250.] GALVANISM. 505 auroral effects are seen at very distant points of the earth's surface. (Sabine.) The occurrence of the aurora is closely associated with sun-spots, magnetic storms, and earth-currents; there is also a period of maximum and minimum frequency, which is identical with that of the sun-spots, namely, 11'11 years. § III. GALVANIC OR VOLTAIC ELECTRICITY. FIG. 202. (250) Galvani's Discovery.-About the year 1790 Galvani made the observation that convulsive movements were produced in the limbs of a frog recently killed, if brought into contact with two dissimilar metals, such as zinc and copper, which were themselves in contact. The experiment may be readily repeated in the following manner-Expose the crural nerve (N, fig. 202) of a recently killed frog, touch it with a strip of zinc, z, and at the same time touch the surface of the thigh, m, with : one end of a bit of copper wire, c; the moment that the other end of the copper wire is made to touch the zinc, the limb is convulsed but the convulsions cease when the two metals are separated from each other, though they are still in contact with the animal tissues. Each time that the zinc and copper are made to touch each other, the convulsion is renewed. A live flounder laid upon a pewter plate shows no particular sign of uneasiness; a silver spoon may also be laid upon its back without any apparent effect; but if the spoon be made to touch the pewter while it rests on the fish, the animal becomes strongly convulsed. If a piece of zinc and a shilling be placed one above and the other under the tongue, no particular sensation is perceived so long as the two metals are kept separate, but if the silver and the zinc be allowed to touch each other, a peculiar tingling sensation or taste is experienced; and if the silver be placed between the upper lip and the teeth instead of under the tongue, each time that the two metals are brought into contact, not only will a taste be perceived, but a momentary flash of light will appear to pass before the eye. These phenomena are all analogous to each other, and have an electrical origin: and by tracing them to this source, a branch 506 ELEMENTARY VOLTAIC CIRCUITS. L250. of electrical science has gradually been developed, which in honour of its first discoverer has been termed galvanism. The term galvanism, or voltaic electricity, as it is also called, in remembrance of the researches of Volta in this field, is applied to electricity which is set in motion by chemical action. It is usually developed by the contact of two dissimilar metals with a liquid. (251) Elementary Voltaic Circuits.-These effects may be traced by very simple means. When a plate of zinc is immersed in diluted sulphuric acid, an evolution of hydrogen gas takes place from the surface of the metal, and the zinc becomes dissolved in the sulphuric acid. But if the surface of the zinc, after it has been cleansed by immersion in the acid, be rubbed over with mercury, a brilliant amalgam is speedily formed over the whole face of the zinc. Such a plate may then be plunged into the acid, and it will remain without undergoing any chemical change for hours. The cause of this inactivity of the zinc is not satisfactorily accounted for, but the fact is continually made use of in voltaic experiments. The addition of a second amalgamated zinc plate, whether it be in contact with the first or be separated from it, produces no change. But if the second plate be of platinum, of copper, or of some metal which is less rapidly acted on by the acid than zinc is, although no action will occur whilst the two plates remain separate (as shown in fig. 203, 1), yet the FIG. 203. HAVG moment that they are allowed to touch each other, either above (2) or beneath (3) the surface of the liquid, bubbles of gas will escape from the surface of the platinum. The platinum, however, is not acted upon chemically in this case; if the two metals be weighed before the experiment is commenced, and again after it is concluded, the weight of the platinum will be found to be unaltered; but the zinc will have been partially dissolved, and will weigh less than it did before. The gas may easily be collected by filling a tube with diluted acid, and, after introducing the platinum plate, inverting the tube in the glass, so that the 253.] ELEMENTARY VOLTAIC CIRCUITS. 507 lower edge of the platinum may touch the strip of zinc (No. 3). On examining the gas which rises in the tube it will be found to be pure hydrogen. It is not necessary. that the two plates should directly touch each other. They may be connected by means of a metallic wire (as at 4, fig. 203), by a piece of graphite, or by any good conductor of electricity: gas will continue under these circumstances to rise from the platinum plate; but if a glass rod, a stick of shell-lac, a bit of gutta-percha, or any electric insulator be made the medium of intercommunication, all signs of action will cease. The length of the metallic wire employed is comparatively unimportant: it may vary from a few centimetres to many miles, and in either case it will enable the action across the liquid to take place. A pair of plates of dissimilar metals in effectual communication, either by direct contact or through the medium of wire, when immersed in a liquid which acts chemically upon one of them, constitutes a voltaic circuit. (252) Activity of the Conducting Wire.