The Thermoelectromotive Force of Copper-Manganese Alloys

Abstract

Thermoelectromotive Force of Alloys of Copper with 5 to 23 Per Cent, Manganese for the Range 0\ifmmode^\circ\else\textdegree\fi{} to 100\ifmmode^\circ\else\textdegree\fi{} C.---The alloys were prepared from electrolytic copper and thermite manganese but contained as impurity about one fiftieth as much iron as manganese. The alloys were cast and then drawn to fine wires. After they had been annealed at 200\ifmmode^\circ\else\textdegree\fi{} C. for 8 hours, the thermoelectromotive forces against fine pure copper wire were determined for various temperatures of the hot junction up to 100\ifmmode^\circ\else\textdegree\fi{} C. The curves, as usual, correspond to the equation $E=\ensuremath{\alpha}t+\ensuremath{\beta}{t}^{2}$. For $t={100}^{\ensuremath{\circ}}$ C., $E$ is positive for all the alloys, especially if allowance is made for the effect of the iron content, though practically zero for alloys with less than 10 per cent. Mn. The alloys as cast and as drawn were more positive than after annealing. However, $E$ showed negative values for the lower manganese alloys and for the lower temperatures, especially when the iron content was increased. In fact, though $\ensuremath{\alpha}$ is probably positive for all pure Cu-Mn alloys, the effect of small percentages of iron, carefully studied to 2 per cent. with a series of 30 alloys, was found to be to decrease $\ensuremath{\alpha}$ so that for 13 per cent. Mn and more than 0.50 per cent. Fe and for less than 11 per cent. Mn and more than 0.20 per cent. Fe, $\ensuremath{\alpha}$ is negative. On the other hand $\ensuremath{\beta}$ is always positive but tends to decrease as the percentage of Mn is increased. Two alloys indicated that the effect of 2 per cent. nickel is to decrease $\ensuremath{\alpha}$ nearly as much as an equal amount of iron. The effect of aluminum seems to be the reverse but the five alloys tested did not give consistent results. Ordinary manganese contains just enough iron as impurity to make its $E$ slightly negative at ordinary temperatures but adding 1 per cent. more iron, as suggested by Hunter and Bacon, would lower it too much.Discussion of Electron Theory of Thermoelectromotive Force.---The electron theory does not account for the fact that $E$ is positive rather than negative for Cu-Mn alloys against Cu, and that while Fe itself is positive against Cu its effect on $E$ for these alloys is negative. It is suggested that the effect of atomic and molecular forces on the free electrons will have to be taken into account.