New kg definition: Kibble Balance
By now, many of you should have heard that the kg is being redefined.
You can read more about it here:
https://www.sciencemag.org/news/2018/11/metric-system-overhaul-will-dethrone-one-true-kilogram
For 130 years, the kilogram (kg) was defined by a cylinder of platinum-iridium alloy in Sèvres, France. You should also know that scientists and standards agencies have been trying to define SI units with measurable techniques based on physical constants. Here’s an example and an explanation of the new kilogram definition from the article above:
The new SI generalizes the trade-off already exploited to define the meter more precisely in terms of the speed of light. Until 1983, light’s speed was something to be measured in terms of independently defined meters and seconds. However, that year, the 17th CGPM defined the speed of light as exactly 299,792,458 meters per second. The meter then became the measurable thing: the distance light travels in 1/299,792,458 seconds. (The second was pegged to the oscillations of microwave radiation from cesium atoms in 1967.)
The new SI plays the same game with the other units. For example, it defines the kilogram in terms of the Planck constant, which pops up all over quantum mechanics. The constant is now fixed as exactly 6.62607015×10-34 kilogram meters squared per second. Because the kilogram appears in that definition, any experiment that previously measured the constant becomes a way to measure out a kilogram instead.
Such experiments are much harder than clocking light speed, a staple of undergraduate physics. One technique employs a device called a Kibble balance, which is a bit like the mythical scales of justice. A mass on one side is counterbalanced by the electric force produced by an electrical coil on the other side, hanging in a magnetic field. To balance the weight, a current must run through the coil. Researchers can equate the mass to that current times an independent voltage generated when they remove the mass and move the coil up and down in the magnetic field.
That’s were the Kibble balance comes in and the linked article below does a better job of explaining it than I could.
https://www.nist.gov/si-redefinition/kilogram-kibble-balance
