What is the temperature scale of a thermometer?

When teaching the "Thermodynamic Temperature Scale", many middle school teachers extend the straight line on the p-t graph to the left after explaining Charles' law (Figure 1) and intersect the horizontal axis at point D, obtaining t=-273℃, and then recite the text in the textbook: "Accurate experiments have proved that 273 should be 273.15, that is, the temperature when the pressure is zero should be -273.15℃... Kelvin created a temperature scale with -273.15℃ as zero, which is called the thermodynamic temperature scale or absolute temperature scale. The temperature expressed by the thermodynamic temperature scale is called thermodynamic temperature or absolute temperature.

The "ruler" of temperature - the temperature scale is a mark of temperature divided according to a certain standard, just like the length of an object needs to be measured with a length ruler - "long scale", which is a man-made regulation, or a unit system. It is more complicated to determine the temperature scale, and we cannot just set the scale interval on the thermometer as we do with the long scale. We must first determine what kind of substance to choose (mercury, hydrogen or thermocouple). The hot and cold states of these substances must be able to clearly reflect the temperature changes of the objective object (the object to be measured), and this change must be reproducible (this step is called selecting "temperature measuring materials"). Secondly, we must know which physical quantities of the temperature measuring material will produce certain expected changes with the change of temperature (this step is called determining "temperature measuring characteristics"). For example, a mercury thermometer uses mercury as a temperature measuring material, and the volume of mercury changes linearly with temperature. This is the temperature measuring characteristic of mercury as a temperature measuring material. Thirdly, we must select two certain values ​​of the physical quantity as reference points (also called reference points), and then determine the method of dividing the temperature interval. The mercury thermometer first made by Fahrenheit was set on the coldest winter day in Northern Ireland. The lowest height of the mercury column was set as zero degrees; his wife's body temperature was set as 100 degrees, and then the length of this interval was divided into 100 parts, each part was called 1 degree. This is the original Fahrenheit temperature scale. Obviously, it is inappropriate to identify air temperature and human body temperature as the standard points for measuring temperature and to divide the scale on this basis. The body temperature of a healthy person often fluctuates during the day, and what if his wife catches a cold and has a fever? Later, Fahrenheit improved the temperature scale he created, setting the melting point of a mixture of ice, water, ammonium chloride and sodium chloride as zero degrees, expressed as 0°F, setting the melting point of ice as 32°F, and the boiling point of water as 212°F, and dividing the interval from 32→212 into 180 equal parts. In this way, the reference point has a more accurate and objective basis. This is the Fahrenheit scale that is still used in many countries today. After the Fahrenheit scale was determined, the Fahrenheit temperature (indicator) came into being.

Later, A. Celsiua also used mercury as a temperature measurement medium, with the melting point of ice as zero degrees (marked as 0℃) and the boiling point of water as 100 degrees (marked as 100℃). He determined that the length of the mercury column changes linearly with temperature, and that it is divided into 100 equal parts between 0 and 100 degrees, and each part, or each unit, is called 1 degree Celsius. This regulation method is called the Celsius temperature scale.

The Fahrenheit thermometer and the Celsius thermometer use the same temperature measuring material (mercury) and the same temperature measuring characteristics (mercury column expands when heated and contracts when cooled). However, due to the different standard points and graduation units, two different temperature scales are created, resulting in two different temperature values.

If the selected standard points are the same, but different temperature measuring materials are used, then the temperature scales will not be completely consistent, because their physical properties may not be the same in different ranges as the temperature changes. The following table lists five types of thermometers. Their temperature measuring materials are hydrogen, air, platinum wire, thermocouple and mercury. Their temperature measuring physical properties are gas pressure, resistance, electromotive force and mercury length. Their reference points are the melting point of ice and the boiling point of water as 0 degrees and 100 degrees. It can be seen that for the same objective temperature (assuming that the indication of a constant volume hydrogen thermometer is used as the standard), the readings of various thermometers are different.

The above specifically illustrates our assertion: no matter what thermometer is used to measure temperature, it only reflects the characteristics of the temperature measuring material and is also affected by the structure of the thermometer. For example, the glass bulb and capillary glass tube of a mercury thermometer will change the zero position depending on whether it contains sodium or potassium or both. Therefore, no thermometer can measure the true temperature of an object. Due to the different selection of temperature measuring materials and temperature measuring characteristics, the selection of reference points and graduation methods are different, so there can be a variety of temperature scales.

In order to end the confusion on the temperature scale, Kelvin (W. Thomson), the founder of the second law of thermodynamics and the most respected physicist, created an absolutely true absolute temperature scale that does not depend on any temperature measuring medium (and of course does not depend on any physical properties of any temperature measuring medium), also called the Kelvin temperature scale or the thermodynamic temperature scale.

The Kelvin temperature scale is based on the Carnot cycle, which uses the heat of the Carnot cycle as a tool for measuring temperature, that is, heat plays the role of measuring temperature. Because of this, we also call the Kelvin temperature scale the thermodynamic temperature scale. The Carnot cycle depicts the basic pattern of an ideal heat engine and has great theoretical significance. The Carnot cycle is like a lighthouse in the fog, giving the upper limit of the efficiency of a heat engine.