Now, assume the current to be flowing and the magnet to be charged, and let us take a piece of metal which has an electrical resistance of 1/100 of an ohm, and lay it across the conductors at any point between the battery and the magnet. The result is, that instead of flowing through 10 ohms resistance via the magnet, it follows the invariable rule, and takes that offering but 1/100 of an ohm; or, more to the point, if we assume the conductors referred to to be one mile of steel rails each (Fig. 3), and again leave their resistance (which would be about one ohm each) out of consideration entirely, leaving that of the magnet as first stated, and assume the bar of 1/100 of an ohm to be an axle and pair of wheels (a) of a train (Fig. 4), which possess the same resistance, we can readily see that the result would be exactly the same, i.e., instead of all the current passing through the magnets, as when the rails were unoccupied, the presence of the wheels upon them would cause 999/1000 of the current to leave the magnet and pass through them; they offering but 999/1000 of the resistance of the magnets, and thus leaving but 1/1000 of the whole current passing through them, which being so small a part of so feeble a current is imperceptible and without sufficient influence to hold the magnet charged. Therefore, it follows that the instant that a pair of wheels enters upon a pair of rails which thus form part of the conductors of an electrical current holding charged a magnet, that magnet becomes practically demagnetized, and consequently loses all power to overcome any opposing force in its armature.
When the armature of a magnet is arranged upon a small lever, by motion of which a second circuit is closed or opened, or two or more circuits are otherwise controlled, the entire device is termed a relay. In all forms of this instrument, as is the case with almost every other electrical instrument, the armature is so arranged as to fall by gravity, or by tension of a small spring suitably arranged, away from the cores of the magnets when they become demagnetized.
Fig. 5
When switches are included in a track section (Fig. 5), it becomes necessary for safety to have them control the track section in such a way that unless they are properly set (and locked, if desired) for the main track, the continuity of the rail circuit is interrupted and the signal is thereby held at danger. To render more certain this result, the circuit controller (switch box) at the switch is arranged in such a way that the track circuit is not only interrupted beyond the switch, but is also short-circuited by it when the switch is not properly set. It is also necessary for safety that the side track from the switch points back to the fouling point (Fig. 5) be included in the track section: thus insuring that all trains on these tracks are out of danger of collision with the main track when a "clear" signal is displayed on it.
In dividing tracks into distinct electrical sections, it becomes necessary to insulate the rail ends at the terminal of each, from those of the adjacent sections. If this were not done the current of each section would traverse the next, and continue on indefinitely, influencing each other so as to interfere with or totally prevent the operation of all.
In order that we may fully comprehend the nature of an insulation, let us make clear a few facts concerning conductors in general. All materials conduct electricity to a certain extent; but some with much more freedom than others. Thus, silver, copper, gold, zinc, platinum, iron, steel, mercury, and other pure metals permit the passage of an electric current through them with but slight resistance, (although all offer a certain amount,) and are therefore termed conductors. The following liquids are classed as conductors: concentrated and diluted acids, saline liquids and water, although they are much less efficient as such than the metals.
To this list might be added the earth itself and the various ingredients forming it, the nature of which ingredients determines very much its efficiency as a conductor. Thus at points abounding in mineral deposits the earth would be far superior as a conductor to those parts in which none exist, but at best should be regarded as a poor conductor.
Next comes a class of materials which offer a great resistance to the current, and which from that reason are termed non-conductors, or insulators; of this class, rubber, glass, leather, resin, wood, brimstone and dry air are the most common.
Wood being a non-conductor, it is very evident that the cross-ties under the steel rails form an insulation between them and the ground; also, that if a piece of the same or similar material be placed between the rail ends, and that if two other pieces of sufficient strength be substituted for the iron fish-plates at that point, a secure insulation will be formed between the rails.