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Specific Heat of . 511 temperature due to its own resistance is measured by a thermocouple inserted in the rod. If a current I produces a temperature change AT in time At, and if m is the mass of the rod and r its resistance, the mean specific heat between T and T + AT is T2a> s = T AT m J * The satisfactory results thus obtained for pure nickel suggested the adoption of this method for the copper-nickel alloys.
Specific Heat of Nickel. 513 The Precision of the Measurements.—Lapp has given an exhaustive account of the errors in her method. The present method differs from hers only in detail, and the error in the final value of the specific heat at a given temperature remains approximately the same, i.e., between 1*5 and 2%.
Specific Heat of . 517 A knowledge of the temperature variation of the coefficients of expansion and of compressibility appropriate to each alloy would therefore be required for a precise evaluation of the correction for dilatation. However, using what values are available for these coefficients at various temperatures for pure nickel and copper, it appears that except in the neighbourhood of the Curie point (where the coefficient of expansion of nickel shows an abrupt change) the corrections for the alloys may be taken as the same as for pure nickel without introducing an error into the excess specific heat S of more than 1%. This assumes that the coefficients vary approximately linearly with com position : the results of Krupkowski* on the coefficient of expansion and of Johansson! on the elastic modulus support this assumption. Lapp’s values of (cv — c„) have therefore been applied to the alloys as to pure nickel, except in the neighbourhood of their Curie points. The abrupt change at the Curie point in the correction for pure nickel will presumably be less in the alloys. It has been calculated on the assumption that its magnitude varies with com position in the same way as the saturation moment, i.e., that it varies linearly with the composition and becomes zero for an alloy of composition approxi mately 40% nickel. The values of (cv — cv) immediately below the Curie point can then be found by a somewhat arbitrary graphical interpolation, which, however, introduces little error.