The unit of electrical resistance, whereby the comparative merits of various materials and sizes of materials as conductors are designated, is called an _ohm_, (just as the unit of lineal measurement whereby the comparative lengths and sizes of various objects are designated, is termed a _foot_) and we will therefore use that term in reference to the resistance of a conductor. [Illustration: Fig. 1] Figure 1 represents an ordinary gravity battery, the conductors from it, and the electro-magnet to which they connect; also the armature as attracted by the magnet and overcoming the spring which tends to withdraw it from the magnet. Now, as long as the current flows through the magnet, this condition of things remains unaltered; but let a second path be presented to the current several hundred times less in resistance than the original one, and the result is that several hundred parts of the current will leave the magnet for the "short circuit," and consequently leave so little remaining in the original one that the effect will be practically to demagnetize the magnets. [Illustration: Fig. 2] Figure 2 will render this very apparent if we will assume the wire of the magnet R to possess a resistance of 10 ohms, and the conductors themselves a resistance so low as to be inappreciable and unworthy of consideration. [Illustration: Fig. 3] [Illustration: Fig. 4] 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