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(by the supposition). Hence the forces of the two galvanised wires are exerted in the same direction ; for the wires are of the same kind, and are propelled the same way.

The conclusion still holds good, when the junction of the wires is heated at D instead of E.

Having thus ascertained, by the former supposition, that the forces of the chemico and thermo galvanized wires are exerted in contrary directions, and that by the latter supposition these forces are exerted in the same direction," I shall leave the theorizing philosopher to adopt which side of this dilemma he may think fit; for it is not my province to predict whether we are to have a thermo-electric force acting in a contrary direction to our old galvanic force, by the former supposition, or whether we are to have these two forces reconciled to each other by the latter; my business being to ascertain some general law whereby to regulate my subsequent experiments.

I have since made experiments, both chemico and thermo, with combinations of other metals, aud the results have given me the greatest satisfaction that the above law is general; for the comparison is, in all that I have made, the same as with copper and zinc. +

Here follows a description of the instrument I have made, for the purpose of exhibiting and comparing chemico and thermo phenomena as influenced by the magnet; by means of which I can make the experiments with the greatest facility and exactness. I have named it the Comparing Galvanoscope.

Fig. 4. A cylindrical glass tube supported by the arm Cc of the brass stand DĚ; on the top of the cylinder is a moveable brass cap ff. To the upper side of this cap is adapted a thick wire, with its bobbin

9 and milled head H. This wire turns in two spring sockets at ii. One end of a piece of untwisted silk is fixed to this bobbin ; the other end is supplied with a finė silver wire hook, which descends through a small hole in the centre of the cap into the glass cylinder. The circular plate o at the top of the stand has a flat moveable rim, which is graduated from 0 to 904 each way. This is for measuring the quantity of deflection ; and hence the instrument in some measure answers the purpose of a galvanometer.

Fig. 5 shews a chemico combination out of the instrument. A is a small glass vessel containing dilute acid. The parallel slips or wires a abb are the dissimilar metals for the experiment. The conducting wire ccc is soldered to their extremities at a and b.

To make the experiment, the glass cylinder is taken off the stand by unscrewing the nut c, fig. 1. The combination is then hooked on and drawn to the top of the inside of the cylinder by turning the pin H. The glass vessel, with its acid, is placed on the circular plate 0; and the cylinder replaced, and the nut screwed tight. The metals may now be let down into the fluid to any depth the experimenter thinks fit. The circle may now be adjusted by bringing o under one of the wires, and the magnet being applied outside of the cylinder will deflect the wire.

* Some time after this was published, I learned that Professor Cumming had previously discovered the same facts.-W. S.

+ This is found not to be a general law with all metals.-W. S.

To make a thermo experiment, the glass cylinder is taken off as before, the combination hooked on, and the whole replaced ; when the wire is at rest (which soon will take place, for the cylinder entirely cuts off the undulations of the external air). The graduated circle is now adjusted, and the lamp applied at L, and the magnet as in the figure.

This method of detecting the galvanic influence in delicate chemico combinations is so efficacious, that a piece of fine silver wire, such as forms a part of what is called gold lace, with a piece of zinc of the same dimensions let into the dilute acid not more than 1-10th of an inch, will cause the connecting wire (on the approach of the magnet) to be deflected 40°, sometimes 50°.

I have a great variety of combinations ready, which are placed between the leaves of a small book made for that purpose, with the names of the metals on their respective leaves.

The whole apparatus packs up in a neat mahogany box.

This instrument differs from the galvanometers I have seen described. I use a powerful magnet for detecting and determining a slight galvanic influence; whereas in the galvanometer a feeble needle is used for that purpose. Many other differences might easily be pointed out, whether advantages or not is not for me to determine.

I am, gentlemen, yours, &c., Artillery Place, Woolwich, Feb., 1824. WM. STURGEON.

Electro and Thermo-Magnetical Experiments. By Wm. STURGEON.

To the Editors of the Philosophical Magazine and Journal. Gentlemen,-Although the experiments I have detailed in my former paper have sufficiently satisfied myself with respect to the directive exertion of the forces of the differently excited wires or machines, yet it is not impossible that some may still doubt of the sufficiency of those experiments to determine the apparently anomalous phenomena : owing, perhaps they may say, to the difference of the construction of the apparatus employed for exhibiting them. I am persuaded, however, that the following mode of making the experiments will, in all probability, be sufficiently decisive to convince the most sceptical on this point.

