149.] Thermo-Electric Diagram. 209 the axis op, and two lines parallel to the axis of temperature, represents the work depending on the corresponding element of the cycle. P Thus NN' represents the passage of a unit of electricity from the lower temperature T to the higher temperature T', Ρ along the conductor B, and the area NN'o'o the energy g taken in in this part of the circuit in consequence of the Thomson effect. T T' The line N'M' denotes the passage of electricity from the conductor B to the conductor A at the temperature T', and the area N'M'P'Q' the energy corresponding to the Peltier effect. The line M'M denotes the passage of electricity along the conductor A from the temperature T' to the temperature T, and the area M'MPP' represents the energy corresponding to the Thomson effect in the conductor A. Finally, the line MN expresses the passage of electricity from the metal A to the metal B through the cold junction at the temperature T, and the area MNQP the energy corresponding to the Peltier effect. The diagram is so drawn that gain of energy corresponds to the passage of a current from a lower to a higher point, and loss of energy to the passage from higher to lower. Consequently the areas QNN'Q', Q'N'M'P', and P'M'MP represent energy gained during the passage of a coulomb of electricity round the circuit, and the area PMNQ represents energy given out. The net gain of energy per coulomb, or the electromotive force in volts, is represented by the difference of these areas, namely, NN'M'M. 149. Measurement of Temperatures by Thermo-Electric Couples. As the electromotive force of a thermo-electric couple is, for a fixed temperature of the cold junction, a function of the temperature of the hot junction, it can be used to determine this latter temperature. A simple arrangement for this purpose consists in twisting together, as at O (Fig. 136), the ends of two wires, AB, forming a couple, and attaching the other two ends to a galvanometer, G, all the junctions from A and from B being at the same temperature. It is necessary to know once for all the curve of the electromotive forces as a function of the temperature, and to be certain that the couple is comparable with itself, and is not altered by being reheated. Care must also B be taken to work below the point of inversion. A couple of pure platinum and an alloy of rhodium with platinum is well adapted for measuring temperatures up to 1200°, and towards this limit indicates temperatures to within 10 or 20 degrees, which in most cases is sufficient. For temperatures below 100°, by the arrangement known as Becquerel's thermo-electric needle (Fig. 137), the curve can be fe FIG. 137. dispensed with. Two identical couples are joined in series in opposition to each other. The junction A being placed at the point whose temperature is to be determined, the junction B is placed in a bath, the temperature of which can be varied until there is no deflection of the galvanometer. The temperature of the two junctions is then the same. For very small differences of temperature, it is advantageous to use a bismuth-antimony couple (Fig. 138). So long as the temperature of 100° is not exceeded, the current is proportional to the difference of temperature of the two junctions. By arranging two ME نائل FIG. 138. Melloni's pile, described above (Fig. 129), constitutes a differential thermometer of extreme sensitiveness. When the two faces are at the temperature of the surrounding air, and heat is allowed to radiate against one end, for instance, if the hand is held in front $149.] Measurement of Temperatures. 211 of it, a difference of temperature is established between them, which is proportional to the radiation, and therefore a current is produced whose intensity is proportional to this radiation. As a further example, may be cited the application of the pile by M. Leroux to measuring the Thomson effect (§ 135). If we suppose two similar bars, AB, A'B' (Fig. 139), placed parallel to each other and joined by a cross-piece, BB', at the heated end, a thermo-electric pile, placed with its faces in contact with two symmetrical points, C and c', will indicate a difference of temperature between these points when a strong current is passed through the bars from A to A' or in the opposite direction. CHAPTER XV. CHEMICAL ACTION OF THE CURRENT. 150. Electrolysis.—If the interpolar wire of a battery is cut, and the two ends are then placed in a liquid (Fig. 140), so that the 息 circuit is completed by a column of liquid, two cases may present themselves. The liquid may act like air, as a perfect insulator, and then no current passes; or the current passes, and then, except in the case of mercury or any melted metal, the liquid is decomposed. A non-metallic liquid never acts like a simple conductor; it never allows any quantity of electricity to pass without a correlated decomposition. This phenomenon is called electrolysis; the term electrolyte is applied to the liquid which undergoes decomposition; and the conductors by which the current enters and leaves the liquid are called electrodes; that in connection with the positive terminal is called the positive electrode, or sometimes the anode, and that connected with the negative terminal is the negative electrode or kathode. FIG. 140. The only bodies which are susceptible of electrolysis are apparently salts liquefied either by solution or by fusion. Perfectly pure liquids, such as water, alcohol, ether, bisulphide of carbon, &c., are not electrolytes. We understand by the term salt a compound formed of a metal united either to an element such as Cl, Br, S, or to a compound radical such as SO4, NO3, . . . &c. The primary decomposition which takes place under the action of the current appears always to consist in the separation of the metal from the simple or compound radical with which it was combined. $151.] Secondary Actions. 213 The constituents into which a body is decomposed never appear in the mass of the liquid itself, but only at the electrodes: the metal at the negative and the radical at the positive electrode. Thus if we use as electrodes two platinum plates, and immerse them in solution of sulphate of copper, copper is deposited on the negative electrode, with its characteristic colour, while at the positive electrode oxygen is given off in the form of gas, and sulphuric acid, SOH2, remains in solution. This may be supposed to result from the action of the liberated SO, on water, thus: 151. Secondary Actions.-When the electrode is not unalterable, the body which is given off may give rise to chemical actions, which are called secondary actions. Thus, in the decomposition of sulphate of copper, if a plate of copper is taken as positive electrode instead of a platinum plate, no oxygen is liberated, but the radical SO4 unites with copper and forms a quantity of sulphate of copper exactly equal to that which has been decomposed; the quantity of copper sulphate in solution remains constant, and in each unit of time the positive electrode loses just as much copper as is deposited on the negative electrode. With an alkaline salt such as potassium sulphate, K,SO4, the decomposition may be supposed to take place in the same way as with copper sulphate; but the potassium set free at the negative electrode, being in contact with water, at once decomposes water, with the formation of potassium hydrate and liberation of hydrogen in the form of gas; thus : : 2H2O + K2 = 2KHO + H2. The result is that hydrogen is liberated at the negative electrode, and an equivalent quantity of oxygen at the positive; at the same time, sulphuric acid is found in solution at the positive electrode, and potassic hydrate at the other. This can be shown by means of a U tube (Fig. 141) containing a solution of potassic sulphate, coloured with infusion of red cabbage, and provided with platinum electrodes, A and B. The liquid, which is at first violetcoloured, becomes red at a where the oxygen is given off, while at B, where hydrogen is liberated, it becomes green; these changes of colour indicating the presence of free acid and base respectively. When solution of common salt is electrolysed, the direct action is similar, but the final result is more complicated. The sodium |