How do companies that make measuring tools like rulers make sure the product is accurate? Is there a universally ruler that is used to check? How do they make sure the measurements are exact?
In: Engineering
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There is an international standard called the [International System of Units (SI)](https://en.m.wikipedia.org/wiki/International_System_of_Units) which defines the base unit of measurement for physical quantities. Base SI units are kilogram (mass), meter (distance), second (time), ampere (electrical current), mole (amount of substance i.e. number of molecules), Kelvin (temperature), and candela (light). The definitions of the units are typically based in physical realizations of the unit… a real physical object or phenomena which represents a unit by definition.
An intergovernmental organization called the [International Bureau of Weights and Measures (BIPM)]
(https://en.m.wikipedia.org/wiki/International_Bureau_of_Weights_and_Measures) based in France manages the SI system and hold the prototype realizations of various units (the official measure). For example, BIPM used to house the official kilogram defined as a platinium-iridium cylinder of a standard height and diameter. Governments from around the world duplicate this prototype cylinder and it is compared to the official to ensure calibration. This is used within countries to calibrate all other weight measuring devices. In the US, there is a federal agency and corresponding state agencies where businesses ensure their kilogram measure is the same as the state one, which is the same as the federal one, which is the same as the official prototype in France.
The same kind of definition and realizations exists for the meter and can be standardized and calibrated similarly. The standard has been for some time to do it this way, but in recent years, these definitions are shifting to physical phenomena rather than objects such as a meter now being defined as the distance light travels in 1/299792458 seconds and mass now being defined by some crazy stuff about Planks constant. It s wild.
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