GALVANISM. replaced by iron, which is nearly as electro-negative in any position. They cannot, however, be su Fig. 16. couples: Bunsen's, 800; Grove's, 780; Daniell's 470; and Wollaston's, 208. The GALVANOMETERS.-The two most reliable evidences of the strength of the galvanic current are, its power to deflect the magnetic needle, and to effect chemical decomposition. To measure one or other of these, is the object of a galvanometer or voltameter. A magnetic galvanometer shews the strength of the current by the amount of the deflection of the needle, and shews its direction The manner by the way in which it deflects. in which a needle should turn when influenced by a current is easily kept in mind by Ampere's rule: Suppose the diminutive figure of a man to be placed in the circuit, so that the current shall enter by his feet, and leave by his head; when he looks with his face to the needle, its north pole always The deflecting wire is supposed turns to his left. always to lie in the magnetic meridian. Astatic Galvanometer, or Galvanometer, is used either simply as a galvanoscope, to discover the existence of a current, or as a measurer of the strengths of weak currents. When a needle is placed under a straight wire, through which a current passes, it deflects to a certain extent, and when the wire is bent, so as also to pass below the needle, it deflects still more. This is easily understood from the above rule. The supposed figure has to look down to the needle when in the upper wire, and to look up to it in the lower wire, so that his left hand is turned in different ways in the two positions. The current in the upper and the lower wire moves in opposite directions, thus changing in the same way as the figure; and the deflection caused by both wires is in the same direction. By thus doubling the wire, we double the deflecting force. If the wire, instead of making only one such circuit round the needle, were to make two, the force would be again doubled, and if several, the force (leaving out of account the weakening of the current caused by the additional wire) would be increased in proportion. If the circuits of the wire be so multiplied as to form a coil, this force would be enormously increased. Two needles, as nearly the same as possible, placed parallel to each other, with their poles in opposite ways, as shewn in fig. 17, and suspended, so as to move freely, by a thread without twist, have little tendency to place themselves in the magnetic meridian, for the one would move in a contrary If they were exactly of direction to the other. the same power, they would remain indifferently as the needles are less Such a or more perfectly n Fig. 17. compound needle is called astatic, as it stands Fig. 18. bobbin, AB, a coil of fine copper wire, carefully 601 GALVANISM. needle, than when it is at zero; the deflecting force of the coil, therefore, differs with the position of the needle, so that the deflections caused by different currents are not in the proportion of the angles of deviation, or their functions; up to from 15° to 20°, it is found for most instruments that the strength of the current is proportional to the angle of deviation; beyond that, the relations of strength indicated by different angles must be ascertained experimentally, which can be done with the aid of a thermo-electric pile. Tangent Galvanometer.-This instrument is shewn in fig. 19. It consists essentially of a thick strip of copper, bent into the form of a circle, from one to two feet in diameter, with a small magnetic needle, moving on a graduated circle, at its centre. When the needle is small compared with the ring, it may be assumed that the needle in any direction it lies holds the same relative position to the disturbing power of the ring. This being the case, it is easy to prove that the strengths of currents circulating in the ring are proportionate to the tangents of the angles of deviation of the needle. Thus, if the deflection caused by one galvanic couple was 45°, and of another 60°, the relative strengths of the currents sent by each would be as the tangent of 45° to the tangent of 60°-viz., as 1 to 173. The needle can never be deflected 90°, for as the tangent of 90° is infinitely large, the strength of the deviating current must be infinitely great, a strength manifestly unattainable. The tangent galvanometer can consequently be used to measure the strongest currents. One great advantage attending its use, is that the current, in passing through the thick copper wire, experiences almost no resistance, and consequent diminution of strength, so that it can measure a current without affecting it. Fig. 19. Voltameter. This was invented by Faraday for testing the strength of a current. Fig. 20 shews how it may be constructed. Two platinum plates, each about half a square inch in size, are placed in a bottle containing water acidulated with sulphuric acid; the plates are soldered to wires which pass up through the cork of the bottle; binding screws are attached to the upper ends of these wires; a glass tube fixed into the cork serves to discharge the gas formed within. When the binding screws are connected with the poles of a battery, the water in the bottle begins to be