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LONDON, EDINBURGH AND DUBLIN

PHILOSOPHICAL MAGAZINE

AND

JOURNAL OF SCIENCE.

[FOURTH SERIES.]

JANUARY 1857.

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I. On the Development of Dynamic Electricity by the Immersion

of unequally Heated Metals in Liquids. By G. Gore, Esq.* 1. T has long been known that electric currents are produced

when two perfectly similar pieces of metal are connected with a galvanometer and their free ends simultaneously immersed in two parts of a conducting liquid of different temperatures, or when they are immersed in a conducting liquid of uniform temperature, and one of the pieces maintained at a different temperature from the other; and the currents so produced have been ascribed sometimes to chemical causes, and sometimes to the ordinary thermo-electric properties of the metals.

2. In order to ascertain more clearly the conditions under which those currents are developed, I have been induced to make some experiments upon the subject. The earliest questions which occurred to my mind were,—1st, Are the currents in such cases due, or partly due, to any thermo-electric properties of the liquids similar to those of single metals ? 2ndly, Are they due, or partly due, to the usual thermo-electric property of the metals employed ? or 3rdly, Are they due, or partly due, to chemical action developed at the surfaces of mutual contact of the metal and liquid by the application of heat ? ON THERMO-ELECTRIC ACTION OF THE LIQUID AS A CAUSE OF THE

CURRENTS. 3. We know that if two pieces of copper, or any other kind of metal, are taken and connected with a galvanometer, and the free extremity of one of the pieces is heated and then suddenly applied to the free and cold end of the other, a feeble current of

* Communicated by the Author. Phil. Mag. S. 4. Vol. 13. No. 83. Jan. 1857.

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electricity is generated, and therefore, in order to obtain an answer to the first of the questions, I made similar experiments with liquids. I took two Florence flasks (fig. 1), one of 3 inches diameter, and the other of 3 inches; the smaller one was cut off evenly at its middle at an angle of about 40 degrees to its largest diameter, and the largest end of the funnel so obtained was accurately closed by a porous earthenware diaphragm eóth of an inch thick, cemented in with marine

Fig. 1.

A glue; the neck was also cut off, and a bent glass tube attached in its place by means of a short piece of vulcanized india-rubber, A, to prevent the heated liquid ascending against the upper wire: the larger one was also cut off at its middle in a similar man. ner. The small end of each GLI of these funnels was closed by a cork, with a piece of slender copper wire projecting about half an inch inside. The larger flask was fixed on a stand in an inclined position with its small end downwards, as in the figure, and was nearly filled with a solution composed of 220 grains of sulphate of copper and 10 ounces of distilled water; the smaller one was then perfectly filled with a portion of the same liquid and its upper end closed by its cork, and the outer ends of the wires connected with a suitable galvanometer. On bringing the wet porous base of the upper flask suddenly in contact with the liquid of the lower vessel, no perceptible electric current was developed by chemical or other differences in the projecting ends of wire. The upper end of the lower liquid was now heated to nearly its boiling-point by means of a small spirit-lanıp, and the whole surface of the porous diaphragm then suddenly immersed in it to about one-eighth of an inch deep: no deflection of the galvanometer needles took place. I repeated this experiment several times with the same result. I also made the same kind of experiment with glass vessels 5 inches in diameter, with bent upper tube, porous earthenware diaphragm th of an inch thick, and the same liquid, but still obtained no current with the upper end of the lower column at 212° F.

4. To ascertain whether the questioned effect was neutralized or prevented by atmospheric air dissolved in the liquid, or by any opposing quality of the diaphragm, I tried again with the liquid pre-boiled, and with a prepared ox-bladder diaphragm 51

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inches in diameter, but still obtained not the slightest current. Each of the two vessels contained about 12 ounces by measure of the solution.

5. In each of these experiments the wires were very carefully prepared, so that there was no interfering current produced by difference of chemical action of the liquid upon them, or by difference of smoothness of their surfaces. The galvanometer was an astatic one (or very nearly so), with very fine silk suspending fibre (I have found a fine platinum suspending wire insufficiently sensitive); its coil contained about 200 turns of moderately fine copper wire (exact diameter 0·0075 inch), and offered no injurious resistance; the same instrument was used in all the succeeding experiments mentioned in this paper.

6. From the foregoing results, I conclude that the currents under consideration are not produced in any material degree by any thermo-electric properties of the liquid alone, similar to those which a single metal is known to possess.

Description of APPARATUS. 7. To test whether chemical affinity takes any part in the production of the currents, also to assist in settling various other questions, I devised and constructed the following apparatus (fig. 2). A is an open glass cylinder*, 2 inches long and 2 inches in internal diameter (with sides of an inch thick), to

Fig. 2.
Vertical section. Scale 1 inch to 2 inches.

