Multiplying this by 425, or Joule's equivalent for the metrical system,
the energy developed in heat is given by
T1 = 425 AbdC(t-t0).
Dividing T1 by T, we obtain the ratio which the energy developed in heat
bears to the total energy of the blow.
With regard to the form of the zone of melting, it was found always to
extend round the edges of the indent produced in the bar by the blow. We
are speaking for the present of cases where the faces of the monkey and
anvil were sharp. On the sides of the bar the zone took the form of a
sort of cross with curved arms, the arms being thinner or thicker
according to the greater or less energy of the shock. These forms are
shown in Figs. 1 to 6. It will be seen that these zones correspond to
the zones of greatest sliding in the deformation of a bar forged with a
sharp edged hammer, showing in fact that it is the mechanical work done
in this sliding which is afterward transformed into heat.
[Illustration]
With regard to the ratio, above mentioned, between the heat developed
and the energy of the blow, it is very much greater than had been
expected when the other sources of loss were taken into consideration.
In some cases it reached 80 per cent., and in a table given the limits
vary for an iron bar between 68.4 per cent. with an energy of 40
kilogram-meters, and 83.6 per cent. with an energy of 90
kilogram-meters. With copper the energy is nearly constant at 70 per
cent. It will be seen that the proportion is less when the energy is
less, and it also diminishes with the section of the bar. This is no
doubt due to the fact that the heat is then conducted away more rapidly.
On the whole, the results are summed up by M. Tresca as follows:
(1) The development of heat depends on the form of the faces and the
energy of the blow.
(2) In the case of faces with sharp edges, the process described allows
this heat to be clearly indicated.
(3) The development of heat is greatest where the shearing of the
material is strongest. This shearing is therefore the mechanical cause
which produces the heating effect.
(4) With a blow of sufficient energy and a bar of sufficient size, about
80 per cent. of the energy reappears in the heat.
(5) The figures formed by the melted wax give a sort of diagram, showing
the distribution of the heat and the character of the deformation in the
bar.
(6) Where the energy is small the calculation of the percentage is not
reliable.
So far we have spoken only of cases where the anvil and monkey have
sharp faces. Where the faces are rounded the phenomena are somewhat
different. Figs. 7 to 12 give the area of melted wax in the case of bars
struck with blows gradually increasing in energy. It will be seen that,
instead of commencing at the edges of the indent, the fusion begins near
the middle, and appears in small triangular figures, which gradually
increase in width and depth until at last they meet at the apex, as in
Fig. 12. The explanation is that with the rounded edges the compression
at first takes place only in the outer layers of the bar, the inner
remaining comparatively unaffected. Hence the development of heat is
concentrated on these outer layers, so long as the blows are moderate in
intensity. The same thing had already been remarked in cases of holes
punched with a rounded punch, where the burr, when examined, was found
to have suffered the greatest compression just below the punch. With
regard to the percentage of energy developed as heat, it was about the
same as in the previous experiments, reaching in one case, with an iron
bar and with an energy of 110 kilogram-meters, the exceedingly high
figure of 91 per cent. With copper, the same figure varied between 50
and 60 per cent.--_Iron_.
* * * * *