portion of the traction current has been circulating around
this coil on the iron brake beam, inducing in the brake shoes
magnetism of like polarity to that in the wheels to which they apply.
They have therefore been _repelled_ from the wheel tires instead of
being attracted to them. Suppose now that it is desired to stop the
motor car; instead of opening the traction circuit, the current
flowing through the helices is simply reversed by means of this pole
changing switch, whereupon the axles are magnetized in the opposite
direction and the brake shoes are instantly drawn to the wheels with a
very great pressure, as the current in the helices and brake coil now
assist each other in setting up a very strong magnetic flow,
sufficient to bring the motor car almost to an instant stop, if
desired.
The same tractive force that has previously been applied to increase
the tractive adhesion now exercises its influence upon the brake shoes
and wheels, with the result of not only causing a very powerful
pressure between the two surfaces due to the magnetic attraction, but
offering an extremely large frictional resistance in virtue of the
molecular interlocking action before referred to. As shown in the
present instance, a portion of the current still flows through the
traction circuit and prevents the skidding of the wheels.
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