510 THE ASTATIC GALVANOMETER. [254. effect. The action of the upper wires upon the needle above them coincides with their action upon the lower needle, with its reversed poles and the effect of a feeble current is thus materially increased by these combined actions. The conducting wire must be covered with silk with a view to preserve each coil duly insulated from the contiguous ones. : FIG. 207. The astatic galvanometer is represented in fig. 207. The needles, n s, s n, are suspended, one within and the other above the coil of wire, w w, by means of a fibre of silk, d, the whole being enclosed within the glass case, G. The parallelism of the two needles to each other is maintained under all circumstances by causing each of them to pass transversely through the same piece of straw, or by connecting them together by means of a piece of fine copper wire; the fibre d is attached to the upper extremity of the straw or the wire. By means of a screw at a, the point of suspension of the silk can be raised or lowered without twisting it, so that when the needles are not in use their weight need not be supported by the silk fibre. C C, is a sheet of copper provided with a graduation on its margin for estimating the angular deviation of the needles; b, b, are binding screws for connecting the extremities of the coil with the wires which transmit the current ;* the apparatus can be levelled by means of the screws. m m; and at 1, a lever is shown by which the coil of wire, w, can be placed accurately parallel with the magnetic needles, so as to make them coincide with the zero of the graduated circle. Such an instrument may be made not only to indicate the existence of voltaic action, but also to measure its amount. When the deviations of the needle are small, not exceeding 15 or 20°, the number of degrees of deviation gives nearly accurately the relative force; but for angles of greater * Instead of binding-screws, it is not uncommon to employ small cups containing mercury as the means of completing the metallic communication between the different parts of the circuit; the ends of the wires should be made perfectly bright before immersing them in the mercury. Copper wires may be easily amalgamated superficially by scouring them with fine emery-paper and moistening them with a solution of mercuric nitrate; the perfection of the contact is thus insured. 254.] THOMSON'S REFLECTING GALVANOMETER. 511 magnitude, this is not the case, because the more the needle deviates from parallelism to the wire, the more obliquely and therefore the less powerfully does the attraction or repulsion act which occasions its motion; and it becomes necessary to determine the value of the degrees by direct experiment. It would require a greater current to move the needle from 20° to 25°, than from 10° to 15° and a still greater to produce a deviation from 30° to 35°; but the current required in each case is definite, and consequently may be estimated and measured.* Thomson's Reflecting Galvanometer.-This is a very sensitive and beautiful instrument, which is now used extensively for telegraphic purposes. The indi cating magnet is attached to the back of a small circular silvered mirror, shown separately at M, fig. 208, the joint weight of the two not exceeding 15 grain Melloni's method of graduating a galvanometer is the following, quoted by Tyndall (Heat considered as a Mode of Motion, 4th edit. 330):-Two small vessels, v, v, fig. 209, are half filled with mercury, and connected separately by two short wires, with the extremities, G, G, of the galvanometer. The vessels and wire thus disposed make no change in the action of the instrument, the thermo-electric current being freely transmitted as before from the pile to the galvanometer. But if, by means of a wire, F, a communication be established between the two vessels, part of the current FIG. 209. will pass through this wire and return to the pile. The quantity of electricity 512 GRADUATION OF A GALVANOMETER. [254. (0'1 grm.). The mirror and magnet are suspended by a few fibres of unspun silk, in the centre of a helix of insulated wire, w; in the central opening of the coil is placed a small lens; the coil is supported in a suitable frame, g, mounted on levelling screws. Upon a perpendicular rod fastened to the top of this frame a magnet, m, slides up and down, so as to enable the operator by its means to neutralize the effect of the earth's magnetism upon the needle of the galvanometer. In front of the instrument a lamp, L, is placed behind a vertical slit, s, which is arranged at a suitable distance from the galvanometer, in a line of the prolongation of the axis of the coil at g. The light of the lamp passes through the slit, s, and is concentrated by the lens upon the mirror, from which it is reflected through the lens upon the scale R; this scale being placed at a distance suited to the production upon its graduated surface of a sharp reflected image of the slit. Since a very slight angular deviation of the needle causes the spot of light to traverse circulating in the galvanometer will be thus diminished, and with it the deflection of the needle. Suppose, then, that by this artifice we have reduced the galvanometric deviation to its fourth or fifth part-in other words, supposing that the needle being at 10 or 12 degrees under the action of a constant source of heat placed at a fixed distance from the pile, that it descends to 2 or 3 degrees when a portion of the current is diverted by the external wire; I say that by causing the source to act from various distances, and observing in each case the total deflection and the reduced deflection, we have all the data necessary to determine the ratio of the deflections of the needle, to the forces which produce these deflections. To render the exposition clearer, and to furnish at the same time an example of the mode of operation, I will take the number relating to the application of the method to one of my thermo-multipliers. The external circuit being interrupted, and the source of heat being sufficiently distant from the pile, to give a deflection not exceeding 5 degrees of the galvanometer, let the wire be placed