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.
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