Into a glass flask of cold water drop a few crystals of potassium permanganate, being careful not to agitate the flask. Apply a flame to the bottom of the flask. As the water becomes heated its density is reduced and it rises, forming convection currents which are coloured by the permanganate and may be distinctly seen.
Convection currents may be formed in any liquid or gas; for instance, air. By means of them heat will be carried from one part of the liquid or gas to another. Thus air heated by contact with a kettle of food will, if allowed to flow freely, carry the heat away from the food.
3. Heat is also lost by radiation.
This takes place less rapidly from a bright, highly polished surface, and for this reason “Thermos” and similar bottles are encased in polished nickle. A cooker-pail with polished outside surface retains heat better than one with a dull finish. In those cookers made with a metal outside retainer, the surface should not be painted or roughened or dulled by any means.
Experiment: Radiation.
Take two empty tin cans of the same size and shape. Wash off the paper labels. Keep one of them bright and shining, but move the other through a candle flame until the entire outer surface is smoked. Into each pour exactly the same quantity of water at the same temperature. Note carefully the temperature and the time. At the end of any given period, say one hour, again take the temperature of each. Which has lost the most heat, that in the bright can or that in the dull can?
4. The effect of different degrees or thicknesses of insulation.
Materials:
The same as those used in the experiment, [section 1], with the addition of boxes of various sizes, some smaller, some larger, than the one used in the first experiment.
Pack the boxes with one or more of the various insulating materials used in the first experiment, so as to allow varying thicknesses of insulation around the cooker-pail. This should be the same or an exactly similar pail in each case. Fill the pail for all tests with an equal quantity of water, boil it for one minute, and leave it in the boxes for an equal length of time. Record the temperature maintained in each test. Keep the record in tabular form.