Searle's Experiment

You know from experience that heat flows from hot to cold. The extent to which materials conduct heat can be quantified by a parameter known as the thermal conductivity, symbol k. Heat conduction is not only determined by the material, but also by its shape, and by the temperature difference between the two ends. You will probably guess straight away that the higher the temperature difference between the ends, the more heat will flow. If you were to grab a longer poker and stick it into the fire, it would take longer for the end you hold in your hands to get warm. However, if you made the poker wider it would be easier for the heat to get across. Finally the heat input is important as well: the more heat you put into one end of the bar, the more heat will flow to the other end. From these considerations it follows that in order to measure the thermal conductivity of a certain material; we need to measure the heat input, its length, its diameter and the temperature difference. In its simplest form, an experiment might look like this: one end of the bar will be heated by means of a steam chamber or power supply, and you measure the amount of energy (heat) going in per second. You measure the length and diameter of the rod, and the temperature difference between the two ends of the rod. There is however one difficulty with this approach: eventually, the heat will spread out throughout the bar, and the entire bar will attain the same temperature. To circumvent this problem we keep the other end of the bar cooled.