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of which may be seen m a paper communicated to the Royal Society. Its immense velocity has been demonstrated long since by the instantaneousness of its arrival at different parts of long metallic tubes, or a series of them, such as are used in pipes for conducting gas. The chemical and other effects of electricity will be found under their proper heads. (Biot's Physique, tome ii.; Pouillet, Elémens de Physique; Murphy's Electricity; Papers by Mr. Snow Harris in the Philosophical Transactions; Turner's Chemistry, fifth ed., &c.) ELECTRICITY, LATENT. [Molecularity.] ELECTRICITY, Medical application of. A supposed analogy between electricity and the nervous power has led to the employment of this agent, particularly in diseases connected with defective nervous energy, and also in cases of defective secretion, perhaps originating in a similar cause. The influence of electricity on the human system differs much according to the manner in which it is applied, the length of time during which it is continued, and the degree of intensity. It also differs in its action according as it is abstracted from, or communicated to, the individual. When applied in a moderate degree of intensity, it occasions an increase of nervous action, of sensibility and irritability, more vigorous circulation of the blood, augmented warmth, and secretion, especially cutaneous transpiration: even the exhalation of plants is much increased by electricity. When the electric principle is more intense, all these actions are heightened, often to a painful degree; while such a degree of concentration as occurs during certain atmospheric changes can occasion instant death. Death occasioned by this means is always followed by rapid decomposition of the body. The diseased states in which electricity has been found most useful are—in asphyxia, from any cause (except organic disease of the heart), but particularly from exposure to irrespirable gases; in certain asthmatic diseases; and dyspepsia, dependent on irregular or defective supply of nervous energy to the lungs and stomach. It is however much inferior to galvanism as a remedial agent in these diseases. (Wilson Philip on the Vital Functions.) In local paralytic affections, when of a chronic character, electricity, duly persevered with, has been found very useful: in a case of dysphagia, from paralysis of the oesophagus, the patient could only swallow when placed on a seat resting on nonconductors and electrified. In deafness and loss of sight, when directed by a competent judge, it has restored the functions of seeing and hearing. Lastly, in defective secretion, especially amenorrhoea, it has proved of service. ELECTRO-CHEMISTRY. The effects of electricity, whether produced by the common electrical machine or the galvanic apparatus, when applied to substances with sufficient intensity, are recognized in their composition or decomposition, their fusion, phosphorescence, &c.; this class of phenomena have been successively investigated by Franklin, Priestley, Cavendish, Wollaston, Cuthbertson, Davy, Van Marum, Gay-Lussac, and Thénard, Becquerel, Faraday, &c., and form an important branch of science closely connected with the nature of chemical affinities, and called electro-chemistry. Though the decomposition of water by common electricity was effected before Wollaston, yet the remarkable simplifications which this celebrated man introduced into every chemical subject with which he was connected accompanied his electro-chemical researches, we shall therefore confine ourselves to the description of the mode he adopted to decompose water. inely-pointed wires of gold or platina are introduced into, capillary tubes, the glass is then heated by a lamp until it becomes soft and completely covers the metal, the uncovered part of the wire is then cut off with a sharp instrument, or else the glass may be ground away until the very point of the wire commences to project; sometimes the platina is first silvered, and when the uncovered part has been cut away, the whole is plunged in nitric acid, which dissolves the silver envelope, }. only a very fine point of platina. Two wires thus prepared are placed in a vessel containing water, bringing the metallie points very close to each other; one wire is now put in communication with the ground, the other with one of the conductors of an electrical machine; when the electricity of the machine is evolved, a series of sparks passes through the water between the metallic
points, the water becomes decomposed into its constituent gases at the points of the wires, and being collected in glass vessels filled with water, they are found in the ratio of 2 to 1 in volume, which is the known proportion of hydrogen and oxygen combined to form water: the finer the metallic points, the greater will be this decomposition; some of those used by Wollaston were of only the 1500th of an inch in diameter. The decomposition of aether, alcohol, oils, &c., have also been effected in nearly a similar manner, by the electrical spark, and the correct proportions of the constituent gases have invariably been obtained; the oxygen in such cases is always found at the negative pole, and the same result is obtained if we use only a single wire, and do not insulate the vessel containing the fluid operated on. In metallic solutions the precipitate on the electrised wire shows visibly the decomposition; thus in a solution of copper, if we employ a silver wire, protected by an envelope of sealing-wax, when the electrical discharge is communicated we find the copper precipitated on the negative wire. When solid substances are inclosed in tubes of glass, such as the oxides of gold, tin, &c., and a strong electrical discharge passed through them, a similar decomposition takes place, the products being deposited on the sides of the glass. When a gas is thus to be decomposed, a glass vessel is filled with mercury, and the wires introduced; the vessel is then inverted in a reservoir of mercury, and the vacuum is sufficiently filled with the gas that the wire points project above the surface of the mercury; the discharge is then effected, and the required decomposition is produced. Erample.