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Experimental Demonstration of Ohm's Law.—An interesting experimental demonstration of the truth of Ohm's law was recently given by Professor Alfred M. Mayer, of the Stevens Institute of Technology, before the New York Electrical Society. This law, as is well known, affirms that in any electrical circuit the current flowing varies directly as the electro-motive force and inversely as the resistance, or, in symbols, C ER, where C is the current, E the electro-motive force, and R the resistance of the entire circuit, including that of the generator. To demonstrate the truth of this law, it is only necessary to show that, the resistance remaining constant, the current increases in the same ratio as the electro-motive force when this is augmented; or that, the electro-motive force being mainlined constant the current varies in the same ratio as the resistance as this latter is raised. In Professor Mayer's experiments the current was measured by means of a Thompson reflecting galvanometer—a delicate instrument in which the deflections of the needle are multiplied to any desired extent by means of a beam of light, reflected from a small mirror attached to the needle, which is received upon a screen. The novel feature of Professor Mayer's demonstration consisted in his mode of obtaining the current so that the electro-motive force could be known with great accuracy, and readily varied. This consisted in generating it by means of the movement of a coil of wire along a bar-magnet. The electro-motive force of the current so produced depends upon the number of lines of magnetic force cut by the moving coil in a unit of time, so that this can be varied by varying the speed with which a given coil is moved, or, the speed remaining the same, by varying the number of coils. Professor Mayer resorted to the latter measure, his apparatus consisting simply of an upright bar-magnet over the end of which a loop of wire could be slipped, the distance which this could slide being limited by a stop. The movable coils consisted of the same lengths of copper wire, in which there were taken one, two, or more loops, the resistance of each of these pieces being the same, so as to maintain that of the complete circuit constant. The coils were placed over the upper end of the magnet, and carried down until they rested upon the stop, the needle of the galvanometer brought to the zero of the scale, and then the coil pulled off the magnet with a quick motion. The deflection of the needle indicated a variation in the current in proportion to the number of loops of wire used, and when the resistance was varied in proportion to the amount of this variation. A better form of the apparatus is one in which the coil, instead of being moved by hand, is drawn up quickly by a spring, when it is released by the pulling of a trigger. With this, Professor Mayer is at present studying the development of magnetism in eloctro-magnets.
More about the Lignified Snake.—Doubts are expressed, in a paper recently read by Professor C. V. Riley before the Biological Society of Washington, as to the so-called "lignified serpent" of Matto Grosso, Brazil, which was described and illustrated in the November number of the "Monthly," being really a serpent at all; Professor Riley rather believes the forma-
