The measurement of the thermal conductivity of a good conductor

The method uses an apparatus called Searle's bar, shown in the following diagram. A specimen of the material in the form of a bar, polished on the outside to reduce radiation losses, is heavily lagged and placed in a wooden box. The length of the bar is large compared with its diameter so that a measurable temperature gradient can be produced.

A heating coil wrapped round one end provides a heat input at a constant rate and the heat is removed at the other end by water flowing slowly through a copper tube which is soldered round the bar at the other end. The temperature gradient is measured by thermometers placed in holes in the bar; these may be filled with mercury to increase thermal contact.

When steady state has been reached the incoming temperature (θ₃) and outgoing temperature (θ₄) of the water is recorded as is its rate of flow (m) in kgs^-1. The cooling water must flow from a constant-head apparatus so that the rate of flow does not vary.

The diameter of the bar is measured at a number of different points using a pair of vernier calipers and a mean value found. From this the cross-sectional area (A) can be found. The value of k is found from the following equation:

where θ₁ and θ₂ are the temperatures of the thermometers in the bar, L is their separation and c_w is the specific heat capacity of the water.