Millimetre of mercury

People use a way to measure pressure.

They use a heavy liquid called mercury. One unit is a tiny bit of this liquid. It is one millimetre high. Doctors use this to check your blood. It helps them see how your body works. This way of measuring is very old. Long ago, a man used mercury to test air. He showed that air has weight. It pushes on things all around us. This is how we measure pressure today.

How do we measure the push of air or liquids? We use a unit called a millimetre of mercury. This is written as mmHg. It measures pressure. Pressure is a push on things.

Long ago, a man named Evangelista Torricelli did a test. He used a tube and a bowl of mercury. He showed that air has mass. Mass means it has weight. This weight creates pressure on everything.

One millimetre of mercury is a very small amount. It is the pressure from a tiny column of mercury. That column is only one millimetre high. Today, we use exact numbers to define it. It is exactly 133.322 pascals. A pascal is another unit of pressure.

Doctors still use this unit a lot. They use it to check blood pressure. They also use it to check pressure in the eyes. It helps them see how the body works. They use tools like a sphygmomanometer to find these numbers. This tool measures blood pressure in mmHg.

A millimetre of mercury is a way to measure pressure. Pressure is the push that gases or liquids make on things. This unit is written as mmHg. It is a very helpful tool for scientists and doctors. Even though it is not a standard unit, people use it often. It helps us understand how much force is pushing on a surface.

To understand how it works, imagine a tiny column of mercury. Mercury is a heavy liquid metal. This unit measures the pressure made by a column of mercury one millimetre high. We define it using exact numbers to keep it steady. One mmHg is exactly 133.322 pascals. A pascal is another unit used to measure pressure. Scientists also use a similar unit called a torr. A torr is almost the same as a millimetre of mercury.

People have wondered about air pressure for a long time. In the 6th century BC, a Greek thinker named Anaximenes made a guess. He thought air changed by different levels of pressure. Later, in the 17th century, Evangelista Torricelli proved air has mass. He used a glass tube and a bowl of mercury. He showed that air pushes on things. This was the first real way to measure pressure.

Other thinkers helped build on these big ideas. Blaise Pascal had his brother-in-law test pressure at different heights. They found that pressure is higher deep in the atmosphere. Mercury manometers were the first tools to measure this accurately. These tools showed the difference in pressure between two liquids. Today, we use different tools because mercury can be toxic. We also use tools that are easier to handle.

You might see this unit used in a doctor's office. Doctors use mmHg to check your blood pressure. They use a tool called a sphygmomanometer for this job. They also use it to check pressure in the eyes. This is called intraocular pressure. It can even be used to check pressure in the brain. This unit helps medical experts keep people healthy and safe.

A millimetre of mercury is a unit used to measure pressure. Pressure is the force exerted by a gas or a liquid. This unit is written as mmHg or mm Hg. It is not part of the International System of Units (SI). However, it remains very common in many professional fields. It is especially important in medical science.

To understand the mechanism, imagine a vertical column of mercury. Mercury is a very dense liquid metal. The unit represents the pressure created by a column exactly one millimetre high. We define this using specific standard values for accuracy. We assume a density of 13,595.1 kg/m³ at 0°C. We also use a standard gravity of 9.80665 m/s². Based on these values, 1 mmHg is exactly 133.322387415 pascals. A pascal is a standard SI unit for pressure.

There is a similar unit called the torr. The torr is defined as exactly 1/760 of a standard atmosphere. One standard atmosphere is equal to 101,325 pascals. This means 1 torr is exactly 133.32236842 pascals. The torr is slightly smaller than the millimetre of mercury. The difference is about one part in seven million. For most practical uses, this difference is considered negligible. You can also divide one millimetre of mercury into smaller parts. These are called micrometres of mercury, or microns.

Humans have wondered about air pressure for thousands of years. In the 6th century BC, the Greek philosopher Anaximenes of Miletus made an early claim. He believed all things were made of air that changed through pressure. He noticed water evaporating into gas. He thought air became more condensed to make objects heavier or colder. In the 17th century, Evangelista Torricelli changed how we understood this. He conducted an experiment using a glass tube and a bowl of mercury. He dipped the tube into the mercury and raised the closed end. The mercury pulled down, leaving a partial vacuum at the top.

Torricelli's experiment proved that air has mass. This was a major discovery because many people, including Galileo, thought air was weightless. This experiment was essentially the first documented pressure gauge. Later, Blaise Pascal expanded on these findings. He had his brother-in-law test pressure at different altitudes on a mountain. They discovered that pressure is higher the deeper you are in the atmosphere. This led to the creation of mercury manometers. A manometer is a tool that shows pressure differences between two fluids. It does this by showing the height difference between mercury levels in connected reservoirs.

While mercury manometers were once the most accurate gauges, they are less common now. Mercury is toxic, which makes it difficult to handle safely. The readings are also sensitive to changes in temperature and local gravity. If you use an actual mercury column, you must make corrections. Gravity can vary by ±0.44% depending on your location. The density of mercury can change by −0.45% at 20°C. If the column is exposed to another fluid, you must also account for that fluid's density. Modern scientists often prefer other types of instrumentation for convenience.

In medicine, the millimetre of mercury is still the standard. Doctors often measure pressure relative to the current atmospheric pressure. For example, if blood pressure is 120 mmHg and the atmosphere is 760 mmHg, the pressure relative to a vacuum is 880 mmHg. A sphygmomanometer is the tool used to measure blood pressure. Doctors also use this unit to measure intraocular pressure in the eyes. Other important medical uses include measuring cerebrospinal fluid pressure and intracranial pressure. It is also used for intramuscular pressure, central venous pressure, and pulmonary artery catheterization. It even helps during mechanical ventilation. This makes the unit vital for understanding human physiology and health.