Kilogram

A kilogram is a way to measure how much something weighs.

It is made of one thousand grams. Long ago, people used water to set the weight. They used the weight of one litre of water.

Later, a special metal cylinder was used as the standard. Now, scientists use tiny parts of atoms to define it. This helps keep the weight the same everywhere. It is a very important tool for science.

We use it to understand our world.

A kilogram is a unit used to measure mass. Mass is how much matter is in an object. One kilogram is equal to one thousand grams.

Long ago, people used water to define the kilogram. In 1795, it was the mass of one litre of water. Later, a metal cylinder became the standard. This was called the International Prototype of the Kilogram. It was made of platinum and iridium.

Scientists kept this metal cylinder for 130 years. But they found the mass was slowly changing. This was a problem for science. In 2019, they changed the way we define it.

Now, the kilogram is based on the laws of nature. We use three special constants to define it. These are the speed of light and the Planck constant. We also use a specific frequency of the caesium-133 atom.

This new way is very steady. It does not rely on a piece of metal. Scientists can use tools like a Kibble balance to measure mass. This helps keep measurements the same all over the world.

A kilogram is the base unit used to measure mass in the International System of Units. Mass tells us how much matter is inside an object. One kilogram is exactly equal to one thousand grams. The name comes from the Greek word for a thousand and a Latin word for a small weight. People often shorten the name to just "kilo." This unit is very important for science and trade around the world.

For a long time, the kilogram was defined by physical objects. In 1795, it was the mass of one litre of water. Later, in 1799, a metal object called the Kilogramme des Archives was used. By 1889, a new cylinder became the official standard. This was the International Prototype of the Kilogram, or IPK. It was made of a mix of platinum and iridium.

Scientists kept the IPK as the standard for 130 years. However, they noticed a problem over time. The mass of the IPK and its copies seemed to be changing. The IPK diverged from its replicas by about 50 micrograms. Because of this, scientists needed a more stable way to define mass. They decided to move away from man-made objects.

In 2019, a new definition finally took effect. Now, the kilogram is defined by three constants of nature. These are the speed of light and the Planck constant. It also uses a specific frequency of the caesium-133 atom. This frequency helps define the second. When these three things work together, they define the mass of a kilogram. This way, the measurement stays the same everywhere in the universe.

This new method uses advanced technology to keep measurements accurate. One tool used is called a Kibble balance. A laboratory can use this to calibrate mass instruments. This makes sure that measurements are consistent and reliable. You can think of these constants like the rules of a game. Even if you change the players, the rules stay the same. This ensures that a kilogram is always a kilogram.

The kilogram is the fundamental base unit of mass in the International System of Units (SI). Mass is a measure of how much matter is contained within an object. The kilogram, represented by the symbol kg, is equal to exactly one thousand grams. Its name is a combination of the metric prefix "kilo-", meaning one thousand, and the word "gram." While people often use the informal term "kilo," the official scientific system requires specific naming rules.

For much of history, the kilogram was defined by physical objects. In 1795, during the French Revolution, it was defined as the mass of one litre of water. Initially, this was measured at 0 °C, but the definition later changed to the temperature of water's maximum density, which is approximately 4 °C. In 1799, a prototype called the Kilogramme des Archives was manufactured to serve as the standard. This object had a mass equal to one cubic decimetre of water at that specific temperature.

In 1889, the scientific community adopted a new standard called the International Prototype of the Kilogram, or IPK. This was a cylinder made from a platinum-iridium alloy. The IPK served as the global standard for mass for 130 years. However, scientists eventually noticed a troubling trend regarding its stability. Evidence showed that the mass of the IPK and its official replicas were changing over time. The IPK actually diverged from its copies by approximately 50 micrograms.

Because man-made objects can change, scientists sought a more permanent definition. They decided to define the kilogram using fundamental physical constants. These are values in nature that do not change. The current definition, approved in 2018 and effective in May 2019, relies on three specific constants. First, it uses a specific transition frequency of the caesium-133 atom to define the second. Second, it uses the speed of light in a vacuum to help define the metre. Finally, it uses the Planck constant to define the mass of the kilogram.

This complex process links mass to time and length through math. The Planck constant, when combined with the metre and the second, allows us to calculate mass. This ensures that the kilogram is no longer tied to a single piece of metal in a vault. Instead, the unit is tied to the laws of physics. This change makes the definition consistent across the entire universe. It also means that any laboratory with the right technology can reproduce the standard.

To make this new definition work in the real world, scientists use advanced tools. One such tool is the Kibble balance, also known as a Watt balance. A properly equipped metrology laboratory uses this device to calibrate mass measurement instruments. The Kibble balance helps turn a weight measurement into a mass measurement by using electrical terms. This process requires very precise measurements of gravity, a field known as gravimetry.

The transition to these universal constants is highly accurate. The current definition remains remarkably close to the original 1795 water-based definition. It agrees with the mass of one litre of water at 4 °C to within 30 parts per million, or 0.003%. This precision is vital for science, medicine, and global trade. By moving from physical artifacts to the constants of nature, we have created a measurement system that is truly permanent.