1.9

## University of Liverpool

Skip to 0 minutes and 5 secondsSo in the last clip, we talked about the zeroth law of thermodynamics, and the idea of temperature arose from being able to define thermal equilibrium. We measure temperature using thermometers, but they are a relatively recent invention. The Swedish astronomer Anders Celsius, who lived from 1701 to 1744, devised the first temperature scale, where water froze at 100 degrees C and boiled at zero. Yes, I didn't get that wrong. His scale was upside down. He thought that water should freeze at 100 and boil at zero. And it was a few years before it was reversed to what we are now used to, boiling at 100 and freezing at zero.

Skip to 0 minutes and 51 secondsThe first mercury thermometer was developed by Daniel Fahrenheit, a German instrument maker who lived around about the same time. And he designed his thermometer to have zero, as the lowest temperature of a salt-ice-water mixture, so it was relatively convenient for him to demonstrate his thermometer. He looked around for what could be 100 degrees, and he decided that his body temperature was a nice portable measure, but, of course, a rather unreliable standard. So on his scale, water freezes at 32 degrees F, and boils at 212 degrees Fahrenheit.

Skip to 1 minute and 29 secondsThe Fahrenheit scale had a temporary advantage at the time it was developed, because negative numbers were rarely needed with the technology of the day. And it's sort of intuitive to set zero as the lowest attainable temperature that we can get to. And so today, we tend to use the Kelvin scale. In the Kelvin scale, zero is thermodynamically absolute zero. And that's defined by the third law, which we'll come across in week five. The degrees are the same size as for the Celsius scale, and so that means water freezes at 273 Kelvin, and boils at 373 Kelvin. There is an alternative that's used in some parts of the world, and that's the Rankine scale.

Skip to 2 minutes and 9 secondsIt uses the same zero, so that's thermodynamically absolute zero, and the degrees are the same size as the Fahrenheit scale.

# Temperature scales

Eann takes his temperature and talks about pioneers in temperature measurement, including Fahrenheit and Celsius. He explains the relationships between the Fahrenheit, Celsius, Kelvin and Rankine scales.