In this blog post, we’ll explore the principles of temperature—which express the heat and cold we experience daily as objective numerical values—along with units of temperature and how various thermometers work.
How can temperature be measured objectively?
Every day, we check the weather forecast, anticipate whether it will be hot or cold, dress accordingly, and head out the door. If temperature were not defined and we had no objective indicator to gauge how hot or cold it is, it would be difficult to know how to prepare for the weather, since everyone has different thresholds for feeling heat and cold.
To measure temperature, first, there must be a property that changes with temperature. Second, these changes must be perceptible. Third, the changes in this property must follow a consistent proportional relationship. Typical examples of such properties include thermal expansion, electrical resistance, and the principle of radiation. Most objects increase in volume when heated and decrease in volume when cooled; this is known as the principle of thermal expansion. When an electric current flows through an object, its resistance changes with temperature, causing a change in current; this is known as the principle of electrical resistance. Finally, all objects with temperature emit radiant energy in the form of electromagnetic waves, and temperature can be measured using the principle that this radiative behavior is related to the object’s temperature.
How were the units of temperature established?
We have explored the properties used to measure temperature. However, these are still insufficient for use as objective indicators because the units of temperature have not yet been defined. The most common units of temperature we use in daily life are Celsius (℃) and Fahrenheit (℉). In science and engineering, the kelvin (K), the unit of absolute temperature, is used. The Fahrenheit scale originated from the temperature scale established by the 18th-century German physicist Daniel Gabriel Fahrenheit. Fahrenheit initially set the freezing point of brine at 0 degrees and human body temperature at 96 degrees; however, as the reference points of the scale were later adjusted, the current scale defines the freezing point of water as 32℉ and the boiling point of water at standard atmospheric pressure as 212℉, with the interval between them divided into 180 equal parts. The unit symbol is “℉.” Some countries, including the United States, still use the Fahrenheit scale today.
Most countries, including South Korea, use the Celsius scale. The Celsius scale originated from the temperature system proposed in 1742 by the Swedish astronomer Anders Celsius. Celsius initially defined the boiling point of water as 0°C and the freezing point as 100°C; however, the scale was later reversed, and the current standard is to use the freezing point of water as 0°C and the boiling point as 100°C. The scale is divided into 100 equal parts between these two reference points, and the unit symbol is “°C.”
Finally, the unit of absolute temperature is the kelvin, denoted by “K.” Absolute temperature is based on absolute zero—the lowest temperature that can theoretically be reached—which is 0 K. 0 K corresponds to −273.15 °C in Celsius. In the past, the kelvin was defined based on the triple point of water, but in the current International System of Units (SI), the kelvin is defined by fixing the value of the Boltzmann constant. The new definition was adopted at the 26th General Conference on Weights and Measures in 2018 and took effect on May 20, 2019.
Since the temperature intervals in Celsius and Kelvin are identical, a temperature difference of 1°C is equal to a temperature difference of 1 K.
The unit of temperature was established historically through several stages. At the 10th General Conference on Weights and Measures in 1954, the triple point of water was selected as the fundamental fixed point for thermodynamic temperature, and that temperature was defined as exactly 273.16 K. Subsequently, at the 13th General Conference on Weights and Measures, the name “Kelvin” and the symbol K were officially established. However, this definition has not been the current official one since 2019; today, a fixed value for the Boltzmann constant is used to define the kelvin.
How do thermometers that use thermal expansion work?
Let’s take a look at a typical thermal expansion thermometer found in everyday life. Early temperature-measuring devices, believed to have been created by Galileo, utilized the thermal expansion and density changes of air. Air has the property of increasing in volume and decreasing in density when heated, and decreasing in volume and increasing in density when cooled. Taking advantage of this property, air is placed inside a long, thin-necked tube and first heated to increase its volume. Next, when the tube is placed upside down in a container of water, the air cools and its volume decreases, causing the water to rise up the tube. In this way, it was possible to compare temperatures by utilizing the property that the state of air changes depending on temperature. However, these devices had the drawback of being affected by atmospheric pressure. Since atmospheric pressure differs between clear, high-pressure conditions and inclement, low-pressure conditions—and consequently, the state of the air and the height of the water can vary—it was difficult to accurately compare temperatures.
Typical examples of thermal expansion thermometers used today include alcohol thermometers containing red dye and mercury thermometers, which were widely used in the past. These are made by filling a thin glass tube with alcohol or mercury and utilize the property of the liquid to expand and contract with temperature changes. A key feature is that the interior of the glass tube is sealed to minimize the influence of atmospheric pressure, a limitation present in Galileo’s early device. When measuring temperature, placing the thermometer against an object causes heat to transfer between the liquid inside the thermometer and the object being measured; the volume of the liquid changes until the two objects reach thermal equilibrium—that is, the same temperature. Once thermal equilibrium is achieved, the volume of the liquid in the thermometer no longer changes significantly; therefore, reading the height of the liquid at this point reveals the temperature of the object being measured. Conversely, if the object’s temperature is lower than that of the thermometer, heat is transferred from the object to the liquid, causing its volume to decrease and the liquid level to drop. In this case as well, the mark on the scale when thermal equilibrium is reached indicates the temperature of the object being measured. Mercury thermometers were widely used in the past, but their use has greatly decreased due to the toxicity of mercury, and thermometers using different methods are now employed in various fields.
What principles are used to measure temperature?
In addition to temperature-measuring devices—such as those known to have been invented by Galileo that utilize the thermal expansion of gases, and thermometers that utilize the thermal expansion of liquids like mercury or alcohol—there are also “bimetallic thermometers” that utilize the thermal expansion of solids, “resistance thermometers” that utilize the property of electrical resistance changing with temperature, as well as “thermistors,” and “infrared thermometers” that measure temperature by utilizing the thermal radiation characteristics that vary with temperature. In particular, digital thermometers measure temperature by utilizing the property of a thermistor’s electrical resistance changing with temperature.
Non-contact thermometers, such as infrared thermometers, measure temperature by utilizing the thermal radiation emitted by an object and its characteristics.
Because temperature is objectively defined and various types of thermometers exist based on this definition, we can compare heat and cold numerically in our daily lives and respond appropriately to different situations. The air temperatures we check every day can also be used as objective information precisely because this system of temperature measurement and units has been established.