—Sulphuretted hydrogen. Hydrogen is disengaged, the sulphur deposited, and the volume remains unaltered. Conversely, the composition of compound bodies from their elements in many cases is effected by the electrical spark, two volumes of hydrogen, and one of oxygen gas being introduced into a stout glass tube filled with mercury, and an electrical spark passed through them by means of the wires, water is formed, accompanied with a loud detonation, and the rising of the mercury in the tube in consequence of the diminution of volume. The instruments used for the combustion of gas are called eudiometers, the principle of their construction, particularly that of Mitscherlisch, is particularly simple; the object being generally to measure the quantity of oxygen contained in airs. The electrical spark will re-light a candle which has been just blown out by the carburetted hydrogen of the smoke with the oxygen of the air, which are then easily inflamed. When electrical sparks are transmitted incessantly through a small given portion of atmospheric air, its volume becomes compressed, and nitric acid formed. (Phil. Trans, vol. 75.) By adding oxygen, we can decompose gaseous bodies (by means of the spark) which contain hydrogen: however, it has been found that there is a limit in such mixtures, beyond which the burning will no longer take place. A table of these limits is given by Turner.
a saline solution, it will cause a separation of its constituent parts; when the current passes through solutions of neutral salt, coloured with vegetable blues, the part of the liquid at the negative pole is green, at the positive, red. When the water contains a solution of a metallic salt, as of zinc, lead, or copper, the negative wire receives a coating of the particular metal; and in the general electro-chemical decompositions, oxygen and the oxides are found at the positive pole, hydrogen and the bases at the negative. Bodies thus susceptible of decomposition by the pile have been lately distinguished by the term electrolytes (nAgorpov Avo): thus hydro-chloric acid, water, &c. are electrolytes; boric acid is not such ; substances are said to be electrolysable when capable of being thus decomposed. These terms have been introduced by Mr. Farady. This is the place to notice the decomposition of the alcalis by means of a powerful galvanic battery. These substances had been previously taken as simple or elementary, but upon being introduced into the circuit of the battery, at the positive pole oxygen was disengaged, while at the negative pole was found the metallic base of the alcali, as sodium or potassium, according to the nature of the alcali employed; these substances burn at the temperature of the air, in oxygen or air, and are ever, capable of being inflamed in water. To preserve them therefore from the contact of the air, Seabeck adopted a process by which they are made to combine with mercury, in proportion as they become disengaged; the amalgam thus produced is afterwards separated by the evaporation of the mercury. Gold-leaf, carbon, &c. placed in the voltaic current between the points of the positive and negative wires of a pile become inflamed, yielding a light of the greatest brilliancy often so intense as to be painful to the eyes. All the chemical effects of voltaic electricity are cateris paribus proportional to the extent of surface of the plates employed, and are also increased by augmenting the number of plates. (For further information on this subject, the reader is referred to the actual memoirs of Davy, Wollaston, and Faraday, in the Transactions of the Royal Society; to the Annales de Chimie ; and to the Traité de l’Electricité, par Becquerel. Several isolated and interesting facts on the same subject will be found in the Annals of Philosophy, and the Edinburgh Journal of Science. See also GAiVANISM.) ELECTRO-DYNAMICS. In ordinary electricity, that fluid when developed takes a position of equilibrium, dependent on the conducting power of the ... on which it is disposed, on the non-conducting power of the medium by which it is enveloped, and on the law of force, whether of attraction or repulsion, between the elementary portions of electricty. The motions of electrised bodies are only results of the statical equilibrium of this fluid, and do not therefore belong to electro-dynamics. The mode of calculating such effects may be found under the head ELECTRIcity. These effects are moreover of the same nature whether the source of electricity be by means of friction, or by chemical action, as in the voltaic pile, the nature of the electricities in these cases differing from each other only in the mode of their Production; but when the contrary, electricities are no sooner produced than re-combined, again reproduced and again re-combined, a new class of phaenomena is produced belonging to electricity as it were in motion. Suppose, for example, that the plate A is a constant source of positive
