Types of Thermometer
6 Minutes
Temperature scales for students: understand Fahrenheit, Celsius, and Kelvin and how each measurement unit was created. An English science lesson.
- Why Fahrenheit set 32 for ice and 212 for boiling water
- How Celsius divided freezing and boiling water into 100 steps
- Why Lord Kelvin created absolute zero and the Kelvin scale
What this lesson teaches
Temperature scales are standardized measurement systems used on thermometers to determine how hot or cold an object is. Thermometers commonly display readings using three primary units: Fahrenheit, Celsius, and Kelvin, each created by a different scientist using different reference marks.
The Fahrenheit scale
Daniel Gabriel Fahrenheit developed this scale in the eighteenth century using an unmarked mercury thermometer. He placed the thermometer into ice, then onto his own body, and finally into boiling water, marking the mercury level at each stage. To avoid working with decimal numbers, he set the freezing point of ice at 32 degrees Fahrenheit and the boiling point of water at 212 degrees Fahrenheit. While he initially measured human body temperature as 96 degrees Fahrenheit, later tests on other people revealed that normal body temperature is actually 98.6 degrees Fahrenheit.
The Celsius scale
Anders Celsius created a simpler temperature scale by using the physical behavior of water as a direct baseline. He placed an unmarked thermometer into ice, marked that level as 0, and then placed it into boiling water, marking that upper level as 100. He divided the space between freezing and boiling into 100 equal steps. This scale is also called centigrade, where centi means one hundred and grade means steps, and it is commonly seen on weather forecasts, air conditioners, and microwave ovens.
The Kelvin scale
- Lord Kelvin designed a new temperature scale because scientists faced difficulties when working with negative values in cold environments like the Arctic and Antarctic.
- He observed that every material has a limit and cannot be cooled beyond a specific point.
- He defined this lowest possible temperature as 0 Kelvin, which is called absolute zero.
- Because nothing can cool below absolute zero, the scale only moves upward step by step and is known as an absolute scale.
- Unlike Celsius and Fahrenheit, the Kelvin scale does not use the word degree, and water freezes at 273 Kelvin and boils at 373 Kelvin.
Why thermometer readings differ
A single patient can show two very different numbers on the same thermometer, such as 39 and 103, because both Celsius and Fahrenheit scales may be present on the device. A reading of 39 degrees Celsius represents the exact same body temperature as 103 degrees Fahrenheit. Because different inventors created separate systems based on different reference numbers, understanding the specific unit is essential to reading a thermometer correctly.