decomposed, and hydrogen and oxygen rise to the surface. If, now, the outer end of the discharging tube be placed in a trough of mercury (mercury does not dissolve the gases), and a graduated tube (fig. 21), likewise filled with mercury, be placed over it, the combined gases rise into the tube, and the quantity of gas given off in a given time measures the strength of the current. The voltameter chooses as a test the work which the current can actually perform, and establishes a uniform standard of comparison. The indications of the tangent galvanometer are comparable only with its own, but the quantity of gas discharged by the voltameter, corrected for pressure and temperature, is something quite absolute. However, by comparing the indications of both instruments with each other when placed in the same circuit, an absolute standard may likewise be got for the tangent galvanometer. If, for instance, the current given by a battery should give 2 cubic inches in a minute, as shewn by the voltameter, and produced at the same time a deflection of 45° in the galvanometer, the ratio of 2 to the tangent of 45°-viz., 2 to 12, is constant, for correct measurements of the strength of currents, however taken, must bear to each other a constant ratio. If the angle of devia tion for another current was 30°, we have therefore only to multiply 2 by the tangent of 30°, to ascertain the amount of gas that would be liberated by a current of that strength in a minute. This found, we know the meaning of a deflection of 30° of the galvanometer in question in a perfectly comparable standard. The plates of the voltameter must be small, for when they are large, a small quantity of electricity is found to pass without decomposing the water. It is found also that a minute quantity of the oxygen forms binoxide of hydrogen with the water, and remains in solution, so that when very great accuracy is required, the hydrogen alone ought to be measured. RESISTANCES TO THE CURRENT.-It is found that the dimensions and material of substances included in the circuit exercise an important influence on the strength of the current. It is of the greatest importance to ascertain the relative amount of the resistance offered by conductors of various forms and materials. The rheostat, invented by Wheatstone, is generally employed for this purpose, and for this object is constructed so as to introduce or withdraw a considerable amount of highly resisting wire from the circuit without stopping the current. It is shewn in fig. 22. Two cylinders, C', C, about 6 inches in length, and 14 inch in diameter, are placed parallel to each other, both Fig. 22. being movable round their axis. One of them, C', is of brass, the other, C, is of well-dried wood The wooden cylinder has a spiral groove cut into it, making forty turns to the inch, in which is placed a fine metallic wire. One end of the wire is fixed to a brass ring, which is seen in the figure at the further end of the wooden cylinder; and its other end is attached to the nearer end (not seen in the figure) of the brass cylinder, C. The brass ring just mentioned is connected with the binding screw, S, by a strong metal spring. The further end of the cylinder C, has a similar GALVANISM. connection with the binding screw, S'. The key, the resistances are enormous as compared with the metals. With copper at 32° F. as 1, the following liquids stand thus: Saturated solution of the sulphate of copper, at 48° F., 16,885,520; ditto of chloride of sodium at 56° F., 2,903.538; sulphate of zinc, 15,861,267; sulphuric acid, diluted to, at 68° F., 1,032,020; nitric acid, at 55 ́ F., 976,000; distilled water, at 59° F., 6,754,208,000. The slightest admixture of a foreign metal alters the resistance very decidedly: per cent of iron in copper wire increases the resistance more than 25 per cent. It has been found also that the resistance offered by a wire increases as its temperature rises. It is almost needless to add, that the conducting powers of metals are inversely as their specific resistances, the least resisting being the best conducting. H, fits the projecting staple of either cylinder, and can consequently turn both. As the brass cylinder, C, is turned in the same direction as the hands of a watch, it uncoils the wire from the wooden cylinder, C, making it thereby revolve in the same way. When the wooden cylinder is turned contrary to the hands of a watch, the reverse takes place. The number of revolutions is shewn by a scale placed between the two, and the fraction of a revolution is shewn by a pointer moving on the graduated circle, P. When the binding screws, S and S', are included within a circuit, say S with the positive, and S' with the negative pole, the current passes along the wire, on the wooden cylinder, C, till it comes to the point where the wire crosses to the brass cylinder, C'; it then passes up the cylinder, C', to the spring and binding screw, S. Ohm's Law. This law is singularly in accordance The resistance it encounters within the rheostat is with experimental results. It assumes that the met only in wire, for as soon as it reaches the large electro-motive force for a particular galvanic pair is cylinder, C', the resistance it encounters