Although everyday measurements here in the U.S. involve pounds, ounces and such, scientists and villains in drug-related television shows use the kilogram, an measure of solid weight adopted by many nations around the world.
The standard kilogram is an egg-shaped piece of metal in a vault in Paris, officially known as the International Prototype Kilogram but nicknamed "Le Grand K." That particular alloy consists of a blend of metals designed to slow erosion (and thus weight loss) and has been the standard kilogram since 1889. It was created to be exactly one thousand times the weight of a gram, which had been defined in 1795 as the absolute weight of one cubic centimeter of water, measured at the temperature of melting ice.
One of the problems, as noted above, is the reality that all matter "erodes" by losing atoms over time. Even the carefully protected confines of Le Grand K and its special construction have not stopped this. The weight loss makes little difference in everyday matters, but when it comes to the precision demanded by scientific experiments -- especially ones conducted by modern super-sensitive equipment -- a century of erosion can affect important measurements.
Another problem is the amount of variation possible in measuring the gram itself. Corralling a cubic centimeter of water may be easy, but do all cubic centimeters of water weigh the same? Can changes in air pressure affect the temperature at which the ice begins to melt? Can these and other factors create uncertainty in the measurements? The answer is yes, and when you realize that any gets magnified in the thousandfold expansion from gram to kilogram you see a whole lot more uncertainty creep in to the mix than scientists want.
Enter the Planck Constant, often designated by h by scientists. It's a figure used to measure action or energy on the quantum scale, deep down in the basic building blocks of the universe. Researchers at the National Institute of Standards and Technology (NIST) in Maryland have a machine that can measure Planck's constant, and over the course of the next couple of years, they will make several. So will other instruments around the world, and the value that crops up most often will be accepted as a value for the constant.
As the story at the Review of Scientific Instruments notes, h is a measurement of energy. The determined value will be fed into Albert Einstein's famous E=mc2 equation because it describes the relationship between energy and mass. When the equation has a number for E (energy) and a number for c (the speed of light) then almost anyone can solve it and find out what m (mass) is. Give me a calculator and even I can do it.
That value for mass will be used to determine the mass of the gram and all of the weights depending on it, such as a kilogram. Which means Le Grand K will become Le Grand Has-Been, although the scientists doing the research figure it'll still be kept around for history's sake.
You may think I miss the point, but one of the most fascinating aspects of this process to me is that there is an actual journal called Review of Scientific Instruments devoted to articles and papers about things scientists use to measure other things, and it's been around since 1930. That's a lot of test-tube articles.