Heat and Thermodynamics

11.4 Experiment of Despretz, Wiedemann and Franz for Comparison of Conductivities of Two Different Materials

In 1822, Despretz compared the thermal conductivities of two materials by heating one end of each of the rods made with the materials, and measuring temperature at three equidistant places of the rods. For this purpose three equidistant holes were made and filled with mercury in which mercury-in-glass thermometers were introduced for temperature measurement.

Let θ₁, θ₂, θ₃ be the temperatures at distances x, x + a, x + 2a from the hot end, respectively; then from Eq.11.19, we can write

 

 

or

 

 

Putting y = e^αa in Eq. 11.29, we have ...