1. Why Thermometers Measure a Changing Property, Not Temperature Itself
You cannot measure temperature directly like you measure length with a ruler. Instead, scientists find a property that changes with temperature and read those changes on a scale. Encyclopedias define a thermometer as a device that uses the changing properties of substances to measure temperature. These are divided into types that use heat expansion, electrical methods, and radiation thermometers that measure without touching [2]. For example, liquid in a tube may look like it is stretching when it is hot, but it is actually expanding. This article looks at these three main types.
2. How Mercury and Alcohol Liquid Thermometers Work
Liquid thermometers trap a liquid, called a temperature-sensitive liquid, inside a thin glass tube. When the temperature rises, the liquid expands, pushing the level up inside the narrow tube. You read the temperature by looking at where the liquid stops on the scale [1]. Mercury and alcohols are commonly used. Mercury is suited for measuring roughly -30 to 300°C, while alcohols are suited for roughly -100 to +100°C. Pentane is sometimes used for even lower temperatures. These ranges are approximate and vary by product [1]. Mercury has a freezing point of -39°C and a boiling point of 357°C. Because the gap between freezing and boiling is wide, it can measure a broad range of temperatures [1].
3. Galileo's Early Device and the Celsius and Fahrenheit Scales
It is said that Galileo created a device using air expansion to measure temperature around the end of the 16th century. He attached a thin tube to a glass container and stood it upside down in water. When the air in the container expanded, the water level in the tube dropped; when it contracted, the level rose [5]. Historical records vary, suggesting 1592 or 1597, so we refer to this as the late 16th century. Adding scales to make it a true thermometer happened in the 18th century. The Japan Meteorological Agency explains that Fahrenheit defined the Fahrenheit scale in 1720, and Celsius defined the Celsius scale in 1742 [6]. The exact year for Fahrenheit is debated, so treat it as an estimate. Water freezes at 0°C and boils at 100°C on the Celsius scale, but freezes at 32°F and boils at 212°F on the Fahrenheit scale [7]. The difference is whether the space between freezing and boiling is divided into 100 parts or 180 parts.
4. How Digital Thermometers Use a Thermistor
Home digital thermometers do not have glass tubes or liquid. Inside the tip is a small component called a thermistor. An instruction sheet for a predictive digital thermometer states that it detects temperature by using changes in the thermistor’s resistance and displays it as body temperature [4]. When the temperature changes, the thermistor’s electrical resistance (how hard it is for electricity to pass through) changes. This change is read as an electrical signal and converted into a temperature reading. Instead of reading the height of liquid with your eyes, the device reads electrical values through a circuit.
5. Why a Digital Thermometer Beeps Before Measuring Ends: Prediction vs. Actual Reading
The same instruction sheet notes that a buzzer sounds when the predictive measurement is established 25 seconds after starting. If you continue holding it, it becomes an actual measurement [4]. It is said that if you keep measuring, it will also report the actual value after 5 minutes. Prediction estimates the final temperature from data while the body temperature is still rising. Actual measurement waits until the temperature stops rising before reading it. The 25-second time is specific to that one product and is not common to all models. Assume the time for prediction varies by product.
6. How Radiation Thermometers Read Infrared Energy
Radiation thermometers detect the amount of infrared energy emitted from an object using an infrared sensor. They calculate the surface temperature from that amount [7]. They can measure quickly without touching the object. Objects emit stronger thermal radiation as they get hotter, and the distribution of wavelengths changes [3]. This relationship is described by Planck’s radiation formula. The sensor works backward from the amount of energy received to calculate the temperature. It is important to remember that this measures surface temperature, not the internal temperature of the body or object.
7. Why Mercury Thermometers Are Banned in Japan
Mercury thermometers and blood pressure monitors are banned from manufacturing and import/export in Japan as of January 1, 2021, due to the Minamata Convention on Mercury and related laws [8]. The convention set a phase-out deadline for products containing mercury in 2020 [9]. According to an industry leaflet, one mercury thermometer contains about the same amount of mercury as 200 fluorescent lamps. However, this figure includes data from around 2010 [8]. The ban applies to making and trading them, not to using ones you already own. However, you must be careful when discarding them and follow local municipal guidelines.
Try Investigating Yourself
We have seen that different thermometers measure different things. So, what is your local thermometer reading? You can gather thermometers from your home, such as body, room, or cooking thermometers. Look for sections labeled "principle" or "sensor" in the instructions or on the device. Predict whether it uses liquid, a thermistor, or infrared, then check to see if you were right. If you visit a school science room or a museum, ask to see a glass tube thermometer. You can verify the relationship between the liquid level and the scale on the real object. For official information in Japanese, check the Japan Meteorological Agency’s "Questions for Children" or the thermometer instruction sheets from the PMDA.