Exactly how big is a kg?

Standards fascinate me. One of the most problematic standard in use almost universally today is the kilogram (kg). The problem is that no one really knows exactly how much mass a kilogram actually has. By extension that means that no one knows how heavy a pound is either since the US government defines it in relationship to the SI kg unit.

Originally the metric system was supposed to be defined in terms of “natural laws” that the common man could measure for himself. The kg was originally defined as a cubic decimeter of water under certain conditions. This is probably what you were taught in school, one of many metric misconceptions (see why everything you know about the metric system is wrong).

But that approach was jettisoned as impractical due to variations in water density, temperature, etc. In 1889 the standard became defined by a set of “physical prototypes” that were manufactured and distributed to major countries. So what was a standard based on “natural laws” became based on an arbitrary hunk of platinum and iridium.

Only that has not worked either (at least not to the number of significant digits desired). The problem is that the different physical prototypes are changing mass by a small but measurable amount. So today there is effectively no precise consistent definition of a kilogram, and thus by extension the pound.

The plan going forwards is to define the kg in terms of basic physical properties, similar to what has been done with the meter and the second. But for now, kg is only an estimate for given levels of precision.

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