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‘The world is getting warmer day by day.’ This sentence has started to crop up every now and then in our day to day lives. But seldom do we wonder what it means. What is warmer? How warm is warm? Is it hotter than hot, or just hotter than cold? To our human senses, temperature is only a subjective evaluation. For an objective and reproducible measurement, we need to quantify the temperature values, and to do that, a suitable measurement device is required. This is achieved through the use of temperature sensors.
Temperature and its Measurement
Simply speaking, temperature is the degree of hotness of the body which is a measure of the heat content in the body. The problem to quantify the heat content of the body on a scale did not arise until the invention of the Steam Engine. The curiosity of scientists to understand the behavior of water at different levels of heat contents gave rise to a formal and better laid out study. One of the first references for ‘temperature’ dates back to 1760, when Joseph Black declared that applying the same heat to different materials resulted in different temperatures. Years of rigorous scientific study led to many theories ranging from the simple ‘Caloric’ concept, which treated heat as a material substance which is exchanged among materials, to Carnot’s description of heat as a form of energy (which laid the foundation of the first law of thermodynamics). However, none of them satisfactorily explained the concept of temperature. It was Maxwell’s theory which offered good reasoning into it. He defined temperature of a body as is its thermal property which provides information about the energy content of the system. It is the measure of the average kinetic energy (energy by virtue of motion) of the molecules of the substance and signifies a heat potential due to which heat flows from higher temperature to lower temperature.
The word ‘temperature’ itself is said to be derived of the Latin word ‘tempera’ meaning ‘moderate or soften’. Moving along Maxwell’s line of thought, the velocity of molecules should be the basis of selecting the value of temperature, with absolute heatlessness being a state where the molecules are totally static. But, this measurement is not possible practically, and hence, other manifestations of the effect of heat are utilized to measure temperature, for example, the geometric expansion of materials. A brief history of temperature sensors with major milestones is shown in the figure below: