We conclude by applying Ohm's law that the electromotive force, E, of the **thermocouple** may be approximately represented for small differences of temperature by the formula E=CR=p(t-t').

Of a copperiron **thermocouple** reaches a maximum when the temperature of.

This law is commonly applied in connecting a **thermocouple** to a galvanometer with coils of copper wire, the junctions of the copper wires with the other metals being placed side by side in a vessel of water or otherwise kept at the same temperature.

- In order to justify the assumption involved in the application of the second law of thermodynamics to the theory of the **thermocouple** in the manner above specified, it would be necessary and sufficient, as Thomson pointed out (Phil.

The method appears to be open to the objection that the difference of temperature reached in so long an interval would be more or less independent of the thermal 0 0 .20 h ?1 capacities of the calorimeters, and would also be difficult to measure accurately with a **thermocouple** under the conditions described.

Returning again to the equations already given in § i i for an elementary **thermocouple**, we have the following equivalent expressions for the E.M.F.

Seebeck found that the metals could be arranged in a Thermoelectric Series, in the order of their power when combined with any one metal, such that the power of any **thermocouple** p, composed of the metals A and B, was equal to the algebraic difference (p'-p") of their powers when combined with the standard metal C. The order of the metals in this series was found to be different from that in the corresponding Volta series, and to be considerably affected by variations in purity, hardness and other physical conditions.

If C is the intensity of the current through a simple **thermocouple**, the junctions of which are at temperatures t and 1', a quantity of heat, P X C, is absorbed by the passage of the current per second at the hot junction, t, and a quantity, P X C, is evolved at the cold junction, t'.

Of a **thermocouple** metal FIG.

- Potential Diagrams of **Thermocouple** on the Contact Theory.