Thermometer

THERMOMETER, a physical instrument for measuring temperatures in arbitrary but fully established units—degrees (see). The possibility of measuring temperature with this instrument is based on the following empirical principle: if one body A is in thermal equilibrium with two other bodies B and C, then B and C are also in thermal equilibrium with each other. This principle allows for comparing temperatures of bodies without bringing them into direct contact with each other, by observing thermal equilibrium through some property of one body (the thermometer) when it is in contact with different bodies. This property ('thermometric') of a body must change unambiguously with temperature, must be independently of the influence of other factors exactly reproducible, and finally must be precisely measurable by a relatively simple and convenient method. To a greater or lesser extent, the following properties of certain substances satisfy these conditions: volumetric expansion (gases, mercury, alcohol, toluene, pentane), electrical resistance of pure metals (platinum, copper, lead), electromotive force of a thermocouple, and radiation. The mercury thermometer is the most widely used instrument. It can be used to measure temperatures from -30° to +700°; it is usually applied for the range from 0° to +360°. It (fig. 1) consists of a glass (or quartz for high temperatures) reservoir a (spherical or cylindrical in shape) and a glass (or quartz) capillary b soldered to it, the upper end of which is expanded into a bulb c (a reserve space for mercury when the thermometer is overheated). Depending on the temperature intervals for which the thermometer is intended and with what accuracy the temperature must be determined, the capillary may have several reserve expansions along its length (a shortened thermometer), which allows for a sufficiently sensitive thermometer even for high temperatures without it being too long. At 0°, mercury completely fills the thermometer's reservoir and part of the capillary, the rest of which must be free of air. The rise in the mercury level indicates the temperature. Graduations in degrees or their fractions are marked either directly on the capillary or on a milk glass scale mounted parallel to the capillary. The thermometer is calibrated according to two main points—0° C and 100° C, with the markings made according to a standard scale, and for normal thermometers according to the mercury scale (see Degree). For the precise determination of absolute temperature (but not necessarily on the absolute scale), the reading taken on a mercury thermometer must be corrected by the following adjustments: for calibration, external and internal pressure, for the main temperature difference, for thermal inertia, for zero depression, and for the standard scale (if the thermometer is calibrated according to the mercury scale). The medical thermometer belongs to the type of shortened thermometers and is intended for determining the temperature of the human body, with a scale graduated from +35 to +42° with values of 0.1°. In addition, it belongs to the type of maximum thermometers, i.e., those in which the mercury level in the capillary does not drop by itself when the thermometer cools, but is held at a certain height corresponding to the maximum temperature of the environment; the mercury level in the capillary can only be lowered by shaking the thermometer. The maintenance of the mercury level in the capillary at a certain height is achieved (fig. 2) by narrowing the capillary b at its beginning, through which mercury can pass only at a certain pressure greater than that which the longest column of mercury of this particular thermometer can exert (the narrowing can be accomplished by means of a rod inserted into the capillary b and fixed at the bottom of the reservoir; the diameter of the rod is smaller than the diameter of the capillary b). Figure Figure щ Figure 2. Mercury thermometer. Figure 2. Enlarged reservoir and part of the capillary of a medical thermometer.

Figure 3. Mica 'spool' for a resistance thermometer. This particular feature of the medical thermometer's design may allow artificially forcing mercury into the capillary to a height not corresponding to the temperature of the medium being measured, and thereby simulating a temperature increase. This can be achieved, for example, by sharp, short blows to the upper end of the thermometer. When this happens, the mercury, jumping in the reservoir, enters the capillary, passes through its narrowing, but does not fall back, for the reason stated above.-In Russia, medical thermometers began to be manufactured only in connection with the needs of the imperialist war. In the USSR, medical thermometers are manufactured at factories of VOTI. According to current laws, medical thermometers can only be put on sale after verification in central laboratories of the Committee for Standardization, which is certified by a certificate and a stamp on the thermometer itself. Verification of the thermometer must be repeated from time to time. Inadequate removal of air from the capillary and 'aging' of the glass may over time cause distortion of the thermometer readings. Resistance thermometers are based on the change in electrical resistance of pure metals with temperature. As temperature increases, resistance increases; as it decreases, resistance decreases. The most common are platinum thermometers, up to the temperature of liquid air; for lower temperatures, lead thermometers are used. The metal is used in the form of wire with a diameter from 0.005 to 0.2 mm and a resistance at 0° C from 2 to 100 ohms, rarely up to 1,000 ohms. The wire is fitted with wires (gold or silver) to conduct current into the thermometer and must be electrically insulated. In most cases, it is wound on mica sheets folded crosswise (fig. 3) and placed in a metal (or quartz) tube so that the wire does not touch the walls of the tube. Generally, the external appearance of a platinum thermometer is not characteristic: consisting of a thin insulated wire, it can be embedded in any instrument, VESSEL AND T. P. in application to the resistance thermometer. Measuring t° with the help of this 1. reduces to measuring resistance, for which the Wheatstone bridge method, the compensation method, and the differential galvanometer method are used. The circuit diagram of the Wheatstone bridge method is given in fig. 4. If the resistances of branches A and B are equal, then the resistance of the thermometer T equals the resistance of branch C when there is no current in the galvanometer branch D and with equal resistances a and a_g. Thus, the measurement reduces to selecting such a resistance in branch C at which the galvanometer D would indicate no current in it.-In practice, it is more convenient to make measurements if the temperature can be judged directly from the deflection of the galvanometer. For this, it is necessary to take the resistances of three branches—A, B, C—as constant, as well as the current source with constant voltage. Then, when the temperature of thermometer T changes, and consequently its resistance, a current will flow through the galvanometer, the strength of which will depend on the change in resistance. Graduations corresponding to degrees of one or another temperature scale are marked on the galvanometer scale. A platinum thermometer is calibrated according to three main points: 0°, 100°, 444.5° (the boiling point of sulfur). According to the provisions of the international temperature scale (adopted by the 7 conference on weights and measures on 4/X 1927), the platinum resistance thermometer is accepted as the standard instrument for measuring temperatures from -190°C to 660°. Thermocouples (see Thermoelectricity, thermocouple). The platinum-platinum-rhodium thermocouple, according to the provisions of the international temperature scale, is accepted as the standard in the temperature range from +660°C to +1,063°C. Optical pyrometers, based on the change with temperature of the wavelength of visible monochromatic rays emitted by a black body, are used exclusively for measuring temperatures above 1000°.