How Thermometers Measure Temperature for Class 7

Thermometers are important tools that help us measure temperature in our daily lives. From checking body temperature to understanding weather conditions, thermometers play a key role in science and everyday use.

They work based on simple scientific principles and are widely used across homes, laboratories, and industries. In this article, we will understand how thermometers work, their scales, and important related concepts.

Table of Contents

• What is Thermometer?
• Different Temperature Scales
• Conversion Formulas
• Solved Numerical

Did you know?

The Fahrenheit scale was developed by Daniel Gabriel Fahrenheit, a German-born scientist, who lived and worked primarily in the Netherlands. Today, the scale is used primarily in the United States and some Caribbean countries. Anders Celsius (1701-1744) was an astronomer who invented the Celsius temperature scale, which is the most widely used scale in the world today. The kelvin (symbol: K) is the S.I. unit of temperature. The Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist, William Thomson, 1st Baron Kelvin. Previously, in the Celsius scale, the boiling point of water was marked as zero degrees Celsius and the freezing point of water was marked as one hundred degrees Celsius.

What is Thermometer?

Thermometers measure temperature by making use of the expansion and contraction properties of substances. This expansion and contraction is measured on a temperature scale.

A thermometer is made up of a thin, long, and uniform glass tube called a capillary tube. It has a bulb at one end. A silver coloured liquid called mercury present in the bulb is used for measuring temperature. Mercury is a thermometric liquid.

Different Temperature Scales

You will find two scales on the stem of a thermometer. One is the Celsius scale, and the other is the Fahrenheit scale. Let’s understand this important temperature scale.

Celsius Scale

Celsius is used in most scientific applications, and it is the most widely used temperature scale throughout the world. The unit of the Celsius scale is indicated by oC.

In the Celsius scale, the temperature at which pure water freezes at sea level is taken as the lower fixed point, whereas the temperature at which pure water boils at sea level is taken as the upper fixed point.

The lower and upper fixed points in Celsius are therefore, 0 °C and 100 °C respectively. The scale between these two points is divided into 100 equal parts, that is, hundred degrees. 

When the bulb of a thermometer is placed in contact with a body, the shining thread of mercury expands and moves up in the narrow capillary tube. You get to know the temperature by reading the graduation corresponding to the mercury thread.

Fahrenheit scale

The unit of the Fahrenheit scale is indicated by °F. Fahrenheit is a temperature scale that takes the upper fixed point as the boiling point of water at 212°F  and the lower fixed point as the freezing point at 32°F. The difference between these two points is divided into 180 parts.

Kelvin scale

The unit of the Kelvin scale is indicated by K. Kelvin is a temperature scale based on the boiling point of water at 373.2 K and the freezing point at 273.2 K. This scale has a point of absolute zero Kelvin. The Kelvin scale is always positive.

Conversion Formulas

1. Celsius to Fahrenheit: °C  =59(F−32)
2. Fahrenheit to Celsius: °F  =(C×95)+32

Where C is the temperature in Celsius and F is the temperature in Fahrenheit.

Solved Numerical

1. The temperature on a cold day was 10 °C. Convert it into Fahrenheit.

Solution: 

°F  =(10×95)+32

= 18 + 32

= 50 °F

2. Arvind had a fever, and his body temperature was 104°F. Convert it into Celsius.

Solution: °C  =59(104−32)

= 59(72)

= 40°C

In this article, we understood what thermometers are and how they measure temperature using the expansion and contraction of substances. We also learned about different temperature scales such as Celsius, Fahrenheit, and Kelvin, along with their fixed points and uses.