Complete Guide on Essential Thermometer Parts: Names & Diagram

What is a Thermometer?

The thermometer is a special medical instrument that measures the temperature. It is one of the important instruments for human beings to get introductory information about body temperature or temperature anywhere. There are too many types of thermometers available. However, liquid-in-glass mercury thermometers are simple and regularly used thermometers. It provides us with a more accurate and repeatable reading, which ranges from -40 degrees Celcius to 350 degrees Celcius (-35 degrees Fahrenheit to 660 degrees Fahrenheit). So, to get an accurate temperature reading, parts of the thermometer are made of special materials.

Thermometer Diagram with Parts

Parts of Thermometer

Parts of Thermometer

  • Spherical Bulb
  • Capillary Tube
  • Expansion Chamber
  • Scale Line
  • Stem

Parts of a Thermometer with Functions

Spherical Bulb

The spherical bulb is the lowest part of the thermometer. It is made up of glass, or sometimes it is also made of stainless steel. The aspherical bulb acts as a reservoir to store the mercury. Mercury inside the bulb stays in a liquid format & is maintained at room temperature.

This mercury is used to calculate temperature readings. When the spherical bulb comes in contact with temperature, mercury moves up. When the temperature decreases, the mercury slows down its position.

Capillary Tube

The capillary tube is nothing but the cylindrical body of a mercury thermometer. This capillary tube is connected to the bulb. It is the route of mercury that allows mercury to travel when the temperature rises. The ending of the capillary tube is called an expansion chamber.

Expansion Chamber

The top of the capillary tube of the mercury thermometer is called an expansion chamber. The function of the expansion chamber is to form a larger volume of mercury if the temperature rises to more than a specific limit. When the mercury hits the expansion chamber, it is the last limit of any mercury thermometer.

Scale Line

A scale line is a series of lines (Division of equal length known as degree ) that shows a temperature reading. This scale line is situated in a capillary tube. It shows the temperature reading when the temperature rises or falls.

In general use, a thermometer displays degrees Celsius and degrees Fahrenheit. Alternatively, a scale named Kelvin is on a special thermometer used by scientists.


The stem is a total glass body of a mercury thermometer. The stem is the longest part of any thermometer.

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