cause. The needle, after receiving the electric explosion, had acquired a singular species of polarity; for its two ends turned, not to the north and south, but to the east and west. Such was the very imperfect state of our knowledge on this important question at the period of the discovery of galvanism. The novel and extensive field of inquiry laid open by this discovery was explored with ardour by philosophers in every part of Europe; and by arming them with new powers and means of investigation, gave a vigorous impulse to the advancement of physical science. It was some time before the real nature of the galvanic influence was perfectly ascertained; and a long controversy arose on the question, whether it was a mere modification of electricity, or a distinct and peculiar power. But all doubt as to its identity with electricity was at length removed by the splendid discoveries effected by the voltaic pile and battery, and the successful imitation of some of their effects by means of common electricity, which was accomplished by Dr. Wollaston*. For the clear understanding of the subject of electro-magnetism, it is requisite that we set out with correct ideas of the distinctions which characterize the different forms of electricity and of galvanism. These distinctions may be traced in the very origin of those agents. The electricity which is produced by friction, as in the common electrical machine, and which is accumulated in insulated conductors, exists in a condition of comparatively high intensity. In order to have a clear notion of what is meant by the condition of tension, or intensity, as contra-distinguished from absolute quantity, when these terms are applied to any physical power, it should be observed that the latter of these conditions has for its measure the total effect which the power in question is capable of producing, from first to last, when the whole of that power is expended in the production of its proper effect. Tension, or intensity, on the other hand, has reference to the degree of effect produced with relation to circumstances which limit the operation of the power in certain ways, so as to allow only of its partial exertion. This distinction is very clearly marked in the case of heat, or caloric. When one body is brought into contact with another which is colder than itself, the quantity of heat it will, during the first moment, impart to the latter will depend, not on the absolute quantity of heat it really contains, but on the tendency of that heat to escape from it; and this tendency is the immediate consequence of its state of tension. But it is well known, that the degree of tension of the heat residing in bodies, and consequently the tendency which it has to quit those bodies, * Philosophical Transactions for 1801, p. 427. bears bears no constant proportion to the absolute quantity of heat they may contain. It is this quality of tension in heat, which is expressed by the term temperature; while the ratio between the absolute quantity of heat in any body, and its temperature, is denoted by the term capacity for heat. A distinction precisely analogous obtains in the case of electricity, with regard to its different modes of efficiency, which it is necessary to keep in view in the estimation of its effects. The tension of the electricity in any body depends, in part only, upon its absolute quantity; for it is also influenced by many other circumstances, such as the vicinity of other electrified bodies, and even the figure of the body which contains it. If the form of the conductor, for instance, be altered, by drawing out one part into a point, the tension of electricity at that part will be increased in a high degree, although the absolute quantity contained in the conductor remains unaltered. The same quantity of electricity, on the other hand, diffused over a more extended space, provided the general form of the body remain the same, will exist in a state of diminished tension. The Leyden jar furnishes a good illustration of this difference between quantity and tension: the two coated surfaces of this instrument are capable, in consequence of the influence of induction, of receiving and of containing a very large quantity of electricity-yet its tension is very small; and the instrument may in this sense be considered as having a great capacity for electricity. It thus gives us the facility of accumulating electricity in vast quantities, and of obtaining some of its most energetic effects. The enormous mass of electricity which may thus be collected in a battery, when suddenly transmitted through bodies, acts with the greatest violence upon their whole texture, combining the operations of intense heat with mechanical separation; the hardest textures of mineral or organized bodies are rent asunder and shattered into fragments; and metals, even the most refractory, are volatilized, or reduced to dust. Philosophers had long been familiar with the powerful but destructive effects of these sudden passages of the electricity which they had collected in their batteries, and which they could fulminate in whatever direction they pleased; but they remained in ignorance of the more delicate operation of a continued stream of the same fluid. The action of the electric battery, although sufficiently energetic, was but momentarily exerted; like the explosive agency of gunpowder, its power was expended as soon as the blow was struck; causes and effects were crowded in too rapid a succession to be traced; and the power was too mighty, to be retarded or controlled by human means. For For our knowledge of a new modification of electric force, namely, that which it exerts while passing along conducting substances, we are entirely indebted to galvanic combinations, which, by developing, during an indefinite time, very large quantities of electricity, enable us to observe its effects with great precision. The voltaic apparatus, in its state of activity, may be regarded as a vast magazine of electricity, capable of furnishing a continued supply, for a very long period, at the demand of the experimentalist, and placing this mighty agent, for all the purposes of scientific investigation, entirely under his control. It may be observed, that the effects of tension, which are manifested in ordinary electric experiments, are no longer perceived in the action of these electric currents, and appear to have no concern in the phenomena. Light bodies in the neighbourhood are neither attracted nor repelled, nor is the electrometer affected by the conducting wire. The galvanic effects of the voltaic battery, such as the decomposition of water and other fluids, which are made to form part of the circuit, proceed without interruption, and appear to be unaffected in kind or degree by the vicinity of electrified bodies, or by any charge of electricity which the apparatus may receive, from a machine. These effects are produced solely by the currents of electricity which are made to circulate in the instrument; and are in proportion to the force of these currents. They cease the moment any interruption occurs in the voltaic circuit, although such interruption is immediately followed by signs of electric tension at the parts where the circuit is broken. These two conditions of electric force, namely, tension on the one hand, and a continued current on the other, produce two classes of effects which are totally different from each other, and appear to be almost in the inverse ratio of each other: the former are those commonly ascribed to electricity; the latter are those generally denominated galvanic, such as the ignition of metals, the decomposition of chemical compounds, certain physiological effects on the nerves and muscles, and lastly, actions on other electric currents, and on magnetic bodies. It is to these two last classes of effects that our attention is now to be specially directed. Ritter stated, that he had succeeded in imparting magnetic properties to a gold needle, by placing it in contact with the extremities of a voltaic circuit. He seemed to have some vague idea of electric terrestrial poles at right angles to the magnetic poles, and also supposed that there was a chemical difference between the two poles; but his experiments attracted little attention, and certainly threw but little light upon the connexion between electricity and magnetism. Soon Soon after this, Professor Ersted, of Copenhagen, published a work in German, of which a French translation, under the title of 'Recherches sur l'Identité des Forces Chimiques et Electriques,' appeared at Paris in 1813; containing many hypothetical views on the nature of electric influence, and its analogies with magnetic action. In speaking generally of these forces, he observes,— Il paroit donc que les forces peuvent se croiser sans se troubler, quand elles agissent sous des formes d'activité différentes. La forme d'activité galvanique tient le milieu entre la forme magnétique et la forme électrique. Les forces y sont plus latentes que dans l'électricité, et moins que dans le magnétisme. Il est donc vraisemblable que les forces électriques exerceront, en les croisant, une influence moindre sur les forces magnétiques que sur les forces galvaniques, Dans la pile galvanique, c'est l'état électrique qu'elle a, comme ensemble de forces, qui se trouve changé par l'approche du tuyau de verre: de même, ce n'est point cette distribution intérieure des forces qui constitue le magnétisme, qu'on peut changer par l'électricité, mais c'est l'état électrique qui convient à l'aimant comme corps en général. Au reste, nous ne prétendons rien décider à cet égard; nous avons seulement voulu éclairer, autant qu'il est possible, un sujet si obscur. Mais dans une question aussi importante, nous serons satisfaits, si l'on juge que l'objection principale contre l'identité des forces qui produisent l'électricité et le magnétisme, n'est qu'une difficulté, et non une chose qui lui soit contraire.'-p. 236. In trying experiments with a view to the illustration of these opinions, Ersted succeeded in obtaining decisive indications of the action of the conducting wires of the galvanic pile, during the passage of electricity, upon the magnetic needle. The phenomena were at first view not a little perplexing; and it was not till after repeated investigation, that, in 1820, the real direction of the action was satisfactorily made out. The conducting wire was found to exert a magnetic force, not in a direction parallel to the wire itself, nor even in any plane passing through that direction, but in one that was perpendicular to it; and with the northern polarity of each turned in one invariable direction, as we follow them round the cylinder. Then will these imaginary magnets indicate the direction and nature of magnetic forces, which emanate from the wire as long as the stream of voltaic electricity is passing through it. The particular direction of this transverse, or, as it has been termed, tangential magnetic force will of course depend upon that of the electric current in the wire, and may easily be traced in all cases by the recollection of the following fact. Supposing the wire to be in a vertical position in which case the planes of the tangential forces will be horizontal-and supposing the stream of positive electricity to be descending along the wire, (which of course implies that the negative electricity is ascending,) then that polarity which exists in the end of the magnetic needle, which naturally turns to the north, will be impelled round the wire in the circumference of a circle in a direction similar to the motion of the hands of a watch; that is, from the north to the east, and then to the south and west. The south pole of a magnet will of course be impelled in the contrary direction. A magnetic body in the vicinity of the wire will, by the influence of this force, tend to assume a position, indicated in the annexed figure, similar to one of the tangential lines we have been describing as placed on the cylinder. But further, the tendency of the electric current DN in the wire is to induce magnetism in soft iron or other bodies capable of receiving it; and the magnetism so induced has the precise direction already indicated as that which a bar previously magnetized would assume by the influence of the wire. This direction is shewn in the figure, where N and S denote respectively the north and south poles of steel bars, situated transversely with respect to a vertical conducting wire, in which the current of positive electricity is descending, as indicated by the arrows. S All the facts which have been brought to light by Ersted are the immediate consequences of the above general law. Mr. Barlow's enunciation of this law, which is as follows, involves too much hypothesis to be strictly philosophical he states 'that every particle of the galvanic fluid in the conducting wire acts on every particle of the magnetic fluid in a magnetized needle, with a force varying inversely as the square of the distance; but that the action of the particles of the fluid in the wire is neither to attract nor to repel either poles of a magnetic particle, but a tangential force, which has a tendency to place the poles of either fluids at right angles to those of the other; whereby a magnetic particle, supposing it under the influence of |