So far, the effect of heating the junction has been considered without regard to the temperature of the remainder of the circuit, and it is necessary, before describing the construction of practical instruments, to consider the laws governing the thermo-electric circuit, the simplest form of which is represented in fig. 2. One of the wires is connected at both ends to separate pieces of the other wire, the free ends of which are taken to the galvanometer Two junctions, A and B, are thus formed, which evidently act in opposition; for if on heating A the direction of current be from A to B, then on heating B the direction will be from B to A. Hence if A and B were equally heated no current would flow in the circuit, the arrangement being equivalent to two cells of equal E.M.F. in opposition. Thermal junctions are formed at each of the galvanometer terminals, but the currents to which they give rise, when the temperature changes, are opposed and cancel each other. The law which holds for this circuit may be expressed thus:— “If in a thermo-electric circuit there be two junctions, A and B, the electromotive force developed is proportional to the _difference_ in temperature between A and B.” [Illustration: FIG. 2.—TWO-JUNCTION THERMO-ELECTRIC CIRCUIT.] It is customary to refer to the two junctions as the “hot” and “cold” junctions; but it is important to remember that fluctuations in the temperature of either will alter the reading on the galvanometer or indicator. A second law, which applies to all thermo-electric circuits, is that “the E.M.F. developed is independent of the thickness of the wire.” This does not mean that the deflection of the galvanometer is the same whether thin or thick wires are used to form the junction. The deflection depends upon the current flowing through the circuit, and this, according to Ohm’s law, varies inversely as the total resistance of the circuit. Consequently, the use of thin wires of a given kind will tend to give a less deflection than in the case of thick wires, as the resistance of the former will be greater, and unless the resistance of the galvanometer be great compared with that of the junction, the difference in deflection will be conspicuous. The E.M.F., however, is the same under given conditions, whatever thickness of wire be used.