Suspend the semi-circular copper arc, with its zinc diameter, as described in my former paper (the extremities of the metals need not be soldered, but twisted together). Wrap one of those joinings of the metals loosely with a piece of tow or unspun cotton. Dip this part of the machine into dilute nitric acid (observe, to counterpoise at the other end); present the north pole of a magnet to the same arm, and it will be projected to the right.

Take off the cotton, wash and dry the machine, and suspend it as before. Apply the lamp now instead of the acid, and the magnet as in the former experiment; that arm of the apparatus will be now propelled to the left.

I have merely pointed out this method of making the experiments, as the most likely to be understood in comparing the phenomena ; but that described with the galvanoscope, for making the chemico experiment, is by far the most efficient and eligible.

The results obtained from the mode of comparing chemico and thermo phenomena, could hardly fail to suggest the idea, that chemico-excited wires would have their electrical tension increased by thermo application at the opposite extremity. And in order to try the suggestion by the test of experiment, í had recourse to the above-described simple apparatus. I twisted the copper wire a good length round the extremities of the zinc, so that as great a metallic surface as possible might be exposed to the action of the acid. After dipping the extremity wrapped with tow into the acid, I suspended the machine in the galvanoscope. On presenting the north pole of the magnet to the arm ascending from the chemico extremity, the latter was deflected to the right about 80°. From that it returned to nearly 200 : thence propelled again to nearly the same distance as before; and so vibrating several times from about 15° to 609, when I changed the pole of the magnet. It was now deflected in a contrary direction (left) to about 70° : from thence it returned as before by the silk, endeavouring to untwist itself, and was again propelled by the magnetic influence ; thus vibrating, still describing a smaller arc, and approximating nearer the magnet.

When it had become so feeble that the greatest distance did not exceed 30°, I applied the lamp at the other extremity. It was soon propelled to above 100% : and by keeping the extremity of the wires warm (it is well known that zinc would soon melt in a strong flame), I could keep it vibrating at about right angles to the pole of the magnet; for when it went further than 90°, the thermo arm became acted on by the magnetic influence, and conspiring with the reaction of the convoluted silk, the machine was frequently driven back again 40° or 50°; but by keeping it moderately warm, it was kept at between 80° and 90° from the magnet.

I now again changed the pole of the magnet, and took away the lamp. The chemico action has now become so feeble as to be just discernible. However, by applying the lamp it soon acquired between 60° and 70°, and could be kept up to nearly the lower point.

I have repeated the experiment, with the same success, in about 62 minutes each time. The copper wire was about 1-60th of au inch in diameter, and the zinc about twice that thickness.

When the lamp is applied before the chemico action gets too weak, this thermo-chemical magnetic experiment, in miniature, is most strikingly decisive. Not having it in my power, at present, to carry on the experiment on a large scale, I am not prepared to say how it might answer. All the phenomena yet exhibited by the differently excited wires, seeming to be so perfectly analogous in every other respect than in the direction of their forces, I have, by parity of reasoning, found no difficulty in producing a thermorotation by the influence of a central magnet.

As this experiment seems to have baffled the exertions of some of your scientific correspondents, it may perhaps be considered of some importance in promoting the advancement of the science. Artillery Place, Woolwich, I am, yours, &c. Feb. 16, 1824.

WM. STURGEON.

Description of a Rotative Thermo-magnetical Experiment.

By WILLIAM Sturgeon. To the Editors of the Philosophical Magazine and Journal. Gentlemen,– Having promised, in a former paper, to communicate to your readers the method I have adopted for rotating a thermocombination, by the influence of a central magnet, the following description of the apparatus I have constructed and employ for exhibiting the experiment, with an explanation of its management, will, I humbly hope, be sufficiently plain to be understood.

