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Page 32
The revolution of the driving axles and wheels is not interfered with
in the slightest, because in the former the axle boxes are outside the
path of the lines of force, and in the case of the latter because each
wheel practically forms a single pole piece, and in revolving presents
continuously a new point of contact, of the same polarity, to the
rail; the flow of the lines of force being most intense through the
lower half of the wheels, and on a perpendicular line connecting the
center of the axle with the rail. In winter all that is necessary is
to provide each motor car with a suitable brush for cleaning the track
rails sufficiently to enable the wheels to make good contact
therewith, and any tendency to slipping or skidding may be effectually
checked. By this means it is easily possible to increase the tractive
adhesion of an ordinary railway motor from 50 to 100 per cent.,
without any increase in the load or weight upon the track; for it must
be remembered that even that portion of the increased friction due to
direct attraction does not increase the weight upon the roadbed, as
this attraction is mutual between the wheels and track rails; and if
this car and track were placed upon a scale and the circuit closed, it
would not weigh a single ounce more than with the circuit open.
It is obvious that this increase in friction between two moving
surfaces can also be applied to _check_, as well as augment, the
tractive power of a car or train of cars, and I have shown in
connection with this model a system of braking that is intended to be
used in conjunction with the electro-magnetic traction system just
described. You will have noticed that in the experiments with the
traction circuit the brake shoes here have remained idle; that is to
say, they have not been attracted to the magnetized wheels. 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.
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 being, made with the object of increasing friction by
means of permanent magnetism, and also with a view to _diminishing_
the friction of revolving and other moving surfaces, the results of
which will probably form the subject matter of a subsequent paper.
Enough has been said to indicate that the development of these two
methods of increasing mechanical friction opens up a new and extensive
field of operation, and enables electricity to score another important
point in the present age of progress. The great range and flexibility
of this method peculiarly adapt it to the purposes we have considered
and to numerous others that will doubtless suggest themselves to you.
Its application to the increase of the tractive adhesion of railway
motors is probably its most prominent and valuable feature at present,
and is calculated to act as an important stimulus to the practical
introduction of electric railways on our city streets, inasmuch as the
claims heretofore made for cable traction in this respect are now no
longer exclusively its own. On trunk line railways the use of sand and
other objectionable traction-increasing appliances will be entirely
dispensed with, and locomotives will be enabled to run at greater
speed with less slipping of the wheels and less danger of derailment.
Their tractive power can be nearly doubled without any increase in
weight, enabling them to draw heavier trains and surmount steeper
grades without imposing additional weight or strain upon bridges and
other parts of the roadbed. Inertia of heavy trains can be more
readily overcome, loss of time due to slippery tracks obviated, and
the momentum of the train at full speed almost instantly checked by
_one and the same means_.
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