For my tenth metrology blog, I will be talking about the Ampere, the SI unit of electrical current.  To begin, a brief explanation of electricity is in order. Electricity is caused by electrons flowing between atoms of metal.  This flow is called current.  The Ampere measures this current with one ampere being equal to one Read More…

In our everyday lives, we are accustomed to hearing temperatures read in degrees Celsius or Fahrenheit.  Yet, science uses the kelvin as the fundamental unit of temperature.  To see where this interesting unit comes from, read on. Early Roman and Greek attempts to measure temperature came in the form of comparative thermoscopes, meaning they could Read More…

The last four units in the SI system, the mole, kelvin, ampere, and kilogram, all form a knotted tangle of definitions.  Until 2019, they were all defined almost independently.  The ampere had its own definition and so did the kelvin.  The mole and kilogram were then related.  However, all these definitions were unsatisfactory, especially the Read More…

This week we will be discussing the candela. The candela is the SI unit of luminous intensity, the power emitted by a light source in a certain direction.  It is closely related to an SI derived unit, the lumen (lm), which measures brightness in all directions.  12.56 lumen is one candela times one steradian.  The Read More…

As promised, I will pick up where we left off last week with a discussion of atom clocks and the current definition of the second. The second is precisely defined as: “The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the cesium frequency ΔνCs, Read More…