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SI and second
The majority of the clocks are caesium clocks ; the definition of the SI second is written in terms of caesium.
The SI unit of charge, the coulomb, " is the quantity of electricity carried in 1 second by a current of 1 ampere ".
Although the candela is now defined in terms of the second ( an SI base unit ) and the watt ( a derived SI unit ), the candela remains a base unit of the SI system, by definition.
The SI derived unit for measuring the catalytic activity of a catalyst is the katal, which is moles per second.
The SI unit for measuring the rate of flow of electric charge is the ampere, which is charge flowing through some surface at the rate of one coulomb per second.
Starting in the 1880s, and more significantly by the mid-20th century, CGS was gradually superseded internationally for scientific purposes by the MKS ( metre – kilogram – second ) system, which in turn developed into the modern SI standard.
While TT is strictly uniform ( being based on the SI second, every second is the same as every other second ), it is in practice realised by International Atomic Time ( TAI ) with an accuracy of about 1: 10 < small >< sup > 14 </ sup ></ small >.
( The SI second as now used for UTC, when adopted, was already a little shorter than the current value of the second of mean solar time.
The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, where one hertz is equal to one oscillation per second.
The ephemeris time of the 1952 standard leaves a continuing legacy, through its ephemeris second which became closely duplicated in the length of the current standard SI second ( see below: Redefinition of the second ).
Close equality between the ET second with the later SI second ( as defined with reference to the cesium atomic clock ) has been verified to within 1 part in 10 < sup > 10 </ sup >.
In this way, decisions made by the original designers of ephemeris time influenced the length of today's standard SI second, and in turn, this has a continuing influence on the number of leap seconds which have been needed for insertion into current broadcast time scales, to keep them approximately in step with mean solar time.
Following this, in 1967 / 68, the General Conference on Weights and Measures ( CGPM ) replaced the definition of the SI second by the following:
This SI second referred to atomic time was later verified by Markowitz ( 1988 ) to be in agreement, within 1 part in 10 < sup > 10 </ sup >, with the second of ephemeris time as determined from lunar observations.
In SI units, the unit of frequency is the hertz ( Hz ), named after the German physicist Heinrich Hertz: 1 Hz means that an event repeats once per second.

SI and was
Another style was to truncate the last 3 digits and append K, essentially using K as a decimal prefix similar to SI, but always truncating to the next lower whole number instead of rounding to the nearest.
A few minutes into the expansion, when the temperature was about a billion ( one thousand million ; 10 < sup > 9 </ sup >; SI prefix giga -) kelvin and the density was about that of air, neutrons combined with protons to form the Universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis.
MKS was in turn extended and replaced by the International System of Units ( SI ).
He developed the concept of horsepower and the SI unit of power, the watt, was named after him.
After publishing in the last issue of the magazine an analysis of the May 1968 revolts, and the strategies that will need to be adopted in future revolutions, the SI was dissolved in 1972.
Originally, the only symbol for the litre was l ( lowercase letter L ), following the SI convention that only those unit symbols that abbreviate the name of a person start with a capital letter.
In European countries where the metric system was established well before the adoption of the SI standard, there is still carry-over of usage from the precursor cgs and MKS systems.
In countries where the metric system was adopted as the official measuring system after the SI standard was established, common usage more closely follow contemporary SI conventions.
This special name for the unit was added in 1971 ; before that, pressure in SI was expressed in units such as N · m < sup >− 2 </ sup >.
This special name for the unit was added in 1971 ; before that, pressure in SI was expressed simply as N / m < sup > 2 </ sup >.
The unit was formerly an SI supplementary unit, but this category was abolished in 1995 and the radian is now considered an SI derived unit.

SI and defined
In the IERS numerical standards, the speed of light in a vacuum is defined as c < sub > 0 </ sub > = in a vacuum, in accordance with the SI units.
The meter is defined to be a unit of proper length, but the SI definition does not specify the metric tensor to be used in determining it.
Like other SI base units, the candela has an operational definition — it is defined by a description of a physical process that will produce one candela of luminous intensity.
The prefix kilo ( symbol k ) is defined in the International System of Units ( SI ) as a multiplier of 10 < sup > 3 </ sup > ( 1 thousand ), and therefore,
The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light.
In the International System of Units ( SI ), the basic unit of length is the meter and is now defined in terms of the speed of light.
However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new International System of Units ( SI ) as equal to 1, 650, 763. 73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.
The kilogram is one of the seven SI base units ; among these, it is one of three ( besides the second and the Kelvin ) which is defined ad hoc, without reference to another base unit, since 1889 by means of the international prototype kilogram.
It is currently disputed whether any changes in dimensional physical constants such as G, c, ħ, or ε < sub > 0 </ sub > are operationally meaningful ;< ref > In SI, c, and ε < sub > 0 </ sub > now are defined numerical values, independent of experiment, so observations now are directed elsewhere .</ ref > however, a sufficient change in a dimensionless constant such as α is generally agreed to be something that would definitely be noticed.
As a result, in 1967 the Thirteenth General Conference on Weights and Measures defined the SI second of atomic time as:
In relativistic terms, the SI second is defined as the proper time on the rotating geoid.
The SI unit of pressure is the pascal ( symbol: Pa ), defined as one newton per square metre.
Other units of pressure are defined in terms of SI units.
The unit of TT is the SI second, the definition of which is currently based on the caesium atomic clock, but TT is not itself defined by atomic clocks.
TT was defined to be a linear scaling of TCG, such that the unit of TT is the SI second on the geoid ( Earth surface at mean sea level ).
Today, these units are defined in terms of SI units.
The Kelvin temperature scale of the SI system is based on the triple point of water, defined as exactly 273. 16 K or 0. 01 ° C.
In astronomy, the Julian year is a unit of time, defined as 365. 25 days of SI seconds each ( no leap seconds ).
TDT is tied in its rate to the SI second, as is International Atomic Time ( TAI ), but because TAI was somewhat arbitrarily defined at its inception in 1958 to be initially equal to a refined version of UT, TT is offset from TAI, by a constant 32. 184 seconds.
Such standards are defined in the International System of Units ( abbreviated SI from French: Système international d ' unités ) and maintained by national standards organizations such as the National Institute of Standards and Technology in the United States.

0.097 seconds.