up to S' may constant, and that the strength of the current it be considered as nothing. When the rheostat is to produces is the quotient which results from dividing be used, the whole of the wire is wound on the it by the resistance of the circuit. This resistance wooden cylinder, C, the binding screws are put into arises from two sources, the first being the resist the circuit of a constant cell or battery along with ance within the cell offered by the exciting liquid, and a galvanometer, astatic or tangent. If, now, the the second the interpolar resistance. If e represent resistances of two wires are to be tested, the the electromotive force; l, the resistance within galvanometer is read before the first is put in the the cell; w, the interpolar resistance; and S, the circuit. After it is introduced, in consequence of strength of the current, or the quantity of electricity the increased resistance offered by it, the needle actually transmitted, the statement of the law for falls back, and then as much of the rheostat wire is The applicaunwound as will bring the needle back to its former one couple stands thus: S T + w place. The quantity of wire thus uncoiled in the tion of the law in a few particular cases will best rheostat is shewn by the scales, and is manifestly illustrate its meaning. If we increase the number equal in resisting power to the introduced wire. of cells to n, we increase the electromotive force n The first is then removed, the rheostat readjusted, times, and at the same time we increase the liquid and the second wire included, and the same un-resistance n times, for the current has n times as winding goes on as before. To fix our ideas, let the quantity of wire unwound in the first case be 40 inches, and in the second case 60 inches; 40 inches of the rheostat wire offer as much resistance to the current as the first wire, and 60 inches of it as much as the second. We have thus 40 to 60 as = = e ne much of it to travel, then S = ne neglected, and so S nl в = で cell gives in these circumstances as powerful a current as a large battery. But if nl be small with respect to was in the interpolar circuit of an electric telegraph battery-nl may be neglected, and S Here we learn that the energy of the ne = -. го current increases directly as the number of cells. We may learn from the same that the introduction of the coil of long thin wire of a galvanometer into such a circuit, introducing but a comparatively small increase of resistance, causes a very slight diminution of the current strength. If, again, we increase the size of the plates of a galvanic pair n times, the section of the liquid is proportionately increased, so that whilst the electromotive force remains the same, the cell resistance diminishes n times; therefore S = the ratio of the resistances of the two wires. The wire of the rheostat, from its limited length, can only be comparable with small resistances; and where great resistances are to be measured, supplementary resistance coils of wires, whose resistances have been ascertained, are introduced into the circuit, or removed from it, as occasion requires, leaving to the rheostat to give, as it were, only the fractional readings. This being premised, it will be easily understood how the following results have been ascertained. It is proved, for instance, that the resistances of wires of the same material, and of uniform thickness, are in the direct ratio of their lengths, and in the inverse ratio of the squares of their diameters. Thus a wire of a certain length offers twice the resistance of its half, thrice of its third, and so forth. Again, wires of the same metal, whose diameters stand in the ratio of 1, 2, 3, &c., offer resistances which stand to each other as 1, 4, 3, &c.; therefore, the longer the wire the greater the resistance; the thicker the wire the less the resistance. The same holds true of liquids, but not with the same exactness. For this reason, the larger the plates of a galvanic pair, and the nearer they are placed to each other, the less will be the resistance offered to the current by the intervening liquid. is thus shewn to increase n times. These are only The following table, constructed by Ed. Becquerel, a very few of the conclusions arrived at by this law. gives the specific resistances of some of the more common substances, or the resistance which a wire of them, so to speak, of the same dimensions, offers at the temperature 54° F.: Copper, 1; silver, 9; gold, 14; zinc, 37; tin, 6-6; iron, 7·5; lead, 11; platinum, 113; mercury (at 57°), 50-7. For liquids, = ne S: With the aid of a tangent galvanometer, which gives the value of S expressed in cubic inches of voltameter gas, we can easily ascertain the value of e and for any pair. By making two observations with two wires of known resistance separately included in the circuit, we have two simple GALVANISM. equations with two unknown quantities, from which and I can be easily found. In doing so, we must adopt a unit of resistance, such as that proposed by Jacobi-viz., that offered by a copper wire 1 mètre (39.3 inches) long, and 1 millimètre (0393 inch) in diameter. The resistance of the liquid of the pair would be expressed in units of this, and