electricity, the plate B in like manner a constant source of negative electricity of equal intensity; that AC, BC are two conducting rods communicating with each; the electricities immediately combine when the conductors are made to touch at C, and for an instant the whole may be conceived to be in the neutral state, but A being the next instant replenished with positive and B with negative electricity, the same combination takes place over again, the same neutrality succeeds, and so on indefinitely. The rod ACB is in a different condition from one in its natural state, since electrical charges are continually pouring through it from A and B; and again it is in a different condition from an electrised rod, since we cannot at any moment say that it is charged positively rather than negatively. Hence we cannot infer that it should attract rather than repel an electrised
D, since these is as much reason for one event as the
other, and in point of fact we find that it neither will attract nor repel D. We have here a positive current of electricity issuing from A and a negative from B, and no effect of attraction or repulsion is produced on an electrised point as in statical electricity. How then is its state recognized? First by touch; for if we touch the rod ACB, a series of shocks is felt, the interval between two ...; ones being inappreciable; and secondly, powerful chemical decomposition may be effected. [GALv ANIsM, Electro-chEMIsTRY.] But thirdly, we may onio it mechanically by presenting to AB another rod A'B' under exactly similar circumstances,
A H B
when the effects of the currents in AB, A'B' will be recognized by the visible motions of the rods, provided they be free to move while their communication with the proper sources of electricity remains unbroken: for example, if their extremities, be immersed in cups of mercury communicating with the constant sources of the positive and negative electricities. The laws of the mutual action of electrical currents constitute the science of electro-dynamics; and previous to its study it would be desirable that the reader should be acquainted with the construction and #. of the galvanic apparatus, the opposite poles of which afford the two constant sources A, B of electricity which we have supposed. These will be found under the head GALWAN is M.
To discover the laws of the mutual actions of electrical currents we must have recourse to experiment. An apparatus similar to that employed by Ampère will be found in Professor Cumming's translation of Dumonferrand's treatise on this subject; together with a description of the mode of performing the various experiments by which these laws have become known. The term direction of a current is convenient when speaking of more than one; for instance, the zinc end of the pile being a constant source of positive electricity and the copper end of negative, a rod communicating with wires connected respectively with these extremities will have a current of positive electricity from the zinc to the copper, and a negative current in a contrary direction; but as it is simultaneously permeated by both, when we speak of the direction of a current we shall understand that of the positive current to avoid ambiguity.
Two parallel currents which are directed in the same way o each other, but when directed in opposite ways, they repel.
When rectilineal currents form mutually an angle, the species of action which they exercise may be thus defined: ‘Two portions of currents will attract if they are both approaching or both receding from the vertex of the angle which they form ; but when one approaches and the other recedes from that angle, then they repel: the same law holds in the limiting case of parallelism.
Let two currents cross each other, as AEB, CED, and suppose the directions of the currents to be those indicated by the arrows in the figure; then, according to this law, the force between CE, EB, is repulsive; and that between CE, EA, attractive; if, therefore, AB is fixed, and CED moveable, we ought to have CE tending towards AE, and for the same reason, DE tending towards EB; the rod CED, therefore, has a rotatory motion impressed on it until it is placed parallel with AB: this is confirmed by experilment.
If we now consider two currents to form a very obtuse
angle, one of them approaching, and the other receding from the vertex, we have repulsion; let the obtuse angle be increased to 180°, and in this extreme case the two currents merge into one: hence it follows that the consecutive parts of one and the same current exercise a mutual repulsion on each other.
The actions exercised by a rectilineal current and by a sinuous current, which have generally the same direction and are terminated at the same extremities, are equal, the intensity of action being supposed the same in both cases; thus, if we suspend a moveable conductor between a rectilineal and a sinuous conductor disposed so as to repel the first, this, after a little oscillation about its mean place, will finally rest in the middle of the interval between the conductors.
Let us now consider the action of an indefinite current AB, on a terminated current CE, which is directed towards E; the direction of AB being that indicated in the figure.
