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 experimental proposition: if one body A is in thermal equilibrium with two other bodies B and C, then both B and C are also in thermal equilibrium with each other. This proposition makes it possible to compare the temperatures of bodies without bringing them into direct contact with each other, but by observing thermal equilibrium through some property of one body (the thermometer) when it comes into contact with various bodies. This property (the "thermometric" property) of the body must change unambiguously with temperature, must be independent of the influence of other factors, be accurately reproducible, and finally be accurately measurable by a relatively simple and convenient method. To a greater or lesser degree, the following properties of certain bodies satisfy these conditions: volumetric expansion (gases, mercury, alcohol, toluene, pentane), electrical resistance of pure metals (platinum, lead, copper), the electromotive force of a thermoelectric couple, and radiation. The mercury thermometer is the most common instrument. It can serve for measuring temperatures from -30° to +700°; it is usually used for the interval 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 spare chamber for mercury in case of thermometer overheating). Depending on the temperature intervals for which the thermometer is intended and with what accuracy the temperature is to be determined, the capillary may have several spare expansions along its length (shortened thermometer), which makes it possible to have a sufficiently sensitive thermometer even for high temperatures, but not too long at the same time. Mercury at 0° completely fills the reservoir and partly the capillary, the remaining space of which must be devoid of air. The temperature is judged by the rise in the mercury level. Divisions in degrees or fractions thereof are applied either directly to the capillary or on a scale of milk glass, fixed parallel to the capillary. The thermometer is calibrated at two main points—0° C and 100° C; divisions are applied according to the standard scale, and for normal ones—according to the mercury scale (see Degree).—For the exact determination of absolute temperature (though not necessarily on the absolute scale) with a mercury thermometer, the reading taken on it must be corrected by the following adjustments: for calibration, for external and internal pressure, for the fundamental temperature difference, for thermal inertia, for the depression of zero, and for the standard scale (if the thermometer is calibrated by the mercury one). The medical thermometer belongs to the type of shortened thermometers and is intended for determining the temperature of the human body; it has a scale with divisions 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 upon cooling of the thermometer, but is held at a certain height corresponding to the maximum temperature of the environment; the mercury level in the capillary can be lowered only by shaking the thermometer. The maintenance of the mercury level in the capillary at a certain height is achieved (Fig. 2) by a constriction o of the capillary b at its beginning, through which the mercury can pass only at a certain pressure greater than that which the longest column of mercury of the given thermometer can exert (the constriction can be carried out using a small column inserted into the capillary b and fixed at the bottom of the reservoir; the diameter of the small column is smaller than the diameter of the capillary b). Figure Figure Figure 2. Mercury thermometer. Figure 2. Highly magnified reservoir and part of the capillary of a medical thermometer.

Figure 3. Mica "cross" for a resistance thermometer.

This design feature of the medical thermometer can allow artificially driving mercury into the capillary to a height not corresponding to the temperature of the measured environment, and thereby simulating an elevation of temperature. This can be achieved, for example, by sharp short blows on the upper end of the thermometer. In this case, the mercury, jumping up in the reservoir, gets into the capillary, passes through its constriction, and does not drop back due to the reason indicated 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 the factories of VOTI. According to current laws, medical thermometers can go on sale only after verification in the central laboratories of the Standardization Committee, which is certified by a certificate and a stamp on the thermometer itself. The verification of the thermometer must be repeated from time to time. Insufficient evacuation of air from the capillary and "aging" of the glass can cause distortion of the thermometer readings over time. Resistance thermometers are based on the change in the electrical resistance of pure metals with temperature. With an increase in temperature, resistance increases, and with a decrease, it decreases. Platinum thermometers are the most common, down to the temperature of liquid air, and lead ones are used for lower temperatures. The metal is used in the form of a wire with a diameter of 0.005 to 0.2 mm with a resistance at 0° C from 2 to 100 ohms, rarely up to 1,000 ohms. The wire is provided with wires (gold or silver) for conducting current into the thermometer and must be electrically insulated. In most cases, it is wound on mica leaves folded crosswise (Fig. 3) and placed in a metal (or quartz) tube so that the wire does not touch the walls of the tube. In general, the external appearance of a platinum thermometer is not characteristic: consisting of a thin insulated wire, it can be built into any instrument, vessel, etc. applied to the resistance thermometer.

The measurement of temperature with the help of this thermometer reduces to measuring resistance, for which the Wheatstone bridge method, the compensation method, and the differential galvanometer method are used. The scheme of the Wheatstone bridge method is given in Fig. 4. If the resistance of branches A and B is equal, then the resistance of thermometer T is equal to the resistance of branch C in the absence of current in the galvanometer branch G and with equal resistances a and a₁. Thus, the measurement reduces to selecting such a resistance in branch C at which the galvanometer G indicates the absence of current in it.—In practice, it is more convenient to make measurements if the temperature can be judged directly by the deflection of the galvanometer. For this, it is necessary to make the resistance of the three branches—A, B, C—constant, as well as the current source with a constant voltage. Then, when the temperature of thermometer T changes, and consequently its resistance, a current will flow in the galvanometer circuit, the strength of which will depend on the change in resistance. Divisions corresponding to the degrees of a particular temperature scale are applied to the galvanometer scale. The platinum thermometer is calibrated at three main points: 0°, 100°, 444.5° (sulfur boiling point). According to the regulations on the international temperature scale (adopted by the 7th Conference on Weights and Measures on October 4, 1927), the platinum resistance thermometer is adopted as a standard instrument for measuring temperatures from -190° C to 660° C. Thermoelements (see Thermoelectricity, thermoelement). The platinum-rhodium platinum thermocouple, according to the regulations on the international temperature scale, is adopted as the standard in the temperature range from +660° C to +1,063° C. Optical pyrometers, based on the change with temperature of the wavelengths of visible monochromatic rays emitted by a black body, are used exclusively for measuring temperatures above 1,000°.