A mercury barometer is an instrument that measures atmospheric pressure using a glass tube filled with mercury. At standard sea level pressure, the mercury column stands at 760 millimeters (29.92 inches) tall, rising or falling as the weight of the atmosphere changes. It was the first accurate tool for measuring air pressure, and it remained the gold standard in weather stations and laboratories for over three centuries.
How a Mercury Barometer Works
The concept is surprisingly simple. A glass tube, sealed at one top end, is filled with mercury and inverted into an open dish (called a cistern) also containing mercury. Gravity pulls the mercury down, but the weight of the atmosphere pressing on the mercury in the cistern pushes back, holding the column up. The space above the mercury inside the sealed tube is a vacuum, so nothing pushes down from that side. What you end up with is a balancing act: the weight of a very tall, invisible column of air is perfectly balanced by the weight of a much shorter, visible column of mercury.
When atmospheric pressure rises, the air pushes harder on the mercury in the cistern, forcing the column higher. When pressure drops, the column falls. You read the height of the mercury against a scale etched or mounted alongside the tube, and that height is your pressure measurement.
Why Mercury and Not Water
Mercury is 13.6 times denser than water. That density is the entire reason it works as a practical instrument. A barometer filled with water instead of mercury would need to be about 13.6 times taller to balance the same atmospheric pressure. That works out to roughly 10.3 meters, or nearly 34 feet. A mercury barometer, by contrast, only needs to be about 33 inches tall. Dense liquid, short tube, manageable instrument.
Torricelli’s Experiment
The mercury barometer was invented by Italian scientist Evangelista Torricelli in the 1640s. In a letter dated June 11, 1644, Torricelli described his device to a friend and wrote one of the most famous lines in the history of science: “We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight.”
Before Torricelli, another Italian named Gasparo Berti had demonstrated that filling a long vertical tube with water could create a vacuum at the top. Torricelli’s insight was to swap water for mercury and, more importantly, to correctly explain why the column stayed up. Previous thinkers believed the vacuum itself somehow “attracted” the liquid upward. Torricelli argued the opposite: the column was being pushed up from below by the pressure of the atmosphere. To prove the vacuum wasn’t doing the pulling, he used two tubes with differently shaped tops. If the vacuum were the cause, the different shapes would have changed its properties and produced different mercury heights. They didn’t. The heights matched, confirming that atmospheric pressure was the real force at work.
Key Parts and Designs
The simplest mercury barometer has just three components: a sealed glass tube (typically about 33 inches long), a cistern of mercury at the base, and a measurement scale beside the tube. But over the centuries, instrument makers refined the design considerably.
The cistern barometer, developed for more precise readings, enclosed the mercury reservoir in a sealed cylinder. A leather bag at the bottom of the cistern rested on a screw. Turning the screw inflated or deflated the bag, adjusting the mercury level. When the screw was advanced far enough to fill the entire tube with mercury, the barometer could be turned upside down and safely transported.
The Fortin barometer became the standard for professional and calibration work. It added a sharp pointer (originally ivory) positioned right at the surface of the mercury in the cistern, plus a vernier scale for extremely precise readings. To take a measurement, you adjust the screw until the pointer just touches the mercury surface, then read the column height on the main and vernier scales. Depending on the measurement range, a Fortin barometer can achieve accuracy within 0.03% to 0.001% of full scale.
Angle barometers, shaped like an inverted “L,” bent the tube near the top of the mercury range so that the last few inches ran nearly horizontal. This stretched the scale, making small pressure changes easier to read with the naked eye. Italian glassblowers, particularly Angelo Lovi in Edinburgh during the late 1700s, used curved and elliptical tubing to magnify the mercury column further.
Reading and Units
Mercury barometer readings are expressed as the height of the mercury column. The most common units are millimeters of mercury (mmHg), inches of mercury (inHg), and hectopascals (hPa), which are identical to millibars. At standard sea level pressure:
- 760 mmHg (also called 760 torr, after Torricelli)
- 29.92 inHg (the unit you’ll hear in U.S. weather forecasts)
- 1013.25 hPa (the international meteorological standard)
A rising mercury column generally signals fair weather, while a falling column often precedes storms or rain. Rapid drops in pressure can indicate severe weather approaching.
Why Readings Need Correction
A raw mercury barometer reading isn’t quite the final answer. Two physical factors can skew the measurement, and professional observers correct for both.
Temperature matters because mercury expands as it warms. A column at 30°C is physically taller than the same amount of mercury at 0°C, even if the actual atmospheric pressure hasn’t changed. The metal or glass scale alongside the tube also expands slightly with heat. Standard practice is to apply a temperature correction that converts the reading to what it would be at 0°C, removing thermal distortion.
Gravity varies by latitude and altitude. Gravity is slightly stronger near the poles and weaker near the equator, which means the same atmospheric pressure will support a slightly different mercury height depending on where you are. A gravity correction accounts for your location’s deviation from the standard value, ensuring readings are comparable across weather stations worldwide.
Mercury Barometers vs. Modern Alternatives
Mercury barometers are highly accurate, but they’re bulky, fragile, and contain a toxic element. Two main alternatives have gradually replaced them.
Aneroid barometers use a small sealed metal chamber with most of the air removed. As atmospheric pressure changes, the chamber flexes, and that movement is transferred to a needle on a dial. They’re far more portable and contain no hazardous materials. The tradeoff is drift: the metal’s elasticity changes over time, so aneroid barometers need regular recalibration to stay accurate. Ships historically carried aneroid barometers that port meteorological officers would check and recalibrate at each stop.
Digital barometers are now the standard in professional weather observation. They use electronic pressure sensors connected to computers for automatic data processing and display. They combine the accuracy advantages of mercury instruments with the portability of aneroid ones, and they can log continuous readings without anyone needing to visually check a column of liquid.
Regulations and Phase-Out
Because mercury is toxic to both humans and the environment, mercury barometers have faced increasing restrictions. In the United States, the EPA issued a rule under the Toxic Substances Control Act requiring anyone manufacturing or importing elemental mercury for use in barometers, manometers, and similar instruments to notify the agency at least 90 days in advance. The rule effectively halted new production. At least eight U.S. states have outright banned the sale of mercury-containing barometers, and since January 1, 2013, the Mercury Export Ban Act has prohibited exporting elemental mercury from the country.
Instruments that were already in service before May 2011 are generally exempt from the restrictions, so antique and vintage mercury barometers can still be used and maintained. New mercury barometers, however, are essentially unavailable to consumers in the U.S. and much of Europe. For most practical purposes, digital barometers have taken over.