-The wire or other medium of communication, during the time that it forms the connexion between the two metals, exhibits signs of activity which it did not before possess; it exerts a variety of influences upon surrounding bodies, and it loses these powers immediately that the contact with the metallic plates is broken. For instance, the temperature of the wire is for the time elevated. This may be proved by causing the wire to traverse the bulb of a sensitive air thermometer, or by making a compound metallic ribbon, such as is used in Breguet's thermometer (140), part of the chain of communication between the plates. If a portion of the wire be sufficiently reduced in thickness, visible ignition of such portion may even be produced. Indeed the quantity of heat given out by the connecting wire may be employed as a measure of the amount of electricity which it is transmitting. (253) Action of the Conducting Wire on the Magnetic Needle. -Another remarkable proof of the activity of the wire which connects the two metallic plates, is exhibited in the peculiar influence which it exerts over a magnetic needle suspended freely at its centre in a direction parallel to the wire (Ersted, Ann. Chim. Phys. 1820 [2], 417). Such a needle tends to place itself at right angles to the wire. If the wire and the needle be previously arranged in the magnetic meridian, the deviation in the needle affords a comparative measure of the electricity which is conveyed by the wire, as the needle ultimately assumes a position of equilibrium between the directive power of the earth's magnetism and that of the wire (301). 508 ACTION OF CONDUCTING WIRE ON MAGNETIC NEEDLE. [253 The movements of such a magnetic needle afford one of the most delicate tests of the development of galvanic electricity, or of electricity in motion. It will therefore be necessary to examine the direction and nature of these movements. The direction of the needle under any circumstances may easily be calculated by recollecting the following rule :-When the wire is placed in the magnetic meridian, with the end connected with the zinc plate towards the north, and the needle is placed below the wire, the marked end will deviate westward. When the needle is above the wire, the marked end will move towards the east. The first effect is shown in fig. 204, 1; the second in 2. On reversing FIG. 204. the attachment of the wire to the plates, the phenomena will in each case be inverted. By means of a simple model, the direction of the needle under any conditions may be readily indicated:— Across a square strip of wood nail a cylindrical piece at right angles; let the square rod represent the magnetic needle, the round rod the connecting wire (fig. 204, 3 and 4), then mark upon the square rod the letters N and s, and on the round rod, ▸ and z, in conformity with the rule just given: by placing the model in any given position, the relative effect of the wire upon the needle under these circumstances will be shown. Another simple method by which the position of a magnet acted on by a current may be remembered is the following:Place the palm of the right hand towards the magnet, and when the current is passing in a direction parallel to the hand and towards the ends of the fingers, the thumb will point to the north-seeking end of the magnet. Even the liquid part of a voltaic circuit acts thus upon the magnetic needle. This may be shown by suspending a needle, n s, fig. 205, by means of a fibre of silk, over a dish of diluted sulphuric acid. On one side of this dish a zinc plate, &, 254.] THE GALVANOMETER. 509 is inserted, on the other, a plate of platinum, P. The needle must be placed so that one of its ends may point towards one plate, and the other end towards the other plate. If the two plates be now connected by a wire, as shown in the figure, the needle will be deflected, and will place itself nearly parallel to the metallic plates. (254) The Galvanometer. Since every part of the circuit acts equally upon the needle, and since it is possible to make several parts act simultaneously upon it, actions may be rendered perceptible which would otherwise be too weak to influence its motion. Fig. 206 will convey an idea of the principle upon which this is effected. Suppose the wire con FIG. 205. FIG. 206. W N S necting the plates P and z to be bent into a loop with parallel sides. If a magnetic needle be suspended between the wires, and parallel to them, the loop and the needle being both in the magnetic meridian, with the end N pointing to the north, the marked end of the needle would be impelled westward under the influence of the force in the upper branch; and as the current returns in the reverse direction through the lower wire, this tendency of the north end westward would be doubled. By increasing the number of coils which are placed around the needle parallel to each other, very feeble actions may be rendered evident. An instrument constructed on this principle is termed a galvanometer or rheometer (or current measurer,' from pew, to flow). The sensitiveness of the galvanometer may, however, be still further increased by placing outside the coil a second magnetic needle with its poles reversed; the directive force of the earth may be thus almost exactly neutralized; its attraction upon the north end of one needle being almost exactly counterbalanced by its repulsion upon the south end of the needle which is parallel to it. A pair of needles thus arranged constitutes what is termed an astatic combination (from äoraroç, unstable). A very feeble current will be sufficient to drive one particular extremity of such a pair of needles to the east or to the west; but the second needle being outside the coil, will be acted upon by the upper wires only, the lower ones being at too great a distance to produce any sensible |