This section is from the book "The American Woman's Cook Book", by Ruth Berolzheimer. Also available from Amazon: The Domestic Arts Edition of the American Woman's Cook Book.
For best results in cooking, exact temperatures should be known and followed. This requires the use of thermometers such as an oven thermometer or an oven-heat regulator for all sorts of baking, and special thermometers for sugar cookery, deep-fat frying, and roasting meats.
Automatic Mechanical Oven-Heat Regulators which control temperature automatically by regulating the supply of heat are available in both gas and electric ranges. These are of great assistance alike to the experienced cook who would always obtain the same results with a given recipe and to the beginner who has nothing to guide her in estimating the length of time required to get the slow, moderate and hot stages in her oven.
Heat Regulators or Temperature Controls must always be built into a gas range at the factory, and they must usually be built into electric ranges. For both types of stove they may be set to control a desired temperature automatically. Once set, they will maintain the temperature to within a few degrees Fahrenheit of that indicated, for an indefinite period.
Time Controls are now quite common on modern ranges and even on fireless cookers, and, in combination with the temperature controls, they are almost uncanny, for they will turn heat on at a definite time and off again at another specified moment. This makes it possible to put a meal in the oven or cooker in the morning and leave it with the assurance that it will start to cook at five o'clock in the afternoon and that the heat will be turned off again at five forty-five. As today's ovens and cookers are thoroughly insulated, the heat retained in the oven wall and in the food will complete the cooking. Moreover, since they are cooking on a decreasing heat, there is little or no danger of burning food, even if you should be delayed beyond the time when you planned to return.
Thermometers That Can be Set in the Oven may be used where an oven heat regulator is not available. A small flash light is useful for reading them in a dark oven.
Other Thermometers may be bought for candy and frosting, for deep fat frying, and for roasting meats. The cost of these thermometers is not large and they will soon pay for themselves in saving of time and food.
If These Devices Are Not Available the next best thing is to seek to develop delicacy of feeling and knowledge of practical tests which will detect differences in temperatures. This, of course, comes only with experience.