and instead of the pulleys or water-wheel we take an iron ring or cylinder also wound with copper wire, and allow a current of electricity to pass through the wire, the iron will become magnetic and the "armature" A will revolve in the direction indicated by the large arrows, because the magnetic force is acting as indicated by the little darts; the dotted darts represent the repelling action, the other darts the pulling action. Now, inasmuch as the armature revolves in the direction shown, because the left hand pole of the field magnet is S and the right N, it follows that if their polarity is changed, the polarity of the armature remaining as before, it will and must revolve in the opposite direction. This change of polarity is easily accomplished by causing the electric current that flows round the cores of the magnets to flow in the opposite direction. How that portion of the armature ring situate at n, s maintains its signs, viz., how the northness and southness remain there whilst the iron itself is being rapidly revolved, that northness and southness being the reason of the revolution, would take too long to explain. Sufficient to say that this is accomplished, and the parts fulfilling the object are called "commutators" and "brushes," two important and troublesome items in electric-traction work. The foregoing description will show how essentially the electric motor differs from the steam-engine, with its cylinder, reciprocating piston, crank and fly wheel, and complicated link motion and double eccentric for reversal. No coal, no smoke, no ashes, no water, no steam, no noise, and considerably less trouble in management, less cost, less risk, less liability to derangement, a few hundredweight instead of a few tons to move along. Let attention be drawn to a point of difference between a steam-engine and an electric motor, not in favour of the latter, a fact, which the failure to fully recognize has led to many failures in the use of electric motors. The revolution of the engine-crank is due to the pressure of steam admitted into the cylinder; if the load be too heavy or the hill too steep, the engine can't move, although the valves are wide open and the full boiler pressure is acting upon the piston. What is the result? Simply that the cylinder containing the steam may be regarded as an extension of the steam-space of the boiler; the parts are all strong enough to bear the pressure, and if the engine does not move, nothing comparatively is wasted. The load must be lightened, or engines with larger cylinders must be employed. The revolution of the armature of an electric motor may be regarded as due to magnetic attraction and repulsion, as previously explained that magnetic action is due to the flow of electricity through the wires that surround the magnets and armatures. The greater the flow of current the greater the magnetic force, and, therefore, the greater the torque, or turning power of the armature. But assuming again that the load is too heavy or the hill too steep, the cars stopped do not, as in the case of the steamengine, stop the waste of energy, but permit a greater waste. There is nothing but the resistance of the windings of the motor to prevent the full flow of the current; and unless the car moves or the electricity is switched off, the wires will probably be burnt through and the motor spoilt. This is one of the reasons why, in my first work, I adopted the gramme ring instead of the drum armature, and why, after much experience, others are now doing the same. If, therefore, a motor restrained from movement permits a waste of electricity, it follows that the quicker the movement the less the waste. Hence all electric motors to be simple and efficient must revolve at a high rate of speed. This is a source of trouble and difficulty in their application to traction work, necessitating the introduction of reducing gear. Perhaps no part of electric traction work has received more attention and absorbed more money than this of gearing. If the important items of first cost and available space were not prohibitory, the motors might be applied to tramcars, having armatures acting directly upon the axles or wheels. Given a new design and structure of car, this may be economically possible, but for the present, gearing of some form must be regarded as a necessary concomitant, no matter how objectionable. If, when a motor is revolving at a high rate of speed, it gives the greatest efficiency, it follows that those who have given attention to this subject should try and devise some sort of graduated gearing that will permit the motor always to run at its best speed and the relation between it and the axles to vary. Say the motor runs at 100 revols. per min., and it is desired to start the car, and for the first half minute the speed of the axle is 10 revols. per min., then the relation would be 1001. At the maximum speed of 10 miles an hour the speed is say 100, then the relation would be 10: 1. Though not stating that this problem is insolvable, it may be safely asserted that no economic method has yet been advanced. Economic in this sense comprises not only the loss in transmission, but also first cost, applicability, wear and tear, maintenance and renewal. Devices for this purpose are so constantly being re-invented, that with the object of preventing thought, time, and money from being expended in their repetition, it would be well to put some of them on record. Let A, A (fig. 4) represent the axles of a tramcar, B, B the X wheels, M the motor having a small pulley on its armature spindle communicating motion by means of a belt to a large pulley on the cone shaft C. A second belt passing from C to C1 will vary the relative speeds of these two shafts, according to the position it is made to assume. When the car is standing and it is desired to start, the belt would be at x; when the car has started the belt may be gradually slided to y. Motion may be communicated to the axle either by belting or gearing ff. This has not only been proposed, but I believe actually tried. Of course the slipping of the belts, the difficulty of sliding to and fro, renders it impracticable. Д B B C y A better method is the following (fig. 5):-M is the motor, having a bevel pinion, 1, keyed on, its armature spindle actuating the two bevel wheels 2, 2, which will of course run in opposite directions, each bevel wheel having on its inner face a smooth disc, 3, 3, pressed inward by an arrangement of springs. These discs nip a small rubber pulley, 4, on the end of a spindle, free to slide in and out of the second bevel pinion, 5, which gears with the wheel keyed on to the car axle. It is obvious that the nearer the pinion is to the centre of the discs the slower its speed, and the further it is drawn out the greater its speed. |