Thermometers are widely used in medicine and science laboratories. The purpose of a thermometer is simple and it is just to give the temperature of the object it is in contact with. Before the advent of the electronic thermometers, people relied on mercury-in-glass thermometers and the accuracy of these devices is good enough that these are not obsolete even today. How does the thermometer give the temperature of an object just by being in contact with it? And how does the level of liquid in the thermometer rise against gravity? Let’s discuss the physics of mercury-in-glass thermometer.
Before getting into the physics of the thermometer, let’s take a close look at its structure. A typical mercury-in-glass thermometer consists of a bulb (the bulged part of the thermometer) that is attached to a glass tube whose diameter is very small. The grades on the thermometer correspond to different levels of temperature. The bulb of the thermometer is filled with mercury. Liquids expand when subjected to heat. The inter-molecular distances increase when liquids are heated and as a result there is an overall increase in the volume of the liquid. The expansion or change in volume is directly proportional to the difference in the temperature or raise in temperature, initial volume of the liquid and the co-efficient of expansion.
When two objects are brought in contact with each other heat from the hotter object flows to the colder object until the temperature of these objects remain the same. Then the two objects are said to be in thermal equilibrium with each other. Transfer of a given amount of heat energy will not effect the same change in the temperature of these objects and it depends on the specific heat of the object. An object in contact with the thermometer results in the transfer of energy between the objects until the temperatures of both the objects remain the same.
A relatively hot object brought in contact with the bulb of a thermometer transfers heat to the bulb and results in the increase in temperature of the mercury in the bulb. The increased temperature causes the expansion of the liquid. The diameter of the glass tube like structure is very small so even a minute change in the volume of the liquid causes a palpable movement of mercury in the glass tube. This is the concept of sensitivity which we discussed during our Physics tuition classes. The expanded liquid moves along the thermometer and since expansion is dependent on the temperature, the extent of the movement of mercury along the glass tube can be correlated with the temperature of the object.