The portion BE of the indefinite current repels EC, in consequence of the contrary direction of the current in the latter. Let us represent this force in magnitude and direction by C& =CG ; also AE attracts CE; the force may be represented by CF, similarly situated with CG ; but C9, the repulsive force of BE, is drawn without the angle BEC; and CF, or the attractive force of AE, must be drawn within the angle AEC. If we now compound the forces CF, CG, they will manifestly produce a resultant CH parallel to the indefinite current AB. Hence the terminated current will be urged by a force parallel to the other, and in a contrary direction; and by similar reasoning it is easily seen that if the direction of the current CE were contrary to that indicated by the arrow, or receded from AB, then the whole force in CE would be in the same direction as the current AB, and parallel to it.
Let us suppose CD to be a conductor moveable round an axis at C in the plane DD'D", and suppose the direction of its current to be from C towards D, and that of an indefinite conductor AB to be similar and parallel; then AB attracting CD will turn it round C into the position CD', and the force on the angle CE'B is then repulsive, and in CE’A attractive; hence CD' will further turn round, and the same direction of rotation will be continued in the upper semicircle; for the force is attractive in the angle D'"E"B, and repulsive in D"E"A. Hence a continued rotation will be produced. This rotation will be in the contrary direction if we change the direction of the current either in AB or CD; or if, without changing the current, we transfer AB to the opposite side of CD: hence if AB be placed so as to meet the axis C, there will be no rotation; hence also if the terminated current be moveable round its middle point there will be no rotation, since both its halves tend to rotate in contrary ways. It is easy to analyse in the same manmer the action of an indefinite conductor on a closed current sy considering its action on each of the parts, the general effect being to bring the moveable conductor into a position
o equilibrium in a plane parallel to the indefinite consluctor. Instead of a single closed circuit we may suppose any number of them connected together after an invariable manner. The action of an indefinite current will still tend to bring that system into a plane parallel to its direction, These systems have been called electro-dynamic cylinders, and also canals of currents. In consequence of the electro-chemical causes which are so widely diffused through the globe, electrical currents are generated, which give its polarity to the magnet, and which, as is well known, are sufficient to generate continued rotation of given currents. It has been found by Ampère that the actions of similar conductors on points similarly situated are equal; and that a closed conductor exercises no action on a circular conductor moveable round a central axis. In seeking for the true laws of elementary action of currents, a decomposition similar to that of the parallelipiped of forces may be employed; that is, for the action of an elementary current we may substitute the actions of the three sides of a parallipiped terminated at the same extremities; for, as before stated, if we preserve the direction of the currents we shall not alter the action by substituting any sinuous for a plane conductor with the same extremities. We will now show how the law of force between the elements of currents may be obtained, which, when once known, will reduce all the phaenomena to mathematical calculation. To determine the law of force tending to or from any element of an electrical current, when points of another current are taken at different distances but in a given direction:— Let &s, Čs' be the elements of two electrical currents, of which the intensities are i, i', their distance a unit, and f the force mutually exercised in the line forming their middle points; hence f = it’ &s és. Let &o 30' be portions of similar currents to the former, but of which the linear dimensions are v times as great, and since, their mutual distance is also v times as great; this force is diminished in proportion to some function of v, as * (v): hence f" = it’ &c. &c'. , (v).
