Ways to measure temperature

When you want to know if you have a fever, how cold it is outside, or if your meal is ready you can do it by measuring temperature. You can easily measure temperature using a thermometer. This is an example of one of the most common thermometers. This little BULB at the end contains a liquid which reacts to changes in temperature. When the temperature is low, the particles that make up the liquid move very slowly and take little space.

When the temperature increases, particles start moving faster and they spread out. As a result the liquid takes up more space - it expands. This phenomenon is called THERMAL EXPANSION. When the liquid expands, it is pushed up a very narrow glass tube, which is connected to the bulb - the CAPILLARY TUBE. Along the capillary tube there are thin evenly spaced lines which allow us to read the result.

These are temperature scales, and can use values in degrees Celsius or Fahrenheit. The level which the liquid reaches inside the tube indicates a certain number of degrees on the scale. The number is equal to the temperature. This type of thermometer is called LIQUID-IN-GLASS. Different liquids react differently to the changes in temperature, and not all liquids are good for thermometers.

Traditional thermometers used mercury. Mercury has a wide range between its freezing and boiling points, expands evenly with temperature, has visible silver colour and doesn’t stick to the walls of the tube. Unfortunately, it’s toxic, so modern thermometers use alcohol instead. Some thermometers use thermal expansion of metals instead of liquids. These are made of two strips of different metals that are bonded together and react differently to changes in temperature.

Such thermometers are called BIMETAL thermometers. When temperature increases, one of the metals expands more than the other. Because of this, the strip bends. At the end of the strip there is a pointer, which moves with the strip and indicates the temperature on a scale. Some thermometers don’t indicate a point on a scale but they show a number on a digital display.

These are DIGITAL thermometers. Some of them have a special electrical part called THERMISTOR. One of its properties - resistance - changes when the temperature increases or decreases. In this way, the thermometer measures the temperature and shows it as a number on a display. All these thermometers show their own temperature - depending on their surroundings they become hotter or colder and they indicate this change.

But how can we measure the temperature of objects which we cannot place a thermometer inside, like a car engine? There are thermometers that can measure temperature from a distance! When you point them at an object, they measure its thermal or infrared radiation. We call them INFRARED thermometers. They also show the temperature on a display so they are another type of digital thermometer.

There are many ways to measure temperature. Different thermometers work differently and therefore have different accuracy, temperature ranges, or time needed for measurement. Some of them will be better for measuring air or body temperature, some for cooking and baking, some for industrial or technical purposes.