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* I have since constantly used a vessel of an hour-yluss form with great success.

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contain the liquid ; it is filled by means of the bent glass tube B. The ends of the cylinder are closed by perfectly similar plates of metal, C, C, varying in thickness from oth of an inch with flexible metals, such as copper, silver, platinum, or aluminium, to tóth of an inch with brittle metals (such as bismuth or antimony), each of which, if of flexible metal, has a short projecting arm D, and wire or strip E (soldered or riveted to the arm), of the same metal, for connecting with the galvanometer ; and if of brittle metal, has a narrow taper rod or arm of the same metal, varying from 10 to 15 inches long (according to the thermic conductivity of the metal), projecting from one part of its edge, with a connecting wire attached for the same purpose; the object of having the arms so long, and of having an arm to each plate, is to prevent the heat applied to the upper plate extending to its junction with the connecting wire, and causing an interfering thermo-electric current; also to allow of the plates being interchanged occasionally.

8. F is a thin cylinder either of glass or metal (the former I have used, but it is apt to be broken by the pressure), open at both ends, of the same depth and internal diameter as the lower cylinder, and with an external flange th of an inch wide upon its lower end. There are suitable washers of vulcanized indiarubber interposed between the metal plates and the ends of the cylinders, in order to make the two vessels water-tight. In some instances, when using plates of cast metal, such as bismuth or antimony, I have had the outer or open cylinder cast in the same piece of metal as the plate; and with sheet copper plates I have sometimes formed the cistern of sheet copper and soldered it to the back. GG is a circular plate of flint-glass ths of an inch thick, with a round bevelled hole in its centre to receive the cistern; and H H is a circular plate of brass gths of an inch thick, with a round hole of the same size (but not bevelled) in its centre; it is covered on its upper surface with a thin sheet of vulcanized india-rubber.

9. The two circular plates are held together and embrace the cylinders, washers, and metal plates firmly, by means of four thumb-screws (with “quick threads”) passing freely through holes in the plate of glass and screwing into the lower plate, of which only two, II, are shown in the figure; washers of vulcanized india-rubber are interposed against those plates to equalize the pressure; there are also gutta-percha washers against the heads of the screws. A small band of vulcanized india-rubber, with a metallic hook at each end, is sometimes used; it

passes over the supply tube at B, and hooks upon two of the screws to keep the tube secure; but generally this is unnecessary, the tube being fitted to the hole in the cylinder by grinding and fixed by

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varnish. There are also three leveling screws (of which only two, JJ, are shown) to enable the apparatus to stand steady. K is a bent copper pipe soldered to the copper cistern, and is used to convey steam into the water. The cistern is provided with a moveable half-lid of metal to admit of the insertion of thermometer, and to prevent the passage of the steam spilling the water.

10. In this apparatus the interference of contact of the atmosphere is prevented; the amount of immersed surface of each piece of metal is kept perfectly uniform; the temperature of the heated metal is under almost perfect command, and is capable of being very closely ascertained.

11. When it is used it is first taken asunder; the plates are then perfectly cleaned by means of fine emery cloth (platinum plates do not require this, but merely to be heated red-hot all over), and repeated rubbings with clean cloths, each being made exactly alike in smoothness and cleaned in a similar manner ; they are then allowed to be face to face in contact with each other for several minutes, to render them more perfectly alike in temperature; the lower cylinder and its washers are also well cleaned, distilled water being used for the washing operations. It is then put together immediately, and the thumb-screws fixed very firmly.

12. The liquid to be examined being previously well boiled, filtered, and cooled to the same temperature as the atmosphere and apparatus, is then poured by the tube B into the cylinder until the latter is quite full, and the tube filled to a level with the top of the cylinder. All air-bubbles are excluded by shaking the apparatus strongly whilst holding it in a suitable position for their escape by the tube.

13. The wires EE are now connected with the galvanometer, and the needles watched to see if

any

deflection arises from a current of electricity produced by any difference of cleanliness, smoothness, temperature, or other causes in the plates or washers. If there is any permanent deflection, the apparatus must be taken to pieces and rearranged: there is generally a small temporary deflection even when all has been properly prepared, especially with liquids which possess a strong affinity for the particular metal in use; but this does not materially interfere with the result in ordinary cases, and may be safely disregarded.

14. The cistern F must now receive about three-quarters of an ounce of water of exactly the same temperature as the inner liquid (the liquid having been previously put into a thin flask and immersed for about ten or fifteen minutes in the water), and the bulb of a thermometer placed in it. A current of steam

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