_Example._—Le Chatelier found that a junction at the temperature of melted aluminium (657° C.) gave 6200 microvolts; at the melting point of copper in air (1062° C.) the figure was 10580. Applying in the above formula log 6200 = A log 657 + B and log 10580 = A log 1062 + B, the value of A is 1·2196 and of B 0·302, as may be found by taking logarithms and solving for A and B. The values of the constants A and B vary for different junctions, and also for different melts of what are reputed to be the same materials. When once determined for a quantity of homogeneous wires, to which the formula applies with sufficient accuracy, it is evident that an indicator with a millivolt scale may be made to read temperatures directly without any necessity for further experiment, although it is always advisable to take one check reading at a fixed point in the working range. [Illustration: FIG. 16.—POTENTIOMETER METHOD OF MEASURING E.M.F.] In order to determine the E.M.F. of a junction at different temperatures, the potentiometer method is used, in which the E.M.F. of the test-couple is balanced against the known E.M.F. furnished by a constant cell. The circuit is shown in fig. 16, in which B is an accumulator which sends a current through the resistances R_{1}, R_{2}, and the calibrated wire DE. The cold ends of the couple are attached at P so as to be in opposition to B, and in this branch of the circuit are included a sensitive galvanometer G and a portion of the wire DE. A standard cadmium cell, S, is connected between R_{1} and R_{2} at one end, and may be put in circuit with the galvanometer through the switch A. In commencing, S is connected to the galvanometer and R_{1} adjusted until no deflection is obtained on G. The switch A is now moved over to the circuit of the couple, and the terminal F moved along the wire until zero deflection is again obtained. The E.M.F. of the couple is determined from the relation E of junction Resistance of DF ────────────────── = ───────────────── . E of standard cell R_{2} By exposing the hot end of the junction to successive standard temperatures, and maintaining the cold ends at a known constant temperature, the necessary data for inclusion in a formula may be obtained. In fixing a permanent temperature scale, calculated from the formula, to a millivoltmeter, it must be remembered that the values given by the experiment are absolute, and independent of the resistance of the circuit composed of the thermo-element and galvanometer. On the other hand, a millivoltmeter is marked to read difference of potential at its terminals; and if in series with a junction and leads of notable resistance, its indications will not be the E.M.F. of the junction. An example will make this point clear.