The History of Pressure Measurement
What Is Pressure?
Pressure is force applied over an area. Atmospheric pressure — the weight of the entire column of air above you pressing down — is something every human lives with but couldn't measure until the 17th century. Understanding pressure unlocked steam power, weather prediction, diving, aviation, and modern industry.
Torricelli's Barometer (1643)
Evangelista Torricelli, a student of Galileo, invented the mercury barometer in 1643. He filled a long glass tube with mercury, inverted it in a bowl of mercury, and found that the mercury column stabilized at about 760 mm — held up by the weight of the atmosphere. He had created a vacuum at the top of the tube, disproving Aristotle's claim that 'nature abhors a vacuum'.
Pascal and the Mountain
Blaise Pascal sent his brother-in-law up a mountain in 1648 with a barometer to prove that atmospheric pressure decreases with altitude (less air above = less weight). It worked — a confirmation that the atmosphere has a definite extent, and that we live at the bottom of an ocean of air. The SI unit of pressure, the Pascal (Pa), is named in his honor.
PSI and Industrial Pressure
Pounds per square inch (PSI) emerged with the steam age. As steam engines became the engine of the Industrial Revolution, engineers needed to measure boiler pressure to prevent catastrophic explosions. PSI gave a practical, intuitive measure: how many pounds of force pushing on each square inch of the boiler wall.
Pressure in Daily Life
Tire pressure is measured in PSI (US) or bar (Europe). Blood pressure is measured in mmHg (millimeters of mercury), a direct descendant of Torricelli's barometer. Weather forecasts use millibars or hectopascals. Scuba divers work in atmospheres (atm). The humble bar of pressure has many faces depending on context.