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 coulomb per second (C/s).  The Coulomb measures electrical charge, with one electron having a charge of 1.602 x 10^(-19) C (called the elementary charge).  With a little math we find that one ampere denotes 1.602 x 10^19 electrons flowing past a point every second, which is about .001% of a mole.  From here we can branch out into the Ohm for electrical resistance and the Volt for electrical force.  The voltage is what pushes the electrons while the resistance tries to hold them back.  How many electrons get through is the current.  It is all summed up nicely with V = IR (voltage = current x resistance).

Now, everyone is familiar with Franklin’s kite experiment of 1752, but the real work of defining electrical units did not begin until 1820.  In that year, Hans Christian Ørsted (Fig 1) discovered that electricity and magnetism are one and the same.  They are both manifestations of electron flow.  This opened up several doors for scientists as it provided a physical way to measure electricity.  The force between two magnets was easily measurable by the scientists of the 1800’s, while measuring electron flow directly was completely new.  A key result of Ørsted’s work was an apparatus that produces a magnetic force between two wires by running a current through them.  The force is proportional to the current (and a few other parameters) and thus allowed current to be measured.

From 1820 to 1920, various electrical societies worked to improve the definition of electrical units.  Many combinations of defining voltage, resistance, and current rose and fell.  By 1920, scientists had reliable ways of producing constant voltages and resistances, but current eluded them.  The CGPM added the Ampere to the metric system in 1920 and arrived at a new definition in 1960 that would do the job until 2019.  Ironically, the 1960 definition harkened back to Ørsted’s original work.  It defined one ampere as the current that produced 2 x 10^(-7) Newtons of force when run through two parallel wires of infinite length (Fig 2).

The problems with this definition are numerous.  First, it’s quite difficult to make infinitely long wires.  Additionally, the force unit, Newton, is derived from the kilogram which was still based on the artifact until 2019.  For all practical purposes, the Volt and Ohm were used experimentally, which made scientists unhappy.

In 2019, the same breakthrough that allowed scientists to redefine the kilogram allowed them to redefine the Ampere.  This now gets into quantum physics, but, long story short, scientists were able to accurately measure the Planck constant.  This proved some relations between quantum values and electrical properties, allowing for a new definition of the Ampere:

“The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 x 10-19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ΔνCs.”

Essentially, new discoveries allowed scientists to robustly prove what I discussed in the first paragraph, and now we have the Ampere.  From the Ampere, many other electrical units are defined.

So, that wraps up six of the seven SI base units.  The last is the kilogram, the big K as some call it.  If passion blogs are continued into the spring semester, I’ll write about it then.  Otherwise, I might just write an extra post; I would not want to leave off on a cliffhanger.  Also, I may do a blog or two about the US Customary system.  It is interesting to learn that an ounce of gold is not a standard ounce, we use both dry gallons and wet gallons, and that the pound only sometimes measures mass.  Until then.