DISTRIBUTION OF HEAT WHICH IS DEVELOPED BY FORGING.
On the 8th of June. 1874, Tresca presented to the French Academy some considerations respecting the distribution of heat in forging a bar of platinum, and stated the principal reasons which rendered that metal especially suitable for the purpose. He subsequently experimented, in a similar way, with other metals, and finally adopted Senarmont's method for the study of conductibility. A steel or copper bar was carefully polished on its lateral faces, and the polished portion covered with a thin coat of wax. The bar thus prepared was placed under a ram, of known weight, P, which was raised to a height, H, where it was automatically released so as to expend upon the bar the whole quantity of work T=PH, between the two equal faces of the ram and the anvil. A single shock sufficed to melt the wax upon a certain zone and thus to limit, with great sharpness, the part of the lateral faces which had been raised during the shock to the temperature of melting wax. Generally the zone of fusion imitates the area comprised between the two branches of an equilateral hyperbola, but the fall can be so graduated as to restrict this zone, which then takes other forms, somewhat different, but always symmetrical. If A is the area of this zone, b the breadth of the bar, d the density of the metal, c its capacity for heat, and t-t0 the excess of the melting temperature of wax over the surrounding temperature, it is evident that, if we consider A as the base of a horizontal prism which is raised to the temperature t, the calorific effect may be expressed by:
Ab x d x C(t-t0);
and on multiplying this quantity of heat by 425 we find, for the value of its equivalent in work,
T' = 425 AbdC(t-t0).
On comparing T' to T we may consider the experiment as a mechanical operation, having a minimum of:
T'/T = (425/PH)AbdC(t-t0).
After giving diagrams and tables to illustrate the geometrical disposition of the areas of fusion, Tresca feels justified in concluding that the development of heat depends upon the form of the faces and the intensity of the shock; that the points of greatest heat correspond to the points of greatest flow of the metal, and that this flow is really the mechanical phenomenon which gives rise to the calorific phenomenon; that for action sufficiently energetic and for bars of sufficient dimensions, about 0.8 of the labor expended on the blow may be found again in the heat; that the figures formed in the melted wax for shocks of less intensity furnish a kind of diagram of the distribution of the heat and of the deformation in the interior of the bar, but that the calculation of the coefficient of efficiency does not yield satisfactory results in the case of moderate blows.--Comptes Rendus.