The Effect of Tension on the Change of Thermoelectromotive Forces by Magnetization

Combined Effect of Tension and Magnetic Field on Thermoelectromotive Force and Resistance of Some Ferro-magnetic Metals.---Wires 10 cm. long and 0.15 cm. in diameter were each held in heavy brass clamps and stretched while placed axially in a vertical solenoid. While determining the electromotive forces, one end was kept at 100^∘{}, the other at 0^∘{} C. In the case of nickel, the effect of tension on the change in resistance (ΔX) and in thermoelectromotive force (ΔE) produced by a longitudinal magnetic field is a decrease for weak fields but an increase for larger fields. For a given tension, the ratios of ΔX to ΔE and to the magnetic change of length (ΔL) are independent of the field strength and therefore seem to depend on the same factor. Pure iron shows a parallelism between the effects of tension on ΔE and ΔL; with increasing tension the curves for ΔE as function of the field shift downward so that for sufficiently large tensions ΔE becomes negative throughout. The curve for ΔE may be considered the sum of two effects, associated with two forms of iron, one of which is proportional to ΔL and is affected by tension more than the other. The behavior of iron-copper alloys is qualitatively similar to that of iron. Iron-nickel alloys behave like iron until the per cent. of Ni is more than 7 when they tend to behave more and more like Ni. For both series of alloys the change of ΔX due to tension is always a decrease.Relation between the Hall Constant, Specific Resistance and Fractional Change of Thermoelectromotive Force Produced by a Magnetic Field in a Series of Iron-copper Alloys is pointed out. The three properties vary with copper content in a similar way, all showing a cusp for 1.5 per cent. copper.Electron Theory of Conductivity and Thermoelectromotive Force.---In a discussion of the above results it is shown that neither the gas-free theory as given by Heaps nor Bridgman's gap theory explains them satisfactorily. The decrease in resistance with increase of length would be expected, however, if the number of free electrons varies in an inverse manner with the atomic frequency; and ΔE would vary with magnetic field and tension if the atomic frequency and the number of free electrons are not the same functions of the magnetic field at different temperatures and tensions.