Rocket holds a kitchen thermometer in a cup of tap water. "Twenty one," he reads. "Twenty one what?"
"Degrees Celsius," Raven says. "Or seventy degrees Fahrenheit. Same water, two numbers."
"Two numbers for one temperature?" Rocket says. "Who decided that?"
Nova hovers over the cup, her light tracing the red line in the thermometer. "Two people, long ago, who each picked a different zero," she says. "What do both scales agree on?"
Raven thinks. "Water. They both use water freezing and boiling as their marks."
"What do you notice about the numbers?" she asks Rocket. He studies the thermometer. "Celsius has a hundred steps between them. Fahrenheit has a hundred and eighty."
Rocket writes both numbers in the Bench Log, side by side.
Thermometers measure temperature using materials that expand when heated and contract when cooled.
A liquid thermometer has a reservoir of liquid that expands when warmed. The column of liquid gets longer as the temperature rises.
A thermometer touched to an object quickly reaches the same temperature as the object. Then its reading tells you the object's temperature.
To give the reading a number, scientists mark the thermometer at fixed points, like the freezing and boiling points of pure water.
The Celsius scale sets the freezing point of pure water at 0 degrees and the boiling point at 100 degrees. The space between is split into 100 equal degrees.
The Fahrenheit scale puts freezing at 32 degrees and boiling at 212 degrees. That leaves 180 degrees between the same two points.
The Kelvin scale starts at absolute zero, the temperature where particle motion theoretically stops. There are no degrees; the unit is just the kelvin.
On the Kelvin scale, water freezes at 273.15 kelvins and boils at 373.15 kelvins. A kelvin is the same size as a Celsius degree.
To change Celsius to kelvins, add 273.15. To change Celsius to Fahrenheit, multiply by 1.8 and add 32.
| Reference point | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | -273.15 | -459.67 | 0 |
| Pure water freezes | 0 | 32 | 273.15 |
| Pure water boils (sea level) | 100 | 212 | 373.15 |
There is only one temperature that reads the same on both the Celsius and Fahrenheit scales: minus 40.
The Fahrenheit scale is typically not used for scientific purposes. Reference tables use Celsius or kelvins.
| Statement | True or false? |
|---|---|
| Thermometers use materials that expand when heated. | ? |
| Water freezes at 32 degrees Celsius. | ? |
| A kelvin is the same size as a Celsius degree. | ? |
| The Kelvin scale starts at the freezing point of water. | ? |
| Minus 40 reads the same in Celsius and Fahrenheit. | ? |
Well measured. Tomorrow is Bench Lab: you will plan a fair test of where ice melts fastest, and time it.