epelled_ 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.

The method thus described is equally applicable to increase the
coefficient of friction in apparatus for the transmission of power,
its chief advantage for this purpose being the ease and facility with
which the amount of friction between the wheels can be varied to suit
different requirements, or increased and diminished (either
automatically or manually) according to the nature of the work being
done. With soft iron contact surfaces the variation in friction is
very rapid and sensitive to slight changes in current strength, and
this fact may prove of value in connection with its application to
regulating and measuring apparatus. In all cases the point to be
observed is to maintain a closed magnetic circuit of low resistance
through the two or more surfaces the friction of which it is desired
to increase, and the same rule holds good with respect to the electric
system, except that in the latter case the best effects are obtained
when the area of surface in contact is smallest.

For large contact areas the magnetic system is found to be most
economical, and this system might possibly be used to advantage to
prevent slipping of short wire ropes and belts upon their driving
pulleys, in cases where longer belts are inapplicable as in the
driving of dynamos and other machinery. Experiments have also been,
and are still be