A method of measuring thermal conductivity in varying state of temperature is described.
A thin and long cylinder is considered, in a large homogeneous medium of uniform temperature. If the surface of unit length of the cylinder is heated at a constant rate of
H cal/sec and if the rise of temperature at the axis of the cylinder from time
t1 to time
t2 is θ
2-θ
1, the conductivity
k of the medium is derived as
_??_
provided that the radius of the cylinder is very small.
Measurements are made electrically and the time needed for one observation is 1_??_30 min depending on the states of material used. Experiments were carried out for values of
k ranging from 5×10
-5 of air to 2×10
-2 of Hg.
By plotting θ
vs ln
t, and finding the minimum time
tmin required for the relation to become linear, the thermal diffusivity
K of the medium is found for the product
Ktmin assumes a constant value. Further, by measuring
k and
tmin for any given material, an estimation of its specific heat
cp can be made by the relation
K=
k/ρ
cp where ρ is the density.
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