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

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

    Wikipedia explains the replacement of the cgs- (centimeter-gram-second) by the mks-system (meter-kilogram-second) by: "The values (by order of magnitude) of many cgs units turned out to be inconvenient for practical purposes. For example, many everyday length measurements yield hundreds or thousands of centimetres, such as those of human height and sizes of rooms and buildings" and "The units gram and centimetre remain useful .., especially for instructional physics and chemistry experiments, where they match the small scale of table-top setups". That does not make sense, does it? 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.
  2. jcsd
  3. f95toli

    f95toli 2,470
    Science Advisor
    Gold Member

    I suspect there are several practical reasons, One being that it would have been extremely difficult to make a 1g artefact (remember that it can only be a base unit if it can be realized, units need to be practical so that we can use them to calibrate instruments).
    Another that it is presumably easier to measure a 1kg artefact than a 1g artefact (accurately).
  4. You run into problems with derived units. For example, the standard unit of force in the cgs system is the dyne which is 105 times smaller than the Newton. That makes it rather useless for everyday scenarios if you consider that the weight of an average person would be something like 70 million dyn.
  5. A.T.

    A.T. 6,456
    Science Advisor
    Gold Member

    I think the question here is not:

    Which mass do we choose as 1 base-mass-unit?

    but rather:

    Why does the name of the chosen base-mass-unit start with "kilo"?

    If the mass of 1 liter water (or whatever else is used) is a practical amount of mass to be used as the base-unit, then why call it kilo[something] and just [something]? I guess it's just "historical reasons" and it doesn't really matter.
  6. The mks system became popular early in the 20th century, without having a name for the kg·m/s2 until 1948 (the Newton). Apparently in everyday scenarios people were happy with the kilogram-force, and then forces are expressed identically in cgs and mks. So I guess the unit of force was not an important motive for introducing the mks-system.
    Last edited: Aug 30, 2011
  7. I agree, I find it annoying. Since the kg is the base unit I am never certain if a thousand kg should be a kkg or a Mg. I usually go with Mg, but it never seems quite right.
  8. Why didn't that count in the 19th century? For example Gauss, who promoted the use of coherent unit systems, chose the millimeter-milligram-second system. Apparently he did not mind that the base unit of mass was a millionth of the artefact. And those 19th century scientists who switched to the cgs system did not mind either.
    Last edited: Aug 30, 2011
  9. As the mks-system was the successor of the cgs-system, the change from gram to kilogram was a choice, not a legacy.
  10. A.T.

    A.T. 6,456
    Science Advisor
    Gold Member

    The change of the base unit was a choice. The names are legacy.

    Ideally you would have neither "kilo" nor "centi" in the names of base units. So the cgs-system was not better here.
    Last edited: Aug 30, 2011
  11. A kilogram just has more everyday uses than a gram. Most non-scientists don't talk about grams of this or grams of that.
  12. Grams of fat or sodium?
  13. f95toli

    f95toli 2,470
    Science Advisor
    Gold Member

    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.

    Two things are worth keeping in mind when discussing the SI (and units in general). Firstly, the SI is controlled by an international organisation. This means that there is no "deeper" reason for some decisions, they are essentially political in nature or were made for historical reasons (which is why the Cd is a base unit). Secondly, the SI is a practical system. There is large international network dedicated to maintaining standards and calibrating instruments. This is why some of definitions in the SI are quite "ugly" (an obvious example is the Ampere, everyone would prefer to define it using the charge of the electron, but experiments based on electron-counting are not accurate enough)
  14. A.T.

    A.T. 6,456
    Science Advisor
    Gold Member

    Kids talk about grams all the time. But if they start talking about kilograms you should check what they really grow in the attic.
  15. I agree, a mgs-system would be preferable from a simple naming convention. The naming convention causes inconsistent prefixes with derived units like the following:

    5 kC / 1 s = 5 kA
    5 Mg / 1 s² = 5 kN

    or weird double-prefixes like the following

    5 kkg / 1 s² = 5 kN
  16. A (small?) correction to a frequent mistake.

    milli stands for 1/1000 not 1/1000000
  17. The kilogram artefact in Paris was made in 1799. Thirty years later, Gauss developed his system of coherent units. His milligram unit was a millionth of the artefact.
  18. 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

    It also seems as though "centi-" is really only used for centimeters, but not much else. Centiseconds anyone?

    Units are a funny thing!
  19. 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.
  20. Sure, but there are fairly compelling practical reasons to use the otherwise arbitrary period of "1 day." :tongue: 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. :biggrin:)

    Centiliters seem to appear in common use. Centistokes in scientific use. Any other examples?

Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?