Thermal Physics

Thermal Energy

Who discovered what makes an object hot?

Benjamin Thompson, Count Rumford (1753–1814), who was born in the Massachusetts Bay Colony, but did most of his scientific work in the Kingdom of Bavaria, now part of Germany, deserves most of the credit. Before his experiments, most scientists thought that hot objects contained an invisible fluid called caloric. Experiments done before Rumford showed that when you heated an object it didn’t gain weight, so caloric must be weightless as well as invisible. This result made many scientists suspicious of the caloric explanation, or theory.

In 1789 Rumford drilled holes in bronze cannons through which a cannon ball would be shot. He found that both the cannon and the metal chips that resulted from the drilling became hot. He determined the amount of water that could be raised to the boiling point by both the cannon bodies and the chips and showed that the caloric theory did not agree with his results. He finally concluded that in hot objects the particles that made up the material moved faster than they moved in cold objects. Using our present terminology, they had more kinetic energy. In their motion they vibrate back and forth; they do not move together like a thrown ball.


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