*

sets of revolving cylinders, one suspended on each pole of an inverted horse-shoe magnet, as represented by fig. 1, plate 3. Upon the usual insertion of the diluted nitric acid, the two sets of cylinders simultaneously enter into rotations in a very interesting and striking manner. This form of the magnet gives increased power on a reduced altitude, and the proximity of the poles materially augments the rotation of the opposed cylinders. The effect is the most pleasing we have ever seen, and was witnessed at the house of Messrs. Jones, opticians, Holborn." The appearance of this apparatus was much improved by the outer cylinders and base being made of brass.

* Prior to this apparatus making its appearance, a straight bar magnet was invariably employed to shew the action of Ampere's cylinders: the action was first shewn on one pole, and then the magnet inverted, to shew the action on the other pole.-EDIT.

Electro-Magnetical Experiments. By WILLIAM STURGEON. To the Editors of the Philosophical Magazine and Journal.

Being desirous to understand the relation that subsists between the chemico and thermo-electric phenomena, as influenced by the magnet, so as to form a comparison of the widely different apparatus for exhibiting those phonomena; and likewise, if possible, to ascertain some general law to be observed in thus comparing the two modes of exciting this influence; the few following simple experiments suggested themselves as the most likely to give satisfaction on this point. As this investigation and comparison* seems to have escaped the attention of every other experimenter, perhaps a detail of those experiments, with their results, and a description of an instrument which I have been led to invent and construct, upon a simple principle, for the purpose of carrying on the comparison to any required extent, may not be thought uninteresting to some of your readers.

Exp. 1. I charged in the usual way, with dilute nitric acid, an Ampere's rotating cylinder, and placed it on a table. I now placed the north pole of a bar magnet on the upper edge of the outer rim of the copper part of that apparatus. I likewise imagined myself coinciding in position with the wire about which the machine turns," and "looking towards the magnet." On bringing one of the wires of the zinc cylinder between me and the magnet, that wire was projected to the left.

66

Let the line ED, fig. 2, be the horizontal part of the wire in this and the following experiments, moveable on a pivot o; then, when the wire is said to be projected to the right, it is meant that the point E is projected towards R, as o R; and when to the left, the point E is projected towards L, as o L.

Exp. 2. I now presented the south pole of the magnet to the wire. The latter was projected to the right.

Observation. This chemico-galvanized wire was evidently the

*There may be said to be one objection; for a comparison has certainly been made by a philosopher of acknowledged skill; and it is with due deference that I cannot subscribe to the conclusions of that gentleman.

ascending wire from the zinc to the copper; or (which is the same thing) the descending wire from the copper to the zinc.

Exp. 3. I now suspended, by a piece of untwisted silk, a semicircular copper wire, with a zinc diameter, as in fig. 3; ccc is a fine copper wire; and zzz a fine slip of zinc, soldered to the former at the extremities zz. The north pole of the magnet was now placed close to one arm of the semicircular arc, as shown in the figure, and the lamp applied at E. The wire was projected to the left. Exp. 4. The semicircle adjusted as before, the lamp was now applied at D. The wire was projected to the right.

Exp. 5 and 6. In these experiments the lamp was applied as before, but the south pole of the magnet was now presented to the wire. The motions were the reverse of the two former experiments. I have tried rectangles instead of semicircles, but there is no difference in the results.

If a semicircle or rectangle be made of platinum and silver wires, the former supplying the place of the copper, and the latter be substituted for the zinc, in the above thermo experiments, the results of the two machines are exactly alike; hence the copper and platinum wires are possessed of the same kind of electricity, and so are the zinc and silver each to each, but of the contrary kind of the other two wires: so if the copper be positive to the zinc, by this magnetic test, we must necessarily conclude that the platinum is positive to the silver.

But this is not all that is to be observed in these experiments; we must compare the results of the chemico with those of the thermo experiments, and endeavour to trace the relation that subsists in the phenomena exhibited by those two modes of exciting the electrical influence.

We will therefore compare our chemico experiment 1, with out thermo experiment 3. We here find the wire projected to the left in both cases.

We will now suppose the immersed plates of the chemico apparatus to correspond with the heated union of the thermo wires, and the most remote extremities of the chemico conducting wires with the coldest part of the thermo apparatus. It will follow, that as the copper wire (by the supposition) is as evidently ascending in the thermo machine, as the zinc wire is ascending in the chemico cylinder; and as they are both propelled in the same direction (left), by the approximation of the north pole of the magnet, the forces of the two galvanised wires must of necessity be exerted in contrary directions. For although they are both propelled the same way yet they are of contrary kinds or names.

Again. Let us now suppose the immersed plates of the chemico apparatus to correspond with the coldest union of the thermo wires, and the most remote extremities of the chemico conducting wires with the heated junction of the thermo machine. In this case, the two wires nearest the magnet of the chemico and thermo machines will in all respects correspond with each other; for they are now both descending from the copper, or both ascending from the zinc

(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.t

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. named it the Comparing Galvanoscope.

I have

Fig. 4. A cylindrical glass tube supported by the arm Cc of the brass stand DE; 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 g 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 fine 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 90o each way. This is for measuring the quantity of deflection; and hence the instrument in some measure answers the purpose of a galvanometer.

A

Fig. 5 shews a chemico combination out of the instrument. 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 o; 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 zine 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, I 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 209: thence propelled again to nearly the same distance as before; and so vibrating several times from about 15° to 60°, 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 had 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 6 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,

Feb. 16, 1824.

I am, yours, &c.

WM. STURGEON.