## Why is the kilogram the base unit of mass, instead of the gram?

 Quote by f95toli I think you are missing the point. You can use whatever system of units you want (and people do, e.g. Angstrom is not part of the SI) as long as they can be derived from the SI. However, the SI is set up so that all the units can be realized (you can perform an experiment to measure it). From a practical point of view this means that the starting point for all calibrations is a primary standard which -by definition- has exactly the value of the base unit OR a calculable fraction of that standard. In the case of the kg this means that the artefact has the mass of 1 kg, and in the case of the other units most primary calibration systems will give you 1m, 1V etc or a calculable fraction of the unit. ...
In reply to your previous post: sure, if someone wants to replace the kilogram artefact by a gram artefact, that is a bad idea.

Just to defend Gauss: he was not just anyone, and he was a masterful experimentalist. The desirability of realizeable base units may have crossed his mind too.

I wonder, isn't the definition of the meter, as the path travelled by light in 1⁄299 792 458 of a second (previously 1,650,763.73 wavelengths of krypton), suitable to define the millimeter and the centimeter as well?

I am still wondering about the replacement of cgs by mks. I am not convinced that rigorous use of the definition of the meter and the kilogram was the motivation.

 Quote by KingNothing I still would think kg are used more often in speech and informal discussions. I guess there's no way to be sure though - I doubt there are any studies on this. Besides, what we really should be talking about is how people tend to use time units like this: microsecond < millisecond < second < minute < hour < day
Sure, but there are fairly compelling practical reasons to use the otherwise arbitrary period of "1 day." 12 hours twice to make a day is maybe a bit more due to tradition, but it too has advantages over, say, 10 "standard hours" (shours? stours?) per day. (And, of course the use of base 60 in circular measurements has a long and glorious history. )

 It also seems as though "centi-" is really only used for centimeters, but not much else. Centiseconds anyone?
Centiliters seem to appear in common use. Centistokes in scientific use. Any other examples?

 Units are a funny thing!
Indeed!
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 Quote by KingNothing It also seems as though "centi-" is really only used for centimeters, but not much else. Centiseconds anyone? Units are a funny thing!
Try pronouncing 2,34 s as 2 seconds, 3 deciseconds, and 4 centiseconds.

Originally the prefix range (milli up to myria) made one step per decade to facilitate a pronunciation similar to 11 pounds, 4 shillings and 6 pence

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hi spareine! welcome to pf!
 Quote by spareine Lengths and masses are expressed identically in cgs and mks, when using the prefixes properly: 5 meters is 5 meters and 7 kilograms is 7 kilograms.
yes, in the cgs system, we can make calculations based on metres and kilograms,

but we would then have to be careful about how many zeros to put in the final answer …

if we use 500 (cm) and 7000 (g), then the answer automatically comes out it dynes (or whatever), without our having to think about it

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 Quote by spareine I wonder, isn't the definition of the meter, as the path travelled by light in 1⁄299 792 458 of a second (previously 1,650,763.73 wavelengths of krypton), suitable to define the millimeter and the centimeter as well?
Sure, in the same way the defintio of the Ampere is used to define mA. However, the "natural" defintion of the meter is quite modern; and for most of the history of the SI it was also based on an artifact. Moreover, engineering length standards are still used quite a lot and a 1m standard is much easier to use than a 1mm standard in practical applications.
Again, the units in the SI need to be practical; and a lot of it is driven by customer requirements, i.e. what the people who rely on accurate measurements actually need (be it in industry or academic research).

 Quote by spareine Why is the kilogram the base unit of mass, instead of the gram?
Cgs was replaced by mks because mks turned out to be the only system of units in which volt, ohm, and ampere are coherent with our units of length, mass, and time.

The cgs, which was adopted by the 1881 International Electrical Congress (IEC), contained three electrical quantities with a prototype: voltage, resistance and current. The coherent cgs-units were abvolt, abohm and abampere. The prototypes were called volt (a specification using a chemical cell), ohm (a thin, long column of mercury), and ampere (current which deposits silver by electrolysis at a certain rate). The cgs-units abvolt and abohm were unpractically small, 1 abvolt = 10-8 volt and 1 abohm = 10-9 ohm. Because of the large difference between these electrical base units and their prototypes, the cgs was unsatisfactory.

Giorgi discovered that the prototype units were coherent in another unit system, mksA. Ampere's Force Law, which relates current to force, got a new coefficient in the mksA system (2·10-7 instead of 2). The mksA system was adopted some time later by the IEC.

Fortuitously, in the new unit system, the base units for mass and length finally coincided with the prototypes from 1799 (kilogram and meter). Just lucky, not on purpose.
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 Tags cgs, kilogram, mass, si-system, unit