CHAPTER XVI.

THERMO-ELECTRICITY.

§ 1. Introductory.-In the cells with which we have up to the present point been concerned, we have had certain arrangements by means of which chemical-potential-energy could be converted into electrical energy; the materials of the cell undergoing permanent chemical change. Any such arrangement (generally of two metals and one or two liquids) was called a voltaic cell.

If we passed a current through such a cell we either lost chemical potential-energy and gained heat energy, or we gained chemical-potential-energy and used up electrical energy that would otherwise have appeared in the form of heat energy; according to the direction in which we drove the current. (We refer to the 'Ce' of Chapter XV. § 9.)

In the present Chapter we shall see that it is very easy to make certain arrangements (consisting simply of metals, without any liquid or other non-metallic body) by means of which heat energy may be converted direct into electrical energy; the metals employed undergoing no chemical change. Such an arrangement, which gives us an electric current by the application of heat only, is called a thermo-cell.

We shall see further that if we pass a current through a thermo-cell we have certain effects (called Peltier and Thomson effects) consisting in disengagement or absorption of heat. These effects are entirely distinct from the Joule effect of Chapter XV. § 4, and answer to the chemical work Ce that is done when we pass a current through an electrolytic-cell (see Chapter XV. § 9).

In what follows we shall discuss in an elementary manner the subject of thermo-cells, taking matters in the following order.

(I.) We shall first give the main facts, established by experi

ment, as to the construction of a thermo-cell, and the dependence of the E.M.F. on the nature of the metals employed and on the temperatures at which the different parts of the cell are maintained (§§ 2 to 7).

(II.) We shall give some account of the facts with respect to the Peltier and Thomson effects referred to above (§§ 8 and 9).

(III.) We shall say something with respect to the theory of the subject (§§ 10 and 11). But, as is usual in elementary books, we shall not deal at all fully with the theoretical part of the subject. § 2. The Simple Thermo-Cell.-If a circuit be made of two metals A and B, having soldered junctions [one of which we de note by AB, the other by B[A], we have seen that the two metals are as a rule at different potentials, but that there is no current (see Chapter XI. § 3); it being assumed as a condition that the whole circuit is at one temperature.

If, however, the two junctions are at different temperatures, we find in general that a current flows through the circuit, or that

there is in the circuit under these conditions a resultant E.M.F. (The reader is requested to notice the simple manner in which we denote the junctions across which we pass from A to B, and from B to A, respectively.)

The figures here given indicate two equivalent forms of the simple thermo-cell. We may either have the two metals A and B only, with AB at t° C. and BA at t° C.; or we may connect A

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and B by a third metal D, having both the junctions AID and DB at ° C. It can be proved experimentally, and it follows from Chapter XI. § 3, that the two forms are exactly equivalent.

$3. The Thermo-Pile.-The E. M.F.s of thermo-cells are very small, while their internal resistance, as they consist entirely of metallic bars or thick wires, is very small. Hence it is usual to couple up many simple cells in series. If n such cells be coupled up we have an E.M.F. that is n times the E.M.F. of one cell, while the internal resistance is still small.

INNI

The figure here given, fig. i., indicates how several cells may be joined in series. It is evident that if we have an E. M. F. in case (ii.) of the last section, then we shall in the present case have the several E. M.F.s acting in the same direction.

In the next figures, figs. ii. and iii., it is shown how such cells may be packed into a very small space. Nobili made his

$ 4. Thermo-Electric Series.-Referring to the simple cell of § 2, it may be stated that the E.M.F. depends on three conditions. They are

(a) The nature of the metals.

(b) The difference of temperature of the two junctions. (c) The absolute temperature of the junctions.

Thus we have (a) a different E.M.F. if we make our cell (cæteris paribus) of Bi and Fe from what we have if we make it of Bi and Sb. Again, we have in general (b) a different E.M.F. if the difference of temperatures of the junctions be 20° C., from what we have if (cæteris paribus) it be 50°C. And, finally, we have in general (c) a different E.M.F. if the junctions be at, e.g., 20° C. and 22°C. from what we have if (cæteris paribus) they be at 50° C. and 52° C.

Thermo-electric series at 20° C.-Hence we cannot give a list of metals showing their thermo-electric relation to one another for all temperatures, but only for some stated temperature.

In the following are given in micro-volts the E.M.F.s of various cells, one metal in each case being lead, and the other the metal opposite to which the number stands. The junctions in each case are at 20° C. and 19° C. respectively; or there is a temperature-difference of 1°C., while the mean temperature is 20° C.

For the metals that occur higher in the list than lead, the current passes from that metal through the hotter junction to lead; for those lower in the list than lead, the current passes from lead to that metal through the hotter junction. This is indicated by the sign of the E M.F. Experiment shows also that the E.M.F. of a cell composed of any two metals is simply the algebraic difference of their respective E. M.F.s with reference to lead. (The list is taken from Lupton's Numerical Tables.)

E. M. F.s, in micro-volts, of cells whose junctions are at 20° C. and 191o C. respectively; lead being one of the metals in each case.

Explanation of table.-The following two or three cases may explain further what the table means.

(i.) If we make a cell of cobalt and lead, the junctions being at 20° C. and 19° C., the E.M.F. will be 22 × 10 volts; and the current will pass from cobalt to lead through the hotter junction.

(ii.) With lead and silver the E.M.F. is 3 × 10-6 volts; and the current passes from lead to silver through the hotter junction.

(iii.) With pure bismuth and cobalt the E.M.F. is (89–22) × 10−6 = 67 × 10-6 volts; and the current passes from former to latter through the hotter junction.

(iv.) With cobalt and pure silver the E.M.F. is [22 − ( − 3)] × 10−6 = 25 × 10-6 volts; and the current passes from former to latter through the hotter junction.

Such a series is called a thermo-electric series at the temperature (i.e. 20° C.) in question.'

§ 5. Thermo Electric Powers.-The effectiveness of any particular cell (ie. a cell composed of any two specified metals), at any specified mean temperature C., may be reasonably measured by the E.M.F. of the cell when the junctions have a temperature difference of 1°, the mean temperature being t° C.

To express this 'effectiveness' we use the term thermo-electric power. The meaning of this term may be seen from the following

statement.

The thermo electric power of any pair of metals at t° C. is measured by the E.M.F. of a cell composed of these metals when there is a temperature difference of 1° C. between the junctions, the mean temperature being t° C. The E.M.F.s are usually expressed in micro-volts.

The more exact meaning of thermo-electric power may perhaps be further explained with some advantage. It is found experimentally that if the junctions are at any two temperatures t° and t1° of which the mean is t°, and if E be the E. M.F. of the cell under these conditions, then