is brought towards a mass of mercury traversed by electric currents, a rapid rotation of this metal is determined around the solid conductor that is plunged in to transmit the current. The experiment succeeds well with the apparatus that is employed for demonstrating the raising of two small cones of mercury above metal points, communicating with the poles of a pile. The direction of the rotation depends upon that of the current, as well as of the pole of the magnet that is brought near to the mercury. The phenomenon is due to the action that is exercised by the magnet upon the currents which, setting out from the metal point that is plunged into the mercury, are disseminated over the surface of this eminently movable metal conductor. Mr. Poggendorff, who has made a special study of the rotation of mercury, has remarked that, when it has occurred for a certain time, it relaxes and altogether ceases. This effect appears to arise from the oxidation of the mercury, which is facilitated by its movement, and which, when it has once occurred, diminishes the liquidity of the metal by rendering it viscous. Priestley had already remarked that simple agitation in contact with air determines an oxidation of this metal, which is manifested by the blackish colour it acquires, and by a diminution of its liquidity. We may also obtain the rotation of mercury by surrounding a capsule filled with this metal with a fixed circular conductor, making several revolutions around the capsule, and traversed by a current. The same current is made to pass through the mercury, making it penetrate by the centre and come out by the borders of the movable disc, constituted of the liquid. This experiment is of the same nature as that of the movable rectilinear conductor (Fig. 108.) set in rotation by the action of the exterior current. Here each filament of mercury plays the part of the movable conductor. In order that the experiment may succeed well, we have merely to take care that the current be very powerful, provided it is to be distributed over the whole surface of the mercury.

An interesting observation is that of the rotatory movements assumed by mercury in the interior of a hollow magnet, and of their comparison with those of mercury placed outside this

magnet. In the interior of a hollow vertical magnet, closed at its lower extremity, we place a certain quantity of mercury, greater or less as the case may be; the current arrives by a metal point plunged in the centre of the circular surface of the mercury, and radiates from the centre toward the inner surface of the hollow magnet. If the level of the surface of the mercury is lower than the plane in which the upper pole of the magnet is situated, the rotation occurs in the direction determined by the direction of the currents of the magnet; it attains its maximum of velocity when this level coincides with the middle of the magnet. If the level coincides exactly with the plane of the pole, the rotation is null; if it is above that plane, it occurs in an opposite direction. This result, which is analogous to what we have already mentioned of the opposed action of the two parts of the same face of a magnet, situated on a different side of the pole, is due, as we have already said, to the particular distribution assumed by the molecular currents at the extremities of magnets. When the hollow magnet is made to plunge into a vessel filled with mercury, taking care that the level of the exterior liquid coincides with that of the interior liquid, we see, conformably with what should follow from Ampère's theory, that the surface of the mercury assumes outside a rotatory movement, contrary, in regard to its direction, to that which occurs within.

Mr. Poggendorff, who has lately studied carefully the phenomena of the rotation of mercury under magnetic and electro-dynamic influence, has also observed, as I had previously done in 1824*, that the direction of the rotation is different according to the part of the magnet with which the surface of the mercury that transmits the current is in contact. He has remarked in particular that this direction changes, according as the section of the magnet plunged vertically into a vessel full of mercury is above or below the pole. If the currents, of which by the theory a magnet is

* Memoirs of the Society of Physics and Natural History of Geneva, tom. iii. 2nd part, p. 127.

constituted, were all, from one extremity of the magnet to the other, quite parallel with each other, and all determined in the same direction, it ought not to be so. In fact, with an electro-dynamic helix or a solenoïd, the rotation occurs in the same direction from one end to the other; but the poles also in these electro-magnets are at the very extremity, whilst in true magnets the poles, as we have seen, are at a short distance from the extremities. It is this same circumstance which causes, as we have seen, the same current to exercise upon the same face of a magnet an attractive or repulsive action according as it acts upon the portion of the magnet situated between the two poles, or upon those which are beyond these poles, even upon the same side. This characteristic difference between a magnet and a solenoïd is very probably due, as we have already seen, to a particular arrangement of molecular currents at the extremities of magnets, itself arising from the rather complex mutual action which these currents exercise upon each other, and from the molecular constitution of the magnetic substance.

Another kind of rotation is that which is obtained by placing between the two approximate poles of a horse-shoe magnet, a metal wheel fixed vertically to a horizontal axis that passes through its centre, and around which it can turn (Fig. 115.). This wheel is tangent, in the lower point of its circumference, to a surface of mercury placed in communication with one of the poles of the pile, whilst the other communicates by means of the axis with the centre of the wheel. The vertical current is attracted by the combined action of the two branches of the magnet; the wheel, obeying this attraction, moves; as the part that was traversed by the current ceases to be so traversed, as soon as it ceases to be vertical, since it is no longer tangent to the mercury, the action of the magnet attracts the new part of the wheel which has replaced the former, and so on; whence results the continuous rotatory movement.

Fig. 115.

Action of the terrestrial Globe upon electric Currents.

We have already seen, in the first paragraph of this Chapter, that a wire bent into a circle or a rectangle, in a word, forming a plane and closed curve, when traversed by a current, and movable around a vertical axis, places itself in a plane perpendicular to the magnetic meridian, and so that the current is directed from east to west in its lower part. Ampère discovered further that, if the rectangle is movable around a horizontal axis so as to be perfectly in equilibrium in all its positions around this axis, which we take care to arrange perpendicular to the magnetic meridian, it places itself in such a position, when traversed by a current, that its plane is perpendicular to the direction of the magnetised dipping needle; the current is always directed from east to west, in the lower side of the rectangle (Fig. 116.).

Fig. 116.

In all these experiments a circular current, a rectangular one, or one of any form, behaves like the section of the magnet, the form and size of which would be those of the figure formed by the wire. A ring, formed of several circular or rectangular turns of the same wire, covered with silk, as is done in M. G. de la Rive's floats, represents several continuous sections of a magnet, and is able more easily than

other combinations of currents to obey the directive action of the earth, under the sway of a very small voltaic force, such as that resulting from a single pair. In these apparatus we have true compasses; they are also generally provided with small card arrows, which, by means of a piece of wood or whalebone rising vertically from the cork, are fixed by their centre perpendicularly to the plane of the current. These arrows, when the floats have acquired the position that is impressed upon them by the terrestrial globe, are found to have exactly the direction of the compass needle. Care is taken so to place the point of the arrow, that it is turned to the north side when the current goes from east to west in the lower part of the ring (Fig. 117.).

Fig. 117.

Finally, a helix of a not very large diameter, (from two to four inches), when its wire is traversed by a current, takes the same direction as a true magnet would take, whose axis should be the same as that of the helix. For this experiment we may employ the helices that we have already described, and of which, under the name of electro-dynamic cylinders or solenoïds, we have determined the properties as being altogether similar to those of magnets. The most convenient apparatus is a float, in which the ring is replaced by a closed helix, the two ends of which return interiorly to the middle, along the axis.

The employment of electric currents for studying the di