SYSTEMS OF UNITS
In the past, the systems of units most commonly used were the Englishand metric, as outlined in Table . Note that while the English system is based on a single standard, the metric is subdivided into two interrelated standards: the MKS and the CGS. Fundamental quantities of these systems are compared in Table 1.1 along with their abbreviations.The MKS and CGS systems draw their names from the units of measurement used with each system; the MKS system uses Meters, Kilograms,and Seconds, while the CGS system uses Centimeters, Grams,and Seconds. Understandably, the use of more than one system of units in a world that finds itself continually shrinking in size, due to advanced technical developments in communications and transportation, would introduce
The meter was originally defined in 1790 to be 1/10,000,000 the distance between the equator and either pole at sea level, a length preserved on a platinum-iridium bar at the International Bureau of Weights and Measures at Sèvres, France.
The meter is now defined with reference to the speed of light in a vacuum, which is 299,792,458 m/s.
The kilogram is defined as a mass equal to 1000 times the mass of one cubic centimeter of pure water at 4°C.
This standard is preserved in the form of a platinum-iridium cylinder in Sèvres.
The second was originally defined as 1/86,400 of the mean solar day. However, since Earth’s rotation is slowing down by almost 1 second every 10 years,
the second was redefined in 1967 as 9,192,631,770 periods of the electromagnetic radiation emitted by a particular transition of cesium atom.
The meter is now defined with reference to the speed of light in a vacuum, which is 299,792,458 m/s.
The kilogram is defined as a mass equal to 1000 times the mass of one cubic centimeter of pure water at 4°C.
This standard is preserved in the form of a platinum-iridium cylinder in Sèvres.
The second was originally defined as 1/86,400 of the mean solar day. However, since Earth’s rotation is slowing down by almost 1 second every 10 years,
the second was redefined in 1967 as 9,192,631,770 periods of the electromagnetic radiation emitted by a particular transition of cesium atom.
Category: electrical, electrical units
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