Pack the box successively with as many of the different packing materials given above as are to be tested, following the directions given on [page 15]; or have several exactly similar boxes packed at the same time. For all tests fill the cooker-pail with water, bring it to the boiling point, let it boil one minute, to permit all parts of the utensil and its contents to reach the same temperature; then put it at once into the cooker-box and leave it for an equal length of time, not less than one hour. Record the temperature of the contents of the pail at the expiration of this period. In order to get a full record and a fair comparison it would be well to repeat this experiment with varying periods of time, taking the temperature, for instance, at the end of one, three, six, nine, and twelve hours. In taking temperatures do not wholly remove the cushion and cover of the pail, but slip them to one side, enough to insert the thermometer. This is, of course, a crude method of taking temperatures, but answers for purposes of comparison. If it is desired to make more accurate records this can be done by boring the cover of the box, the cushion and the pail cover, and inserting a thermometer through corks which are used to close the bored holes. The temperature can then be read while the apparatus is closed. However, the first method, if carefully done, will give probably within one degree of the correct temperature. Record the results in tabular form.
Which material do you find gives the best insulation?
Winkelmann,[4] Duff,[5] and other writers on physics give tables of the conductivity of felt, asbestos paper, paper, cotton, flannel, and other materials; but as different figures are shown, from different sources, for the same material, it is likely that the insulating power of any material used for packing a cooker will depend as much or more upon the way it is packed as upon the material used.
[4] “Handbuch der Physik.”
[5] “Textbook of Physics.”
Experiment: Conductivity of different materials.
Take a piece of copper wire about six inches long in one hand, and a piece of steel wire of the same length and thickness in the other. Put one end of each piece in a flame, holding the wire by the extreme end. Notice which first becomes too hot to hold at the end farthest from the flame. This illustrates the different conductivity of the two materials, steel and copper. There is not a great deal of difference in the conductivity of different materials, but metals are relatively good conductors, and air is a very poor conductor.
2. Heat is carried from the pail partly by convection
, except where solid insulating material, such as wood or indurated fibre, is used; and that manner of packing which best entangles the air and prevents air currents will, therefore, most increase the effectiveness of the insulation.