Temperature is a fundamental physical property that measures the average kinetic energy of particles in a substance. When particles move fast, we feel heat. When they move slowly, we feel cold. This molecular motion is what we call temperature.
Temperature measurement is essential in many aspects of daily life. We need it for cooking food safely, predicting weather, diagnosing medical conditions, and controlling industrial processes. However, our human senses are unreliable for accurate temperature measurement. We cannot precisely tell the difference between similar temperatures by touch alone.
A thermometer is an instrument designed to measure temperature accurately. The basic principle behind most thermometers is thermal expansion - materials expand when heated and contract when cooled. In a typical liquid thermometer, the liquid in the bulb expands as temperature rises, causing it to move up the narrow tube, giving us a temperature reading.
There are several types of thermometers designed for different applications. Liquid-in-glass thermometers use mercury or alcohol and are the most traditional type. Digital thermometers use electronic sensors with digital displays for easy reading. Infrared thermometers detect heat radiation and allow non-contact temperature measurement. Bimetallic thermometers use two different metal strips that bend with temperature changes to move a pointer on a dial.
Let's examine how a liquid-in-glass thermometer works in detail. When heat is applied, it increases the kinetic energy of liquid molecules. The molecules begin moving faster and spreading apart, causing the liquid to expand. As the liquid expands, it takes up more space and rises in the narrow tube. The height of the liquid column directly corresponds to the temperature, giving us an accurate reading.