Question

Describe the working principle of a liquid thermometer. What are the properties of mercury and alcohol that make them suitable to use as thermometric liquids? Write a short note on Digital thermometer.

Accepted Answer

The thermometer is the most common instrument to measure temperature. In liquid thermometers either mercury or alcohol is used as thermometric liquids. When heat is supplied to the thermometer bulb containing mercury by the hot body whose temperature is to be measured, the mercury expands and rises in the glass tube of the thermometer. This fact is used in measuring the temperature of the hot body (or hot object). When the thermometer is in contact with a cold body, the mercury in the capillary tube contracts and falls down. Hence, we can say that a liquid thermometer works on the principle of thermal expansion of liquids.

The following are the properties of mercury that makes it suitable to use as a thermometric liquid:

1) Its expansion is uniform (For equal amounts of heat it expands by equal lengths)
2) It is opaque and shining.
3) It does not stick to the sides of the glass tube.
4) It is a good conductor of heat.
5) It has a high boiling point ( $357 ° C$ ) and a low freezing point ( $— 39 ° C$ ). Hence, a wide range of temperatures can be measured using a mercury thermometer

The following are the properties of alcohol that makes it suitable to use as a thermometric liquid:

1) The freezing point of alcohol is less than $— 100 ° C$ . So it can be used to measure very low temperatures.

2) Its expansion per degree Celsius rise in temperature is very large.

3) It can be coloured brightly and hence is easily visible.

Nowadays, digital thermometers are quite frequently used in place of mercury thermometers to measure body temperature. These thermometers do not contain mercury or any other liquid. Instead, they have a thermistor. Current passing through a thermistor changes with a change in temperature. Another small device inside this thermometer measures the current passing through the thermistor and sends the information to a liquid crystal display, which displays the temperature in digits. The major advantages of digital thermometers over mercury thermometers are that they are inexpensive. It is very easy to take the reading in a digital thermometer. This type of thermometer requires very little maintenance. They provide faster readings with high accuracy. The digital thermometer is having disadvantages also. They may get damaged easily if dropped. Thus, the battery powering them eventually runs out.

	Column A		Column B
$1$	Temperature	A	Solids
$2$	Conduction	B	Insulator
$3$	Mica	C	Thermometer
$4$	Land hot, Sea cooler	D	Poor heat absorbers
$5$	Bright and Shiny	E	Sea breeze

Describe the working principle of a liquid thermometer. What are the properties of mercury and alcohol that make them suitable to use as thermometric liquids? Write a short note on Digital thermometer.

Important Questions on Heat-Measurement

Learn and Prepare for any exam you want !