A thermometer is a general term for a device that can accurately determine and measure temperature. It is designed based on the phenomenon of thermal expansion and contraction of solids, liquids, and gases due to temperature changes. There are various types of thermometers available to choose from, such as kerosene thermometers, alcohol thermometers, mercury thermometers, gas thermometers, resistance thermometers, thermocouples, infrared thermometers, and photothermal thermometers, among others. However, it is important to use them correctly and understand their characteristics to make the most of them.

The Principle of Various Thermometers

1. Gas thermometer: It is often used to measure temperature with hydrogen or helium because their liquefaction temperatures are very low, close to absolute zero, so its measuring range is very wide. This type of thermometer is very accurate and is often used for precise measurements.
2. Resistance thermometer: It is divided into metal resistance thermometers and semiconductor resistance thermometers, both of which are made based on the change in resistance value with temperature. Metal thermometers mainly use pure metals such as platinum, gold, copper, nickel, and alloys of rhodium iron and phosphor bronze; semiconductor thermometers mainly use carbon and germanium. Resistance thermometers are convenient and reliable to use, and have been widely applied. Its measurement range is about -260°C to 600°C.
3. Thermocouple thermometer: It is a temperature measurement instrument widely used in industry. It is made by utilizing the thermocouple phenomenon. Two different metal wires are welded together to form the working end, and the other ends are connected to the measuring instrument to form a circuit. When the working end is placed at the measured temperature, a potential difference will appear when the temperature of the working end and the free end is different, thus causing a current to flow through the circuit. By measuring electrical quantities, the temperature at another known location can be determined by using the temperature at that location. It is suitable for measuring the temperature between two substances with large temperature differences, and is often used for high-temperature and low-temperature measurements. Some thermocouples can measure temperatures as high as 3000°C, while others can measure temperatures close to absolute zero.
4. High-temperature thermometer: This refers to a thermometer specifically designed to measure temperatures above 500°C, including photothermal thermometers, colorimetric thermometers, and radiative thermometers. The principles and construction of high-temperature thermometers are quite complex, and they are not discussed here. Their measurement range is from 500°C to over 3000°C, and they are not suitable for measuring low temperatures.
5. The needle thermometer: It is a thermometer shaped like a dashboard, also known as a thermometer, used to measure room temperature, which is made by using the principle of metal's expansion and contraction with temperature changes. It uses a bimetallic strip as the temperature-sensing element to control the needle. The bimetallic strip is typically riveted together with a copper sheet on the left and a iron sheet on the right. Since the expansion and contraction of copper with temperature changes is much more obvious than that of iron, when the temperature rises, the copper sheet pulls the iron sheet to the right, causing the needle to deviate to the right (pointing to high temperature) under the action of the bimetallic strip; Conversely, when the temperature drops, the needle deviates to the left (pointing to low temperature) under the action of the bimetallic strip.
6. Glass thermometer: A glass thermometer measures temperature by utilizing the principle of thermal expansion. Due to the different expansion coefficients of the measuring medium and its boiling and freezing points, the common glass thermometers include kerosene thermometers, mercury thermometers, and red ink thermometers. Their advantages are simplicity in structure, convenience in use, relatively high measurement accuracy, and low cost. However, their disadvantages include limitations on the measurement range and accuracy due to the quality of the glass and the properties of the measuring medium. Moreover, they cannot be used for long-distance measurement and are prone to breakage.
7. Pressure-type thermometer: A pressure-type thermometer measures temperature by using the expansion or pressure change of a liquid, gas, or saturated vapor within a sealed container as a measurement signal. Its basic structure consists of a temperature sensor, a capillary, and an indicator. The advantages of a pressure-type thermometer are: it has a simple structure, high mechanical strength, and is not affected by vibration. It is also inexpensive and does not require external energy. However, its disadvantages include: it has a limited temperature range, typically between -80°C and 400°C; it has a large heat loss and a slow response time.
8. Mercury thermometer: A mercury thermometer is a type of expansion thermometer. The freezing point of mercury is -38.87°C, and the boiling point is 356.7°C. It is used to measure temperatures within the range of 0-150°C or 500°C, and it can only be used as a local supervision instrument. Using it to measure temperature is not only simple and intuitive, but it can also avoid the errors of external remote temperature meters.