Mercury-in-glass thermometers have not become obsolete yet but with the introduction of electronic thermometer, the usage the old school methods may not hold the same legacy. Electronic thermometers are relatively easy to use with an LED screen to display temperature. And since mercury is not used in these thermometers, the contribution of heavy metal pollution is reduced with a switching from mercury-in-thermometer to electronic thermometers. The working mechanism of a mercury-in-thermometer was elucidated and is relatively simple. How does an electronic thermometer work?
To understand the working of an electronic thermometer, let’s discuss Seebeck effect. Though the name of this effect is not tested at the Cambridge O and A level examinations, it is the principle behind the workings of the thermocouple which we have learnt during our Physics tuition classes. Thomas Johann Seebeck, a Germany born physicist, observed it for the first time and the effect is named after him. He connected two conducting wires of different materials at both ends and maintained the temperature of one of the junctions hot and the other cold (relatively). He observed a deflection in the compass and initially thought that the maintenance of different temperatures at the two junctions resulted in magnetic field. Later it was discovered that the magnetic field associated is generated because of flow of current in the circuit. This phenomenon of generation of current in a circuit, formed by wires of two different materials whose junctions are maintained at different temperatures, is called as Seebeck effect.
The circuit formed by joining two wires of different materials as mentioned above is called as thermocouple. The same can be observed in the case of semiconductors as well. The amount of current or voltage generated in the circuit is proportional to the temperature difference between the heat and the cold junctions. So, if one knows the temperature of one of the two junctions and also the voltage generated in the circuit and the proportionality constant ‘a’ one can estimate the temperature at the other junction using the equation V=a*(Th -Tc). ‘Th’ and ‘Tc’ stand for the temperatures of the hot and cold junctions respectively and ‘V’ is the voltage generated in the circuit.
An electronic thermometer consists of a thermocouple. The temperature difference between the reference and the object whose temperature needs to be measured drives current in the circuit and depending on the amount of current generated one can calculate the voltage and subsequently the temperature of the object. This whole calculation process is done in the background and all you see a reading of temperature in the corresponding units.