of the distance, as in statical electricity; but we must observe that the directions of the currents are here supposed to make given angles with the joining line v. The following theorems are taken from Mr. Murphy's ‘Electricity,' to which we refer for the demonstrations, which are by no means difficult to persons a little acquainted with the differential and integral calculus. Let a right line v join the middles of the elements &s, Čs' of two currents, being inclined respectively to those elements at the angles 0, 9', the planes of which angles are mutually inclined at an angle p, and let p, p' be the intensities of the currents; the mutual action of these two elements will then be represented in all cases by the formula f f o sin 0, sin 9'cos ? – 3 cos 0 cos w}
exerts no longitudinal action on 3 s'; only a normal force. This coincides with what has been before shown for the action of an indefinite current on a terminated conductor. The same property holds true for a closed current, since in this case 6 = 9, R = Rr. From hence it is easy to find the total action of a fixed current, or a moveable rectilineal current. The action of a closed current, or an element of another current, which is turned in all possible positions round its middle point, lies in an invariable plane. The mutual action of two small closed conductors, containing areas A, X', the centres of which are at a distance v, exercise on each other a force directly as the plane areas, and inversely as the fourth power of the distance. The action of a uniform canal of currents indefinitely extended in one way varies inversely as the square of the dis; tance of its extremity from the element acted on, and directly as the sine of the angle which that distance forms with the element, and is in a direction perpendicular to the plane passing through the element and the extremity of the canal. When two uniform and indefinite canals of currents act on each other, the canals being supposed terminated at one extremity only, the resultant is in the line joining their extremities, and the force is inversely as the square of this line: hence the action of finite canals may be easily estimated, as being the difference between two indefinite canals. With respect to the nature of the force, it will be attractive or repulsive as before described. The simplest mode of observing the actions of a canal of closed currents is by twisting a wire in the form of a helix having but small intervals between the successive convolutions, the action of each portion of the helix being then very nearly the same force as that of a portion of a circle or closed current. Ampère imagined an ingenious manner of calculating the actions of any plane closed conductors. Conceive one such to be divided into an infinity of small compartments by right lines parallel to the rectangular axes of co-ordinates, and the periphery of each compartment to be traversed by currents, in the same manner as the whole curvilineal side which encloses the area; then it is easily seen that all the internal sides of the compartments, being traversed by two currents in opposite directions, will have no electro-dynamical action, and therefore the sole remaining current is that which circulates in the periphery of the given figure; but by this division into compartments we can calculate the mutual actions of the two closed conductors from the very simple law which we have already given for the action of small closed conductors on each other. Voltaic conductors, of which the centres of gravity are supported, undergo terrestrial action, similar to that produced by a canal of currents. We should infer, by the position which the moveable conductor takes, that the direction of the terrestrial currents is nearly from east to west, having the north magnetic pole situated on their right. Since the action of closed currents on an element of a conductor is perpendicular to that element, hence a straight conductor fixed at one extremity, and free to move in a horizontal plane, will receive a continued rotation from the influence of the currents of the earth; but if the conductor were supported by its centre of gravity, it would be brought by their action into a fixed plane, and an electro-dynamic o: would come into a position perpendicular to that plane. All these results of theory are confirmed by experiments, and are shown in the lecture-rooms of gentlemen who profess this branch of science. There are few works expressly on this subject beside those quoted, the subject being itself the most modern addition to the exact sciences. ELECTRO-MAGNETISM. The first important discovery in point of time, which laid the foundation of this new science, was made by Professor Oersted of Copenhagen. By reference to the article ELECTRo-DyNAMIcs it will be seen that when the wires which communicate with the poles of a galvanic battery are connected by a conductor or by being brought into contact with each other, the opposite electricities thus continually made to combine acquire a power of action on another ...i. under similar circumstances, though latent with respect to common electrical action; but this discovery of Ampère was preceded by that of Oersted, who found that the electrical current thus generated acted upon * magnetised bar, and tended to turn it round as if exer
cising a tangential force. Bofore this time a connexion between electricity and magnetism had been suspected, or rather believed, by Franklin, Beccaria, and others, from the well-known circumstance that the A. of the compassneedle had been frequently reversed during thunder-storms, and that the same effect could be produced by electrical discharges. In most experiments which were then made these discharges were unnecessarily strong; but to Oersted's discovery, followed up as it has been by Ampère, Faraday, Barlow, Arago, &c., we must ascribe the source of those accurate data by which the actions of the earth on magnets, of magnets on each other, of conducting wires on magnets, and of the earth on conducting wires, are reducible to similar and simple principles of action. When a magnetic needle is placed near a conducting wire in the plane of the magnetic meridian, and the battery is powerful, it is observed that the needle will turn round, placing itself at right angles to the direction of the current; the same effect, which we have seen in the preceding article, would be produced by the same conductor on a canal of currents. If we suppose that a man with his face turned to the needle is himself the conductor, with his feet at the positive pole, the north pole of the needle will turn towards his right. This must be understood as only meant to illustrate the direction of rotation. In order to discover the law of action of a current on a magnetic element, Biot and Savart used a small magnetic needle, guarded from the agitations of the air, and having the action of terrestrial magnetism neutralized by a bar, thus subjected only to the immediate action of the conductor. Having acquired the position indicated by Oersted, the times of its small oscillations were observed, which we know by the principles of Dynamics must be inversely proportional, cacteris paribus, to the square root of the accelerating force impressed. By observing the times in which, for instance, ten oscillations of the needle took place, at different distances, it was deduced, without difficulty, that the electro-magnetic force exercised by the whole conductor was inversely as the distance of the needle from the conductor: this of course supposes that the current may be regarded as indefinite, compared with the dimensions of the needle. Hence it easily followed, as was shown by Laplace, that the force exercised by each element of the conductor on the magnetic needle must, like all known forces, vary inversely as the square of the distance; and Biot showed that, when the distance was given, the force was then proportional to the sine of the angle formed by each element of the current with the right line joining the middle of that element with the middle of the needle. It has been shown by means of the multiplier that the electrical intensity of the current at different points of the same conductor is constant. We may observe that the primciple of the multiplier consists in bending the wire in the form of a helix, but returning upon itself so as to form a closed circuit, the wire being covered with silk to prevent communication at the crossings; the action of such a spiral being similar to that of closed circular currents equal in number to the spiral convolutions. It was afterwards found that the magnetic needle of the multiplier could be acted on by electrical discharges from a Leyden jar; and Mr. Faraday showed conclusively that, with the condition of time, ordinary electricity can produce a continued deviation of the needle; this oil. he fulfilled by making the electricity pass through imperfect conductors. Arago observed that small fragments of soft iron were attracted by the conductor of the galvanic pile, and the same current imparted permanent magnetism to small needles of steel. The needle should be placed perpendicularly to the joining wire or current, or, which is better, be introduced in a helix, the discharge of the current through which instantaneously magnetises the needle. Nobili observed that needles placed between the isolated spires of a plane spiral of copper wire were, by an electrical discharge, magnetiscd in opposite ways, when near the centre and when near the circumference. Savary also observed that when needles were placed horizontally with their middle points vertical over a horizontal current and the needles perpendicular to the direction of the current, they were differently magnetised according to their distances. These experiments he has varied relatively to the
length of the needles, the length and diameter of the con ductor, &e
The magnetising force of the current is transmitted without sensible loss through isolating media, as glass, wood, &c., but is much altered by the interposition of conducting plates, a result similar to the development of ordinary electricity by the influence of electrised bodies. Thus:— A large plate interposed between the conductor and the needles weakens the magnetising effect of feeble discharges, while it augments strong ones; and for a given charge, a thin and a broad interposed conducting plate may produce contrary effects, and with a certain determinate breadth the effect would be unaltered, and in general the two surfaces of the same plate exercise contrary actions. (Savary.) When a bar of soft iron, bent in a horse-shoe shape, is encompassed by a helix covered with silk and always turned in the same way, it may be made to receive a powerful magnetism under the influence of a current through the helix discharged from a voltaic battery. Mr. Watkins has made some valuable experiments on the conservation of the magnetic power in soft iron, for which see Phil. Trans., 1833. The discovery of the currents produced by volta-electric induction is due to Mr. Faraday. With about 203 feet of copper wire he formed each of two helices, and twisted them about a cylinder of wood, making one in communication with a galvanometer and the other with a powerful voltaic pile. The moment the communication was established, the galvanometer deviated; then, after some oscillations, returned to its place, and again deviated the instant this communication was broken: hence the directions of the inducing and induced currents are contrary, while that generated at the interruption of communication or cessation of the inducing current is directed the same way with the latter. The same philosopher has also succeeded in producin currents by the influence of magnets, his experiments .