Whenever a patient is brought to a doctor, the doctor normally measures his body temperature. The device the doctor uses to measure the body temperature is thermometer. There are different types of thermometers for different purposes.
The thermometer that a doctor uses to measure the temperature of human body is called clinical thermometer. The thermometer for measuring temperature in science experiments is called laboratory thermometer and the thermometer that the meteorologists use for determining the maximum and minimum temperature during a day is called as maximum - minimum thermometer. These days they are using digital thermometers for different purposes.
Normally mercury-in-glass thermometer is conveniently used in day to day applications. In this type of thermometer there is a thin walled bulb attached to a thick walled capillary. The bulb and to a certain height the capillary are filled with mercury by repeated heating and cooling. The capillary above mercury level is evacuated and its upper end is sealed.
Then the thermometer is calibrated (marked) to measure temperature. For calibration lower and upper fixed points are marked respectively by burying the bulb first in melting ice and then in steam for sufficient time, so that mercury level in the stem remains fixed with time in each case.
Why use of mercury is preferred as thermometric liquid? The reasons are many. Mercury acquires the temperature of the body, it is kept in contact with very quickly; it absorbs very little heat from the body in contact and has large uniform expansion over a wide range. It is opaque and does not stick to the walls of the container. These properties make mercury the most appropriate liquid for accurate temperature measurements over a wide range.
Giving different values to the lower fixed point and upper fixed point and dividing the space between these two marks in equal number of divisions different scales are developed for measuring temperature.
Three scales are: celsius scale, fahrenheit scale and kelvin scale.
In celsius scale the lower fixed point (ice point) is marked as 0, the upper fixed point (steam point) is marked as 100 and the intervening space is divided into 100 equal parts. In fahrenheit scale the lower fixed point is marked as 32, upper fixed point as 212 and the intervening space is divided into 180 equal pats. In case of a kelvin’s scale the lower fixed point is marked as 273, steam point as 373 and the space between them is divided into 100 equal parts. SI Unit of temperature is kelvin (K).
The three scales are related by the formula:
C/100 = (F-32)/180 = (K-273)/100