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_.
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