In 1809, Joseph Gay-Lussac showed that if a fixed mass of gas is kept at a constant volume in a closed system, its pressure is directly proportional to the absolute temperature, as shown:

• Mathematically (Fig 1)
• Graphically (Fig 2)

Again, take the example of gas in a sealed cylinder but with a constant volume. If heat is applied, the rise in pressure in the cylinder is directly proportional to the rise in absolute temperature (Fig 3).

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