# the great magnets of the Royal Society proving most manifestly the disengagement of electricity by the influence of ordinary magnetism. The extraction of the electrical spark from the magnet is now pretty jo, exhibited, as also the continued rotations produced by terrestrial magnetism. The theory of Ampère, which supposes electrical currents to exist round the component particles of magnetised substances, and round the mass of the earth, is perhaps the most satisfactory explanation yet given of the cause of magnetic action, and has been greatly strengthened by the discoveries of Faraday on electro-magnetic induction, by which many objections that had been urged against this theory are removed. This branch of science is daily receiving constant accessions, and it is gratifying that much of its progress is eminently due to our countrymen. The labours of the French and German philosophers have also been far from unfruitful. The following works may be consulted on this subject: Gilbert's Annalen ; Memoirs by Erman of Berlin, Prechtl, Hansteen, &c.; and in Poggendorf the papers by Seebeck, Kupffer, &c.; the recent volumes of the Philosophical Transactions, containing Faraday's Researches, Professor Cumming's Electro-Dynamics, and his papers in the Annals of Philosophy; Barlow's labours described by himself in an article of the Encyclopaedia Metropolitana, &c. ELECTRO'METER. This term strictly applies only to instruments adapted to measure electricity; it has however been applied in a more extended sense to those which only indicate the presence of that fluid; but these are more correctly denominated electroscopes. Of the former kind is the Balance of Torsion invented by
Coulomb, to which we have had occasion to refer in the articles Elasticity and ELECTRicity. The following is a description of this delicate instrument. A very fine metallic wire, or, which is better, a single thread of silk taken from the cocoon, is fixed at the upper extremity, and at the lower it supports horizontally a fine needle made of a good non-conducting substance, as gumlac, to one of the ends of which is attached the body to be electrised, as for instance a small ball of elder-pith; at the top of the suspended string there is placed a plate moveable with friction on a glass cylinder, in which the thread is contained, by which any requisite torsion may be given to the thread, which is shewn by an index on a micrometer screw; the body of the large cylinder which encloses the needle is also surmounted with a graduated brass circle. In electrical experiments the index of the micrometer is on 1ts division zero, and the plate is turned round to bring the needle and pith-ball to the zero of the graduated circle on the string. Again a second ball is attached to the extremity of a fine isolating cylinder inserted in the apparatus so that both balls may be in contact without pressure. The balls are then electrised by communication with some isolated and electrised body, and acquiring similar electricities repulsion immediately takes place. That attached to the needle being moveable with it, carries it round through a certain angle, and after some oscillations settles at a definite position with respect to the fixed ball, this angle being indicated by the graduated arc; the elastic force of torsion is then in equilibrium with the moving force of repulsion between the balls, and hence a measure of the latter can be obtained. In such experiments only a very small electrical charge is communicated to the balls. Coulomb, in seeking the law of electrical action, found that in the first instance of his experiment the needle deviated by 36°. Then, communicating a torsion to the thread in a direction tending to diminish this deviation, he found that the micrometer index traversed 126° to reduce the angle of deviation to 18°, and 56.7° of torsion was necessary to bring it to 83°; the thread being twisted by forces applied at both ends it is evident that the entire torsions in the two latter cases are 126°4-18°= 144° and 567+8}=5754°, while in the first case it is only 36°. By comparing the deviations with the torsions, it was easily seen that the force of repulsion varied inversely as the square of the distance between the balls. It should be remembered in such experiments that if the torsion of the thread be too great, its elasticity will act imperfectly, and be no longer proportional to the angle of torsion. #o In like manner the law of attraction of differently electrised balls was ascertained, the torsion being then employed in resisting the attraction. We may observe here that the results thus deduced are necessarily approximative, and not exact, because the neutral electricity of the balls being partly decomposed by the mutual influence of the electricities communicated, the small forces thus arising interfere with the actions which should be due to the latter only. The attractive and repulsive forces may also be estimated by disturbing the needle a little from its position of equilibrium, and observing the number of oscillations which it makes in a given time, as was adopted by Biot in determining the law of electro-magnetic action of a galvanic Current. The proof-plane also used by Coulomb was o al small disc of gilt paper fastened to an isolating handle; this he employed to discover the distribution of electricity on the surfaces of bodies by touching them with the plane at various points, and observing by means of the torsionbalance the quantity of electricity taken up by contact, which he assumed to be proportional to the quantity of electricity at the point touched. Mr. S. Harris has lately thrown doubt on the exactness of this assumption. Various instruments have been constructed for estimating approximatively the total quantity of electricity in the charge of an electrised body, such as Lane's, Henley's, and Cuthbertson's electrometers. The most precise instrument of this description is one recently invented by Mr. Harris, who is always distinguished by the beautiful precision of his experiments; its description will be found in, his paper on electricity in the “Philosophical Transactions. Electroscopes indicate the presence of very small quantities of electricity, and therefore are generally used with a condenser; as the gold-leaf electroscope, consisting of two small portions of gold-leaf laid flat together; and when