In 1894 Congress asked the National Academy of Sciences to define standards for electrical measurements. In July they passed into law definitions for the amp, volt, joule, watt, coulomb, henry and ohm that brought the US into compliance with international standards. This began a long process of defining measures that continues to this day. Since Congress is busy with other matters, it has delegated standards authority to NIST, the National Institute of Science and Technology that employs some 3,000 scientists, engineers and technicians. NIST is working with other international agencies such as International Bureau of Weights & Measures to redefine a problematic measure, the kilogram.
A little background might help here. There are seven independent base units from which all others can be obtained. For example, the meter is currently defined as the ‘length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second’ and the second is defined as ‘the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.’ Thus speed can be derived from those two measures. The base units in the SI system include: meter for length, kilogram for mass, second for time, ampere for electric current, kelvin for temperature, candela for luminous intensity and mole for the amount of substance. Ideally, the base units should be indestructible and be invariable over time and place. They must be easily measured, reproducible and precise. To paraphrase one book author, an ideal measure is one where you can email instructions to another lab on how to construct an instrument that can reproduce that measure to better than one part per billion. The kilogram has none of the desired properties.
Historically, the definition of a kilogram was a liter of water. Now it is based upon a sample residing in a lab in France from which there are a dozen near copies (seven in the US). The problem is, however, all of these samples have different weights and all of the weights are changing with time. (No, this is not measurement uncertainty. They are actually changing weight due to evaporation, cleaning, handling, deposition and other mechanisms.) There are several proposals for the new kilogram that are independent of a physical sample. Some require counting atoms of carbon or silicon, but practical problems remain how to do this. The most promising seems to be the watt balance that should be able to not only be reproducible anywhere in the universe without a physical sample, but also to improve on the current uncertainty of 40 parts per billion.