Temperature and its Measurement — Class 6 Science

This chapter explains how we measure how hot or cold a body is. It opens with the insight that our sense of touch is unreliable: after dipping one hand in warm water and the other in cold, the same water in a third container feels warm to one hand and cool to the other. Therefore we need a reliable measure of hotness or coldness, called temperature, and a device that measures it, called a thermometer. The chapter describes the clinical thermometer used for body temperature, noting that modern digital thermometers have replaced toxic mercury ones, and even infrared (non-contact) thermometers that measure temperature from a distance, as used during the COVID-19 pandemic. The normal temperature of a healthy human body is taken to be 37.0 °C (98.6 °F), though it varies slightly with age, time of day, and activity, and the human body normally stays between 35 °C and 42 °C. Temperature is measured on three scales: the Celsius scale (degree Celsius, °C), the Fahrenheit scale (degree Fahrenheit, °F), and the Kelvin scale, whose unit, the kelvin (K), is the SI unit of temperature; the conversion is . The chapter then introduces the laboratory thermometer, which has a liquid (alcohol or mercury) in a sealed glass tube with a Celsius scale, and explains how to read its range (typically –10 °C to 110 °C) and the value of its smallest division. It teaches the correct way to use it: the bulb must be immersed without touching the bottom or sides, the thermometer held vertically, the reading taken while still immersed and with the eye level with the liquid. A clinical thermometer cannot measure boiling water or ice because these lie outside its range. Air temperature, measured by room and weather thermometers, varies daily and is used for weather forecasts.