the electromotive force in cubic inches of explosive gas with a circuit offering a unit of resistance. THE EFFECTS OF THE GALVANIC CURRENT may be classified under physiological, mechanical, magnetic, heating, luminous, and chemical. The mechanical effects relate to the mutual attraction or repulsion of one current to another, or to a part of itself. These, along with the magnetic effects, will be found treated of under MAGNETO ELECTRICITY. The heating and luminous effects have been partly discussed under ELECTRIC LIGHT. We shall here only further refer to the heating of wires, and to the galvanic spark. The luminous effects of galvanic electricity of very high tension will be given under INDUCTION COIL. The chemical effects have been already referred to, but a fuller consideration of these will now be given under the head Electrolysis in this article. The physiological effects, as shewn by the convulsions of Galvani's frog preparation, were the first observed manifestation of the current. Frog-limbs, as prepared by Galvani, when included in a circuit, form a galvanoscope of excessive sensibility, which rivals the finest galvanometer in delicacy of indication. There is one peculiarity in their action which deserves to be noted. The limbs contract only when the circuit is completed and broken, and remain undisturbed so long as the current passes steadily through them. The more frequently, therefore, the current is stopped and renewed, the greater is the physiological effect. The same is experienced when a current is passed through the human body. When the terminal wires of a battery are lifted one by each hand, except it consist of a very large number of cells, almost the only sensation felt is a slight shock on completing and breaking the circuit. Du Bois Reymond, the great authority on animal electricity, states that the nerves of motion are affected only by changes in the electric tension of the current, whereas the nerves of sensation are affected not only by these, but also by the steady continuance of the current, and that the excitation of the nerves dependent on the changes of tension increases with their frequency and suddenness. Frictional electricity in this way owes its superior physiological power to the instantaneous nature of its discharge. It is only currents of great tension which affect the ordinary human nerves. The poles of a battery of 50 Bunsen cells, capable of giving a brilliant electric light, for instance, may be handled without much inconvenience. This may be attributed partly to the non-conducting nature of the skin. If the current enter the body by a cut or wound, the sensation is affected even when the current is weak. The physiological effect is also much heightened by moistening the hands with salt and water, or by holding metal handles instead of wires, so as to improve the conducting connection. Another cause of this insensibility may be attributed to the fact that the current is not restricted, as it is in part of the frog preparation, to the nerve, but passes through all the conductors of the system. The nerves of the palate can be affected by a very feeble current; that of sight by one proceeding from a battery of one or two cells, and that of hearing by a battery of some 30 cells. See ELECTRICITY, MEDICAL Heating Effects.-When a strong current passes through thin wires, an intense heat is produced, sufficient to bring them to a white heat, and to fuse them. This is turned to practical use in exploding gunpowder, in engineering and mining operations. Two wires of a battery placed at a safe distance are insulated from each other, and their ends, which are connected by a fine iron wire, are sealed up in a tin cartridge filled with gunpowder, and laid in the exploding charge. When all is adjusted, the battery connection is completed, and the current making the iron wire red hot, ignites the gunpowder in the cartridge, and that again the charge. In this way, all danger is avoided. Experiments on the heating effects of the current through wires have proved that the heat developed is proportional to the resistance of the wires, and to the squares of the strength of the currents; and that the strength of the current being the same, any length of wire may be heated to the same redness. Galvanic Spark.-When the wires connected with a powerful galvanic battery are brought together, no current passes except they are made to touch, or nearly so; and if then separated, the current continues with the evolution of sparks, though removed for some distance. Jacobi found that the poles of a battery of twelve Grove's cells could be brought as near as '00005 of an inch without a spark passing. In Gassiot's water battery of 3520 well-insulated cells, however, a spark passed when the poles were brought to 02 of an inch, and continued to do so uninterruptedly for weeks and months together. When the galvanic spark is examined with a microscope, it is found that the light only appears at the negative pole. Electrolysis is that branch of the science of galvanism which treats of the laws and conditions of electro-chemical decomposition. As this decomposition is generally attended by electro-chemical combination, it is sometimes difficult to