1 Absolute zero from Charles's law and from the pressure law

• The line on the volume–temperature (°C) graph, when extended, cuts the temperature axis at −273 °C (the volume appears to vanish there).
• The line on the pressure–temperature (°C) graph (Jolly's experiment), when extended, also cuts the temperature axis at −273 °C (the pressure appears to vanish).

Absolute zero (zero kelvin) is therefore the lowest theoretically attainable temperature, −273 °C; the temperature at which the volume of an ideal gas becomes zero at constant pressure, or the pressure of an ideal gas becomes zero at constant volume.

Example of the method. For a gas whose volume is 273 cm³ at 0 °C and 373 cm³ at 100 °C, the line V = 273 + t reaches V = 0 at t = −273 °C.