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.
But by far the greatest proportion of the increased friction is purely the result of the change in position of the iron molecules due to the well known action of magnetism, which causes a direct and close interlocking action, so to speak, between the molecules of the two surfaces in contact. This may be illustrated by drawing a very thin knife blade over the poles of an ordinary electro-magnet, first with the current on and then off.
In the model before you, the helices are fixed firmly to, and revolve with, the axles, the connections being maintained by brushes bearing upon contact rings at each end of the helices. If desired, however, the axles may revolve loosely within the helices, and instead of the latter being connected for cumulative effects, they may be arranged in other ways so as to produce either subsequent or opposing magnetic forces, leaving certain portions of the circuit neutral and concentrating the lines of force wherever they maybe most desirable. Such a disposition will prove of advantage in some cases.
The amount of current required to obtain this increased adhesion in practice is extremely small, and may be entirely neglected when compared to the great benefits derived. The system is very simple and inexpensive, and the amount of traction secured is entirely within the control of the motor man, as in the electric system. It will be seen that the car here will not, with the traction circuit open, propel itself up hill when one end of the track is raised more than 5 inches above the table; but with the circuit energized it will readily ascend the track as you now see it, with one end about 13½, inches above the other in a length of three feet, or the equivalent of a 40 per cent. grade; and this could be increased still further if the motor had power enough to propel itself against the force of gravity on a steeper incline. As you will notice, the motor adheres very firmly to the track and requires a considerable push to force it down this 40 per cent. grade, whereas with the traction circuit open it slips down in very short order, notwithstanding the efforts of the driving mechanism to propel it up.
The resistance of the helices on this model is less than two ohms, and this will scarcely be exceeded when applied to a full sized car, the current from two or three cells of secondary batteries being probably sufficient to energize them.
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.
Read before the American Association for the Advancement of Science. New York meeting, 1887.