N1 Engineering Science|The Easy Way! 71


A suitable scale is fixed onto or next to the thermometer. The tube is often thickened on one side to act as a magnifying lens to magnify the thin column of liquid inside the tube.


Operation An increase in temperature will cause the liquid to expand andmove further up the bore. This can then easily be read against the scale. A decrease in temperature will cause contraction of the liquid and a shortening of the column of liquid will take place. Experiments and demonstrations with thermometers are simple to carry out.


4.4 Heat transfer


Heat can be transferred (propagated) in three ways; • conduction, • convection, and • radiation.


4.4.1 Conduction


Te thermal conductivity of different materials can vary considerably. In general, metals are good conductors of heat whereas materials such as wood, plastic, paper and wool are poor conductors.


Experiment 4.1 Conduction


Place a piece of wood and a piece of iron, each with a diameter of about 15mmand a length of about 10 cm, with half their lengths in a pot of boiling water, fig. 4.3.


Te outer end of the steel rod will soon become much hotter than that of the wood.


Experiment 4.2 Convection


A thin-walled glass jar is filled with water and conveniently placed. A drop of stamp-pad ink (used with rubber stamps) is dropped into the water. The ink sinks to the bottom(fig. 4.4).


On heating the jar with a candle in the centre at the bottom, it will be seen that the liquid rises fromthe flame,mushrooms (spreads out) at the top and sinks down the sides.


Candle Fig. 4.4 Convection g. 4.4 Convection Glass jar Wood Iron


Fig. 4.3 Conduction


igure 4.3 Conduction


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