from v to v; the needle falls to 1°5. The connexion between the two vessels being again interrupted, let the source be brought near enough to obtain successively the deflections: 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°. Interposing after each the same wire between v and v, we obtain the followlowing numbers :— 1°5, 3°, 4°5, 6°·3, 8°4, 11°2, 15°*3, 22°4, 29°·7. Assuming the force necessary to cause the needle to describe each of the first degrees of the galvanometer to be equal to unity, we have the number 5 as the expression of the force corresponding to the first observation. The other forces are easily obtained by the proportions: 5 (that is to say, one reduced current is to the total current to which it corre sponds, as any other reduced current is to its corresponding total current), where a represents the deflection when the exterior circuit is closed. We thus obtain 5, 10, 15°2, 21, 28, 37°3, for the forces corresponding to the deflections 5°, 10°, 15°, 20°, 25°, 30°. In this instrument, therefore, the forces are sensibly proportional to the arcs, 255.] GRADUATION OF A GALVANOMETER. 513 the entire length of the scale, the equidistant graduations may be taken as exactly proportional to the power of the current. By dividing the helix into coils of different lengths, which portions may be used independently, and by shifting the position of the magnet many degrees of sensitiveness may be obtained with the same instrument, rendering it applicable to a large number of experiments, in which the quantity of the current varies within wide limits. (255) Allusion has already been made to the physiological action of the current, in consequence of which, if a living animal, or a part of one recently killed, such as the limb of a frog, be included between a pair of plates, muscular contractions are produced; similar effects occur if a portion of the human body, such up to nearly 15 degrees. Beyond this the proportionality ceases, and the divergence augments as the arcs increase in size. The forces belonging to the intermediate degrees are obtained with great ease either by calculation or by graphical construction, which latter is sufficiently accurate for these determinations. By these means we find Degrees Differences Degrees ... ... ... 13°. 14° 15° 16° 17° 18° 19° 20° 21° 22° 23° 24° 25° 28 I'I I'2 I'2 1'3 26° 27° 28° 29° 30° 235 249 26'4 In this table we do not take into account any of the degrees preceding the 13th, because the force corresponding to each of them possesses the same value as the deflection. The forces corresponding to the first 30 degrees being known, nothing is easier than to determine the values of the forces corresponding to 35, 40, 45 degrees, and upwards. The reduced deflections of these three arcs are 15°3, 22°4, 29°7. Let us consider them separately, commencing with the first. In the first place, then, 15 degrees, according to our calculation, are equal to 152; we obtain the value of the decimal o'3 by multiplying this fraction by the difference II, which exists between the 15th and 16th degrees; for we have evidently the proportion The value of the reduced deflection corresponding to the 35th degree will not therefore be 15°3, but 15°2+0°3 = 15°5. By similar considerations we find 23°5 +06 = 24°1 instead of 22°4, and 36°7 instead of 29°7 for the reduced deflections of 40 and 45 degrees. It now only remains to calculate the forces belonging to these three deflections-15°5, 24°1, and 36°7—by means of the expression 3333 a; this gives us Comparing these numbers with those of the preceding table, we see that the sensitiveness of our galvanometer diminishes considerably when we use deflections greater than 30 degrees. 514 THE VOLTAIC PILE. [255 as the tip of the tongue, be included between two interrupted points of the conducting wire. But in addition to the heating, magnetic, and physiological effects, another remarkable series of phenomena, those of chemical decomposition, may be exhibited at the interrupted points of the conducting wire. These, however, are more distinctly shown when a number of pairs of plates is employed. FIG. 210. (256) The Voltaic Pile.-In prosecuting the experiments of Galvani, Volta discovered that by using a number of similar metallic pairs moistened by a saline or by a feebly acid liquid, many of the effects already described were greatly increased; and in the year 1800 he published a description of the apparatus which he had contrived, and which has perpetuated the name of its inventor under the designation of the Voltaic Pile. This important instrument is represented in fig. 210. It consists of a succession of pairs of plates of two dissimilar metals, such as zinc, z, and copper, c, or zinc and silver, each pair being separated on either side from the adjacent pairs by pieces of card or of flannel, F, moistened with salt and water, or with very weak acid these plates may be supported by a frame of dry wood. The effects produced by : such an apparatus were soon seen to be of an electrical character. If the ends of the pile or the wires connected with them were touched, one with each hand previously moistened, a sensation similar to that of the electric shock was experienced. Sparks could be obtained between two pieces of charcoal attached to the ends of the wires; divergence of the gold leaves of the electroscope was produced when one wire touched the cap of the instrument, whilst the other wire was in communication with the earth; and other electrical effects were obtained. In arranging the plates of metal it is necessary strictly to observe a certain order in their succession; thus, if a plate of zinc with a wire attached to it form the bottom of the pile, a piece of wet flannel must be placed upon it, then a piece of copper, then a piece of zinc, then flannel, then copper, then zinc, then flannel, and so on, till the pile terminates at the top with a plate of copper to which a wire is attached. By soldering together the zinc and copper in pairs, a considerable improvement is effected; complete contact of the two metals is insured, and the apparatus can be mounted with more rapidity. Many practical inconveniences, however, |