236

FLUIDITY AND PLASTICITY

ity of metals and alloys. The work of Andrade on the differ-
ent types of flow in metals may be referred to.

Werigen, Lewkojeff, and Tammann (1903) measured the rate
of outflow of various metals and arranged the metals in a plastic-
ity series as follows: potassium, sodium, lead, thallium, tin,
bismuth, cadmium, zinc, antimony. They observed that with
equal pressures and openings, the efflux increases by about 100
per cent for every rise of 10° in temperature. This is shown by
the following table:

TABLE LXII.—THE RELATIVE EFFLXJX OF METALLIC LEAD THROUGH A SMALL
OBIFICE AT VABIOUS TEMPERATURES (AFTER WERIGEN, LEWKOJEFF

AND TAMMANN)

Temperature, degrees
Efflux (relative)
Temperature, degrees
Efflux (relative)

0.5
0.8
60.3
42.4

10.4
1.2
70.0
84.3

20.5
2.3
79.3
157.5

30.4
4.7
89.6
211.5

50.7
22.9

When a wire, which is stretched by a weight, is subjected
to torsional vibrations, the amplitudes of the vibrations form a
series in geometrical progression, and therefore the logarithmic
decrement of the amplitude is a constant. A part of the energy
of vibration is given to the surrounding atmosphere and a part
is transmitted to the support, but a portion of the energy is
dissipated within the wire itself. It is generally agreed that this
loss is due to the lack of perfect elasticity in the wire. In other
words, the wire when subjected to shearing stress suffers per-
manent deformation even though the stress is not equal to the
elastic limit. This deformation causes a shift in the position
of rest, so that as the pendulum passes from its new position of
rest to its old position of rest, it does so at the expense of its own
momentum and there is thus a loss of energy. This flow is
entirely analogous to the flow of various plastic materials such
as clay slip and paint, which we have already considered, when
the shearing stress is less than the friction.