Temperature is one of the measurable aspects of nature that can be felt by human body. We have always been able to differentiate the hot from the cold or have associated with it in one form or the other. We know that a lot of natural phenomenon in this universe is directly dependent on the temperature of its environment. From the coldest of places on earth to the hottest of natural furnaces in the universe we know temperature plays an important role. Since temperature is a relative measure of hot and cold it was at one point important to have a standard such that we can understand and communicate temperature on an objective level.
Measuring temperature and designing ways to measure temperature has been one of the most interesting fields of study in physics, and it is also one of the key chapters taught during our Physics tuition classes. We know that changes in temperature can cause change in the way nature works. Temperature changes can cause changes in the rate of a particular reaction; it can either slow down the process or quicken it. We know that changes in temperature can bring about changes in the physical forms of a particular body and hence all the characteristics associated with it. Like the resistance of a wire, the density of a substance, humidity in the air, comfort and discomfort in the human body, speed of sound, etc.
The science of measuring temperature is called thermometry. It was Anders Celsius a professor at the Uppsala University in Sweden who suggested a centigrade scale to measure temperature by making 100 divisions between the boiling point of water and the melting point of water. Although many other scales were proposed since, the two popular scales used in any thermometric measurement today are the Celsius and the Fahrenheit scales. The fundamental principle of thermometry is to measure the variation in some property of the thermometer with temperature. There are three main ways to do that. A platinum resistance thermometer is based on the variation of the electrical resistance of high purity platinum wire with temperature.
Another way to measure temperature is using a thermocouple where, the two junctions between two different materials like copper and constantan are maintained at different temperatures. This difference in temperature in the two substances produces an emf across the thermocouple leads. Another interesting and widely using technique in thermometry is the diode thermometer. Where, when a small forward current is sent through diode (about 1 mA) in the forward direction a voltage drop occurs across the diode. If the current through the diode is kept constant, this forward voltage decreases with increase in the temperature over a limited temperature region. As a reverse process, the diode thermometer can be used to control temperature. Since the diode thermometry are not particularly accurate, platinum resistance thermometers and thermocouple thermometers are more widely used.