When he had discovered this fundamental law, he thought it proper to publish the discovery, in order that it might be as soon as possible perfect ed by the co-operation of other philosophers. Ap prehending that others might lay claim to this dis covery, he sent a short Latin description of his ex periments to the most distinguished philosophers and learned bodies; and though, by this means, he has not avoided the pretensions which have been made to his discovery by others, still he has rendered them ineffectual. It deserves, perhaps, to be no ticed, that the above-mentioned Latin description, consisting of four pages in 4to., of which the first gives the introduction and the description of the apparatus, the last the conclusions, contains upon the two intermediate pages, the results of more than 60 distinct experiments. From this brevity, it has happened, that some philosophers have thought that he had treated his subject in a super ficial manner.
As the details of this discovery, and of all those which have originated from it, will be exhibited in this article, we shall in the remainder of this his torical sketch, in order to avoid repetitions, confine ourselves to the most striking and leading facts, and insert the other historical notices in the doc trinal part.
The first discovery to which that of Professor Oersted gave occasion, was that of Ilk. drnpere, member of the French institute. He found that a conductor, conveniently suspended, is attracted by another, when both are transmitting an electrical current in the same direction; but that they repel each other, when the two currents have opposite directions. Professor Schweigger at Halle, invent ed at the same time, an electromagnetical multipii cator, which is of very extensive use. Ilk. *drag° found that steel can be magnetized by the electrical current. Ilk. Gay Lussac at Paris, and Professor Ermann at Berlin, discovered, that when the cur rent has passed perpendicularly through the plane of a steel ring, or through a steel plate, it shows no magnetical effect, before the circumference was in terrupted.
The most remarkable of all the discoveries, to which that of Oersted has given occasion, is no doubt the thermoelectricity, discovered in 1822 by Dr. Seebeck,member of the Royal Academy at Berlin.
In the same year, the rotation of a magnetical needle around an electrical current, and of a body, which transmits an electrical current around a magnet, first imagined by Dr. Wollaston, was ex hibited in a series of ingenious experiments by 111r. Faraday.
Effect of the Electrical Current upon the Magnetic Needle.
The galvanic battery was the first apparatus, by which the magnetic effects of electricity were de monstrated. In order to make it give its mag netic action, its two poles must be joined by a con ductor, commonly a metallic wire, which, for bre vity's sake, we shall call the uniting conductor, or the uniting wire.
When not closed, the galvanic circle produces no effect upon the needle of a compass.
When the uniting wire is approached, and placed parallel, or nearly so, to a properly suspended mag netical needle, it is caused to deviate from its ordi nary direction.
The magnetical effect of the electrical current is not interrupted by the interposition of other bodies. Already the first experiment showed that it passes like the magnetism of a loadstonc through metals, glass, resin, wood, stoneware, water, Ste.; even when the magnetical needle was placed in water, it was affected by the electrical current.
When the conducting wire is placed parallel to a conveniently suspended magnetical needle, the direction of the needle is changed.
1. If the needle is above the wire, and the posi tive electricity passes from the right to the left hand of the observer, the north end of the needle will go from the observer.
2. When the needle is below the wire, the direc tion of the needle is changed in the opposite way; its north end approaches to the observer. it is not
necessary, in this and the preceding experiment, that the needle is in the same perpendicular plane as the conducting wire; it is only required that the needle shall be sufficiently near the wire, and in the first experiment, in a plane above, in the last in a plane below it.
3. When the needle is in the same horizontal plane as the wire, and is placed between the observer and the wire, the north end is elevated.
4. If the needle is upon the opposite side, the north end is forced down. in these two experi ments, the needle must be very near to the wire.
From these facts, Professor Oersted concludes, that the magnetical action of the electrical current describes circles round the conductor. It will perhaps not be out of place to quote here his own words, which have been overlooked by several authors, who have written the history of this discovery.
In the original publication he says, "ex observa tis colligere licet, hunc conffictum (the electrical current,) gyros peragere; nam hoc esse videtur conditio, sine qua fieri nequit, u t eadem pars fill conjungentis (conducting wire,) qua; infra polum magneticum posita cum orientem versus ferat, su pra posita eandem occidentum versus agat." For the sake of brevity we shall, in the following pages, denominate the direction of the current after the system of Franklin; or, to speak according to the system of two electricities, after the direction of the positive electricity in the current. If we now suppose that the electricity of the current enters the conductor at the right hand of the observer, the austral magnetism (the same which predomi nates in the north-end of the needle,) will, upon the superior surface of the conductor go off from the observer; on the side most distant from the obser ver, the austral magnetism goes downward; on the inferior surface it goes towards the observer; on the side nearest the observer it goes upwards. This is represented in Plate DXXII. Fig. 1. where B A is the conductor in which the direction of the current is A B, the circle c d ef represents a plane perpendicular to the conductor, in which the mag netical circulation takes place. This plane is here and in the other figures represented as if it were material and opaque. The little arrows show the direction of the austral magnetism. We can make the application of this law to experiments, in a very commodious manner. For this purpose take a piece of paper (Fig. 2,) upon which the arrows and letters, there represented, are drawn. This piece of paper is to be wrapt around a cylindrical body, for instance a pencil, in such a way that the arrows lie in a plane perpendicular to the axis of the cylinder. We have thus an electromagnetical index, which, put in the place of any part of the conductor, shows the direction of the magnetical powers in it. The sharp ends of the arrows indi cate the direction in which the austral magnetism (and consequently the north-end of the needle,) is repelled, and the contrary attracted; the opposite ends of the arrows indicate also the direction in which the boreal magnetism (and consequently the south end of the needle) is repelled, and the con trary attracted. The reader may understand with out trouble the most complex facts we are here to explain, if he has at hand two such cylinders, dur ing the experiment. The same thing may be ex pressed in different. ways. Air. Hill, lecturer of mathematics at the University of Lund, in Sweden, has proposed one of the best. Let us imagine, says he, that the observer swims upon the electri cal current, with his face turned outwards, (with his back turned towards the axis of the current,) and his head towards the origin of the current, the direction of the austral magnetism of the current will always proceed from his left to his right hand.