The fact that fuels are expensive and that the supply of some fuels is diminishing, makes it advisable to conserve heat. This can be done in no more satisfactory way than by means of a fireless cooker.
It has been said that future historians in summing up the great achievements of the first quarter of the twentieth century will probably name as the most important, wireless telegraphy, aviation, and fireless cookery. The fireless cooker cannot be used with all methods of cooking, but its possibilities are many.
In Experiment 2 it was found that wood did not transmit heat rapidly, while tin did. Another familiar illustration will show the difference between wood and metal in transmitting heat. A metal door knob feels very cold on a winter day, because the metal conducts the heat away from the hand rapidly, while a wooden knob is comfortable to touch. Wood is termed a poor conductor of heat. Metals are good conductors of heat.
Paper, hay, excelsior, sawdust, cork, wool, feathers, and many other materials are poor conductors of heat. If any hot substance is surrounded by any of these poor conducting materials, the heat of that substance is retained for some time. Also, if any cold substance is surrounded by a poor conductor, the substance remains cold. In throwing a piece of carpet or newspaper over an ice cream freezer, to prevent the ice from melting, one makes use of the latter principle.
The walls of a well-built refrigerator consist of a number of layers of non-conducting materials.
To understand the principle involved in "cooking without fire," try the following:
Fill 2 tin measuring cups half full of boiling water. Immediately inclose one cup of water in a paper bag or wrap paper about it so there will be considerable air space between the cup and paper. After 15 minutes, insert a thermometer into the water in each of the cups. Which is hotter? What has "kept in" the heat of the hotter water?
The fireless cooker is a device containing cooking kettles which are surrounded by some poor conductor. When food is heated thoroughly, the heat can be retained for a number of hours by placing the hot food in the fireless cooker.
In the ordinary fireless cooker it is possible to cook all foods that can be cooked in water at a temperature below the boiling point of water, i.e. simmering temperature. Another type of fireless cooker has a metallic or an enamel lining and is provided with movable stone disks. Both the stones and food are heated on a range and then introduced into the cooker in such a way that the stones are under and over the kettle of food. By this arrangement, foods can be cooked at a higher temperature than in the ordinary fireless cooker.
There are also electric fireless cookers. Such cookers are equipped with a heating element which is placed in the bottom of the insulated box. With these it is not necessary to heat the food before placing it in the cooker. The uncooked food is put into the cooker and the current turned on. By means of a clock arrangement the current may be cut off when the desired length of time of heating has passed.
The principle of the fireless cooker is used on some of the modern gas and electric ranges. The walls of the ovens of these ranges are surrounded by insulating materials. When an oven is heated and has reached the desired temperature, the gas or electricity is cut off, but the baking temperature is retained for some time. The top burners of some gas ranges have a fireless cooker attachment in the form of an insulated hood. The food is first heated over the burner, then the hood is lowered over the food, and the gas is cut off. The food continues to cook, however, by the retained heat.