distinguish electrolysis from the more general subject of Electrochemistry, which embraces all chemical changes resulting in or from the galvanic current. In one case, however, the application of the term is strictly correct-viz., where decompositions are effected by electrodes (poles, see ANODE), which are not attacked by the elements of the electrolyte (the substance decomposed) discharged at them. Throughout the article, there have been frequent allusions to electrochemical changes, but here we shall discuss more particularly the laws of electro-chemical decomposition. No substance is decomposed by the current so long as it is in a solid or gaseous state, and it must first be brought to a liquid state, either by solution or fusion, before the current acts on it. The decomposition of water by platinum plates is always taken as the type of electrolytic action. Fig. 23 represents a very convenient apparatus for the purpose. A glass basin is made so as to admit a cork below, through which two wires pass having slips of platinum plate soldered to them above. Two glass tubes, open below, are hung over the plates, to hooks projecting from an upright support. The bowl is Fig. 23. oxygen GALVANISM. of their equivalents. Thus, if the same current pass filled with acidulated water; and the tubes, after being filled with the same, are inverted, and hung with their lower ends enclosing the plates. When the wires projecting downwards from the cork are connected with the poles of the battery, hydrogen rises from the negative, and oxygen from the positive electrode, to fill each its separate tube. As the decomposition proceeds, twice as much hydrogen is When the tubes are filled, liberated as oxygen. The oxygen they may be removed and examined. thus obtained smells strongly of ozone. Hydrogen is here the type of the metals or other electro-positive substances (cations), which, during electrolysis, are always disengaged at the negative electrode; and of the salt radicals, chlorine, iodine, sulphur, &c., which, being electro-negative (anions), always appear at the positive pole. Moreover, the proportions of the volumes of the two gases being that of their chemical combining volumes, reminds us that, when a body is decomposed, its components are always separated in the proportions in which they were united, viz., those of their chemical equivalents. If the tubes of this apparatus were graduated, it would serve for a voltameter. If, instead of one such voltameter included in the circuit, we had several, we should find that, whatever amount of gas was liberated in one of these, the same amount would be liberated in all, and that independent of the size of the plates, and amount of acid in each. We learn, therefore, that the chemical power at every point of of the current is the same the circuit where it is manifested. If, instead of two or three voltameters in the circuit, we had one and two decomposing cells of the following description. A test tube, having a platinum wire, on which the glass has been fused, passing through the bottom, is partially filled with protochloride of tin, which is kept fused by the heat of a spiritlamp. The platinum wire at the bottom of the tube forms one electrode, and one descending from the top forms the other, dipping below the fused chloride. If, then, this cell be included in the circuit along with the voltameter, and a similar cell containing fused chloride of lead, so that the current enters the tubes by the upper electrodes, and leaves by the lower, the water, protochloride of tin, and chloride of lead, are decomposed simultane-secondary action-viz., that of sodium on water. The ously by the current passing through each. In the voltameter, hydrogen and oxygen are disengaged; in the tubes, metallic tin is deposited at the lower electrode of the one, and lead at the other; whilst chlorine is liberated at the upper electrodes of both. If, now, the quantity of hydrogen, tin, and lead thus set free be weighed, it will be found that their weights are in the proportion of their chemical From such experiments as these, equivalents. Faraday made the first grand electrolytic generalisation to the following effect: When the current passes through a series of binary electrolytes, consisting of an equivalent of each of the elementary bodies, the quantities of the separated elements of the electrolytes are in the same proportion as their chemical equivalents. It is not only in cells exterior to the battery that this law holds, but in the cells of the If the battery which effected the battery itself. above decomposition consisted of six cells, for each equivalent of hydrogen, tin, and lead separated without the battery, one equivalent of zinc in each cell would have been dissolved, and an equivalent of hydrogen disengaged at each of the copper plates, The above law holds if the cells were one-fluid. also for binary compounds, whose elements do not stand in the relation of an equivalent of the one to ELECTRO-METALLURGY is the art of depositing, an equivalent of the other, but with this modification, that the weights of the electro-negative elements alone, separated in the action, are in the ratio | electro-chemically, a coating of metal on a surface 605 |