Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/104

Rh E L E C T K I I T Y [ELECTROMOTIVE FORCE. Grove s element Cells of Bunseii, ko. See- beck s dis covery, crystals of copper sulphate ; the narrow end of this neck dips into the small beaker, the copper sulphate runs slowly out, and being specifically heavier than the zinc sulphate, it collects at the bottom about the coppei ring. Yet another form of Dauiell s element is the tray cell of Sir William Thomson, which consists of a large wooden tray lined with lead, the bottom of which is covered with copper by electrotyping. The zinc is made like a grating, to allow the gas to escape, and is enveloped in a piece of parchment paper bent into a tray- shape, the whole resting on little pieces of wood placed on the leaden bottom of the outer tray. Sulphate of copper is fed in at the edge of the tray, and sulphate of zinc is poured into the parchment. The zincs in these elements are some 40 centi metres square, so that the internal resistance is as low as 2 ohm. One of the best known in this country, and perhaps the most used of all the two-fluid cells, is the element of Grove. This differs from Daniell s element in having nitric acid with a plati num electrode in the porous cell, instead of the copper solution and the copper electrode of Daniell s element. The hydrogen evolved at the platinum is oxidized by the nitric acid, and the polarization thus avoided. The nitrous fumes given off by the chemical action are very disagreeable, and also very poisonous, so that it is advisable to place the battery outside the experimenting room or in a suitable draught chamber. The electromotive force of Grove s cell is a good deal higher than that of Daniell s, and its internal resistance is very much less, &quot;25 ohm being easily attained with a cell of moderate dimensions. On this account the cell is much used for working induction coils, generating the electric light, and so on, notwithstanding that it is troublesome to fit up, and must be renewed every day. In Bunsen s element the platinum foils of Grove are replaced by carbon. The prime cost of the battery is thus considerably re duced, the more so now that carbons for the purpose have become articles of commerce. The electromotive force of the element thus altered is as great as, or with good carbons even greater than, in Grove s original form ; but the internal resistance is greater. There is a difficulty sometimes in obtaining good connection with the carbons, and trouble arises from their fouling ; but the fact that this cell is a universal favourite in Germany proves its practical utility. It is comparatively little used in this country. In the cell of Marie Davy, which is, or was, much used for tele graphic purposes in France, the copper solution and copper plate of Daniell are replaced by a watery paste of protosulphate of mercury, into which is inserted a carbon electrode. The electromotive force of this cell is said to be about 1 5 volts, 1 and its internal resistance to be greater than that of Daniell s cell. Besides these, various bichromate elements of merit might be described ; but we have dwelt long enough on this subject already. The following table of Latimer Clark s, quoted by Maxwell, will give the reader an idea of the relations as to electromotive force of the commoner elements : Daniell H 2 S0 4 + 4Aq CuS0 4 1-079 Do. H 2 S0 4 + 12Aq CuS0 4 0-973 Do. H,SO, + 12Aq Cu(NO,), 1-000 Bunsen H 2 S0 4 +12Aq HN0 3 1-964 Do. H 2 S0 4 +12Aq HN0 3 (sp.g. 1-38) 1-888 Grove H,S0 4 + 4Aq HN0 3 1-956 The electromotive force is stated in volts, and the solutions in the third column are concentrated, unless it is otherwise stated. Thermoelectricity. We have already alluded to the law of Volta, according to which there can be no resultant electro motive force in a circuit composed solely of different metals ; and it will be remembered that we added the condition that all the junctions must be at the same temperature. Seebeck was the first to discover 2 that this law is subject to exception when the junctions are not all at the same temperature. If we form a circuit with an iron wire and a copper wire, and raise the temperature of one of the junctions a little above that of the other, a current floAvs round the circuit, passing from copper to iron over the hotter junction ; similarly, if we solder together a piece of bismuth and a piece of antimony, and connect the free ends with the copper wires of a galvanometer, then when the junction of the bismuth and antimony is heated the galvanometer indicates a current passing from bismuth to antimony over the hot junction. It will be perceived that the second of 1 Jenkin, Electricity and Magnetism, p. 225. 2 Pogg. Ann., vi. 1826. The discovery was made about 1821 or 1822. our two illustrative instances is more complicated than the first, inasmuch as three metals enter into the circuit in stead of two. Nevertheless the experimental result is not altered by the intervention of the copper wire (abstraction being made of its resistance), provided the temperatures of the points where it joins the bismuth and antimony re spectively be the same. It is easy to give a direct experi mental proof of this assertion by inserting between the pieces of bismuth and antimony a piece of copper wire so that the circuit now is Bi.Cu.Sb.Cu.Bi. ; if the junctions of the inserted wire with the bismuth and antimony be raised to the same temperature as the BiSb junction in our second experiment, and the junctions with the copper wire of the galvanometer be at the same lower temperature as before, the total electromotive force in the circuit will be the same ; and, provided the resistance of the circuit has not been sensibly increased by the interpolation of the copper wire, the galvanometer indication will also be the same as before The same result is obtained however many different metals we insert between the bismuth and antimony, provided the temperatures of all the junctions be the same and equal to that of the BiSb junction in the original experiment. The law of Volta therefore still holds if stated thus : A scries of metals whose junctions are all at the same temperature may be replaced by the two end metals of the series without altering the electromotive force in any circuit of which the series forms a part. It is not unlikely that the above statement of the fundamental facts concerning thermoeleetromotive force has suggested to the reader two notions : 1st, that the phenomena may be completely explained by a contact force at the junctions of the metals which is a function of the temperature of the junction; and 2d, that this con tact force is the true contact force of Volta. It is perhaps as well to mention even at this early stage that the first of these notions is certainly not correct, and that the second is not admitted by some of the greatest authorities on the subject. Seebeck examined the thermoelectric properties of a large Th&amp;lt; number of metals, and formed a thermoelectric series, any e!e ? metal in which is thermoelectrically related to any following sei one as bismuth (see above) is to antimony, the electro motive force in a circuit formed of the metals being ceteris paribus greater the farther apart they are in the series. The following is a selection from Seebeck s series : Bi.Ni.Co.Pd.Pt.Cu.Mn.Hg.Pb.Sn.Au.Ag.Zn. Cd. Fe. Sb. Te. This series has only a general interest, and is not to be re garded as in any way absolute. Seebeck himself showed the great effect that slight impurities and variations of physical condition may have on the position of a metal in the series. Some specimens of platinum for instance come between zinc and cadmium. Another instance of the same kind is afforded by iron : Joule 3 found the following order to hold cast iron, copper, steel, smithy iron. Thermoelectric series have been given by Hankel, Thomson, and others, but we need not reproduce them here. It may be well, however, to direct the attention of the reader to the properties of metallic sulphides and of alloys which in many cases occupy extreme positions in the thermoelectric series. Alloys present anomalies in their thermoelectric properties somewhat similar to those already noticed in our discussion of their conductivity. These properties have been much studied with a view to practical applications in the construction of thermopiles. Consider able progress has been made in this direction (see above p. 11), notwithstanding the fact that many of the alloys most distinguished for their thermoelectric power are very brittle and have a tendency to instability under the con tinued action of heat. 4 3 Phil. Mag., 1857. 4 For further information consult Wiedemann, Galv., Bd. i 593 &c.