The work of Charles tells us that the volume of a gas at constant pressure is directly proportional to its absolute temperature, T (not to its centigrade temperature):

V = k' T

Here k' is a constant that relates V and T, and is the slope of the lines in the plots at the top of page 36.

We also can write Charles' law as




If two sets of experimental conditions at the same pressure are being compared, 1 and 2, then Charles' law can be written as




It is important to remember that this equality holds only at constant pressure.