The following thermometers should be a part of the equipment of every household:
To give a fair measure of the temperature of a room, a thermometer should be placed about four feet from the floor, away from a stove, radiator, or ventilation flue and not on an outside wall. Under certain conditions fifteen minutes or more may be required to show the correct temperature if a thermometer is moved.
To indicate the real temperature of outdoor air as given in the weather reports, a thermometer must be mounted in a specially well ventilated house or box four feet from the ground and so built as to shield the thermometer entirely from direct sunlight. Nearly the same results may be secured by placing a thermometer in an open shady place, possibly at the north of a building, several feet away from the walls and four feet from the ground.
The usual clinical thermometer is a "maximum" thermometer, that is, the mercury in the stem registers the highest temperature reached and does not return when the thermometer is cooled, but must be shaken back before another temperature can be measured. For this reason the thermometer may be removed from the mouth and read later.
The usual type of "lens-front" thermometer is so made that the front of the glass tube acts as a lens magnifying the width of the mercury thread. To read such a thermometer, it should be held in the hand and turned until the mercury column suddenly appears magnified to considerable width. This will occur when the clear corner of the triangular tube is directly in front. The reading can then be made, remembering that the smallest divisions of the scale are usually 0°.2. The mercury should then be shaken back into the bulb by holding the thermometer firmly between thumb and forefinger, bulb outward, and giving a few very brisk shakes from the wrist, or with the arm, and then seeing that the thermometer reads as low as 96° F. or 35°.5 C. The thermometer should never be tapped against a hard substance, as this is almost certain to break the bulb.
Bath thermometers usually have their scales printed on paper or milk glass contained in a large glass tube which incloses the thermometer capillary. They are often protected by a wooden cage to prevent breakage. When thus protected, it may take some time to obfig. 40. - Comparison of Centigrade and Fahrenheit scales for measuring temperature.
A bath thermometer should be read while it is in the water because the readings will change very rapidly when the thermometer is taken out of warm water.
Milk thermometers are useful in measuring the temperature of milk or cream, for the control of pasteurizing milk, churning cream, whipping cream, and the like. These processes are best carried out at definite temperatures. Some of these milk thermometers are purposely made large and light so that they will float, making their use more convenient.
These thermometers are for use in making candies, boiling sirups, and the like. The thermometer should not be too near the bottom or the sides of the kettle, nor yet should it be at the point where boiling is most violent. Some of the temperatures at which boiling sirups should be removed from the fire to make different kinds of candies, as well as other useful information as to temperature, are given in Table II.
A candy-making thermometer may be tested for accuracy as follows: First find the boiling point for any altitude from Table I (thus at 2,000 feet elevation the average temperature is 208° F. or 98° C.); then hold the thermometer with its bulb well immersed in a dish of briskly boiling pure water, and read the highest temperature reached. If this differs from that found in the table, the thermometer is too high or too low by this difference.
Various kinds of thermometers are used for reading oven temperatures. One kind is placed in the oven door and has a dial with a hand for indicating the temperatures. These thermometers may not indicate the true temperature of the oven because the door never becomes as hot as the remainder of the oven, and often takes much longer in heating up. However, since temperatures sometimes need not be known more accurately than within 10 or 20 degrees, such thermometers are useful and are more convenient than those which have to be hung inside the oven. One kind of thermometer which reads up to 550° F., is made to screw into a special opening in the range. It may be hung inside the oven, but in this case the door must be opened or a window provided in order to read it.