NS, in the figure 1, plate 4, is the magnet; Pc P, a piece of platinum wire, bentinto the form of a semi-circle, or other convenient curve; Ps, Ps, are two pieces of silver wire, twisted to the former at the extremities PP. The other ends of the silver wires are bent downwards at s s, and made quite sharp and smooth at the points. These points descend into the metallic cell F E, which contains pure quicksilver, with which the points communicate. A descending point c, soldered to the platinum wire, forms the pivot on which the moveable part of the machine turns. A small concavity, well polished at the bottom, is made in the point of the magnet, for the purpose of containing a small globule of mercury, and likewise for the rotating pivot to work in.

The point c being amalgamated, when it is placed in this globule of mercury, forms a communication with the magnet; and the other part of the magnet, which passes through the cell, communicates with the mercury in that cell : and the points of the silver wires being immersed in this mercury, the metallic circuit is thus rendered complete; first, through the platinum wire from P to c; thence through the pivot to the top of the magnet, and along that part of the magnet from the top to the quicksilver in the cell FE: and, lastly, along the silver wire, from the point s to the extremity at P, where it joins the platinum.

The other part of the wire machine being on the same principle as that described, the platinum arms of this apparatus, when heated by a spirit lamp or otherwise at the extremities PP, are in every respect assimilated to the arms of the rotating cylinder of Ampere ; for the electric fluid is transmitted, in the same direction, through both arms of the apparatus ; and hence the rotating tendency is constant round a central magnet; and not impulsive, as in other rotations with an external magnet.

The moveable part of this machine (which is the platinum and silver wires only) will rotate with a facility proportioned to the delicacy of the suspension, the difference of temperature of the parts P and c of each arm, the power of the magnet, and the dexterity of the experiments. And I must here warn the reader, that this last requisite is not the least to ensure success in the experiment; for had I not been satisfied that the apparatus was constructed upon principle, I probably might not have persevered sufficiently to attain my object. However, a slight modification of the apparatus con

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siderably facilitates the experiment, and renders it more permanent and beautiful.

A circle of lamps are placed on a stage of the same figure, in such a manner that they may coincide with the periphery of the circle, described by the points P P of the wire part of the machine, so that the latter may constantly be kept at nearly the same temperature in every part of their revolution. And the shoulder of those arms, or that part of the platinum wire to which the pivot c is soldered, is kept at as low a temperature as possible, by means of ether or other cooling liquid.

If, instead of lamps, a circular flame of ignited hydrogen be substituted, and regulated by a stop-cock, this part of the apparatus may perhaps be considered as its acme of perfection.

Another improvement is, by having a conducting wire from the pivot c to the metallic cell F E, in the same manner as the conducting wire of the copper part of M. Ampere's rotating cylinders ; through the upper part of this conducting wire passes a screw with a milled head, made into the form of a cup. The pivot c runs in this cup, at the bottom of which is a small globule of mercury, for the better ensuring the contact. The cup is then filled up with ether, and may be supplied during the experiment in proportion to the evaporation.

The lower end of this screw rests in the hole in the top of the magnet; and by turning the milled head to the right or left, the points ss of the silver wires may be heightened or lowered at pleasure; and, consequently, their contact with the mercury in the cell F E may be regulated to the greatest nicety; the attainment of which was the only embarrassment I had to encounter with the original apparatus. However, by means of this improvement, my anticipations were soon agreeably realised, by witnessing the first thermo rotation ever produced by the influence of a central magnet.

I must here beg leave to observe, that the only attempt I ever heard of (and the only one perhaps on record) was with the apparatus of Professor Cumming, and a similar attempt by Professor Barlow, with a combination upon the same principles.

The latter gentleman, however, has candidly confessed the failure of the experiment, and sufficiently accounted for the inefficacy of the apparatus upon the principle of its construction.

I am, gentlemen, yours respectfully,
Artillery Place, Woolwich.

WM. STURGEON. P.S.-April 13. I have since succeeded in forming a sphere of galvanized wires, to rotate by the influence of both poles of an internal magnet.

This experiment was suggested on reading the late Dr. Halley, on the theory of the earth ; and although it may not be considered as a proof of that philosopher's notion of terrestrial magnetic variation, yet, perhaps, it may tend in some measure to strengthen the hypothesis. A description of the apparatus shall be the subject of another paper.

W. S.

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