The most common way to measure the temperature of an object is with a thermometer. These are easy to find and can be used for food, water, or air. Thermometers use a simple mechanism: when an object is hot, its particles vibrate faster and it has more internal energy. When these particles vibrate more slowly, an object is cold and has less internal energy. Energy flows from the warm object to the cooler one. This flow is what we call heat.
Scientists also use another temperature scale, called Kelvin, which uses a different reference point than Celsius. Zero on the Kelvin scale is a very cold temperature that has never been achieved, but it does represent the lowest possible temperature, the point at which all molecular motion stops.
If we want to measure the temperature of an object that isn't giving off visible light, like a star in outer space, we can use other actual physical observations that depend on its temperature, such as its radiation spectrum graphs. These are calculated using Max Planck's theory of black-body radiation.
In the laboratory, scientists can also measure temperatures directly. They can use a specialized instrument called a pyrometer. This device detects the infrared radiation emitted by an object and reports its temperature. A less expensive alternative is a color changing sticker that can be applied to surfaces and read by a thermal imaging camera. This is used in remote sensing applications, such as monitoring semiconductor performance or next-generation nuclear reactors.
