Simply put, Charles’ Law states that: At constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature (in Kelvin) increases or decreases.
Although the formulas and final concept of Charles’ Law were developed by Joseph Gay-Lussac in 1802, Jacques Charles came up with most of the ideas first and hence, the name of the Law.
Jacques Charles was an avid balloonist – flying in hot air balloons was pretty hot stuff back then. In an effort to improve the performance of the hot air balloons, Charles studied the relationship between the air in the balloon in its normal state and the air in the balloon when heated. Detailed measurements were taken and recorded. He took great care to keep all properties of the gas constant (i.e. unchanged) except for temperature and volume. Using mercury (the stuff in thermometers that rises when the thermometer is heated) and warm bath water, he was able to determine that the volume of the gas increased as the temperature of the gas was increased.
You may have noticed this effect when you toss a floatie into a swimming pool. The inflatable pool float may seem under inflated after a few minutes in the pool. This isn’t due to a leak in the float but rather, due to the temperature of the water in the pool cooling the float and reducing the volume of gas in it.
In the diagram below, notice that when the fire heats the gas (the yellow stuff with what Reeko figures is black pepper in it), the weights (green things) are forced upward as the gas expands. The graph to the left shows the constant increase in volume (V) and temperature (T).
An interesting note about Charles – he made his first flight of a hydrogen balloon in 1783. It landed outside of Paris and was destroyed by terrified peasants who had no clue what the strange device was.