Friday, December 14, 2018

Ohm, My…

We all know what an ampere is, right? According to Ohm's Law, one amp (I) equals one volt divided by one ohm (E/R), or some variation that fits the basic equation. But Ohm's Law defines the relationship between these units, not what defines the units themselves. And those definitions have just changed.

In November, the international body that defines each of these units adopted new definitions, not only for electrical measurements but those for mass (the kilogram), temperature (Kelvin), quantity (mole) and luminosity (candela).  In a statement, the General Conference on Weights and Measures said the new definitions are based on "seven physical constants (for example the speed of light, the Planck constant and the Avogadro constant) and are therefore inherently stable." The goal is to eliminate the need for refining definitions in the future as measurement techniques become more precise. 
The need for a change, according to report on the vote in the New York Times, was made evident by "Le Grand K," the platinum-iridium cylinder housed in a vault in Paris that has defined the kilogram since 1889. The problem, it was discovered in 1990, was that somehow, Le Grand K had become 50 micrograms lighter than six official copies. The new definition of the kilogram, 28 years in the making, is based on the mathematical formula known as Planck's constant rather than on a physical object.

The ampere and other electrical units will also be redefined on the basis of Planck's constant, which has been linked by physicists to electrical voltage and resistance, according to the Times. According to the General Conference announcement, the changes will affect only those users who need "the highest level of accuracy," noting that "(f)or the vast majority of measurement users, no action need be taken as the volt will change by about 0.1 parts per million and the ohm will change by even less."

The new definitions take effect on May 20, 2019.