Ask Me Everything5

Why do we need units?

Standard units of measurement are essential so that everyone can agree on what time it is, how big and heavy something is, and what the temperaturę is, so that parts of machines madę in different places fit together. Some units that we use are thousands of years old, while others were invented morę recently.

the meter

01: The meter was irwented in France.

02: There, it became the standard unit of length in 1791.

03: It was defined as one ten millionth of the distance between the North Pole and the eąuator.

04: Today, the meter is defined as the distance light travels in a vacuum in 1/299,792,458 of a second.

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Work like an Egyptlan

When is a galion not a galion?

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Standard meters, yards, pounds, and so on were madę in metal.

Each country kept its own set of standards.

■ Many of the earliest units of length were based on parts of the body. The palm, foot, and cubit (from the fingertips to the elbow) were used in Ancient Egypt.

Most measurements have been standardized Meters, kilograms, feet, and pounds are the same all over the world, whoever uses them and wherever they are used. But, British and American gallons are different. An American galion is equivalent to 3.78 liters, but a British galion is equivalent to 4.54 liters.

The standard meter was a metal bar with a meter marked on it. All meter rulers were copied from this standard meter.

■ The standard unit of length in Ancient Egypt, the cubit, was around 18 in (45 cm) long. It was not the only cubit—the Royal Cubit was 20.6 in (52.5 cm) long.

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Tell me morę: origins of units

■ Later units of length were also based on the body— an inch was 1/12 of a foot, and a yard went from the nose to the tip of an outstretched hand (or three feet).

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Palm

£ Foot ^	V
Yard

Ancient Egyptians and Babylonians divided the day and night into two 12-hour periods.

The 12-hour period was chosen because there are 12 Moon cycles in a year.

Metric units were invented to replace an estimated 250,000 different units that were used throughout France in the 1700s.

' During the French Revolution in the late 1700s, a ten-hour day was introduced. Each hour had 100 minutes and each minutę had 100 seconds.

Does it matter if units get mixed up?

In 1999, the Mars Climate Orbiter space probe broke up in the Martian atmosphere instead of going into orbit around Mars because of a mix-up with units. The spacecraft was programmed to receive navigation information from Earth in metric units. The information was actually sent in nonmetric units, which sent the spacecraft off-course.

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Ten units named after people

Andre Marie Ampere	Alessandro Volta	George Simon Ohm	Michael Faraday	Charles-Augustin
(1775-1836)	(1745-1827)	(1789-1854)	(1791-1867)	de Coulomb
amp	volt	ohm	farad	(1736-1806)
electric current	electric potential	electric resistance	electric capacitance	coulomb

electric charge