“The world is changed.” – opening narrative, “The Lord of the Rings: The Fellowship of the Ring”
The international kilogram mass, or Le Grand K, will no longer the world’s standard. There will no longer be a physical object protected from human touch and wayward air currents under triple bell jars. Now the standard mass for the kilogram will be defined as something ethereal.
I think the first time I heard about “Le Grand K I was in 3rd grade. An article in my Weekly Reader spoke of a weight stored in a vault outside of Paris. I was fascinated! Since the time I was small, I had seen weights used on scales at feed stores and general stores in old rural North Carolina. They were handled by hardy men and women as they talked about the weather and their families. They moved them onto and off the scale-balance with careless ease as they weighted out feed or flour or nails. But this weight in Paris was completely different. This was a piece of metal that had been carefully made and even more carefully protected. It defined, for all other weights in the world, what a true kilogram actually was. It was handled not with bare, sweaty hands but only with gloved hands. It was covered in a glass bell jar. That bell jar was covered by another, larger glass bell jar. And that bell jar was covered by a larger, third bell jar. Le Grand K did not hang on a nail on the wall next to the scale. It was in a vault, kept behind closed and locked doors.
But over the years corrosion and dust and decay have affected the weight of this world standard. There had to be a better way. So now, all these years later, the old metal standard is just a museum piece. Perhaps if I wander far enough I may find it in a flea market in some distant corner of Europe. It is now detritus of a past age.
There is no longer a physical presence of a defining Kilogram. There is no more tactile experience of grasping the bell jar and lifting it away, being careful not to hit the precious mass. There is no more grasping the mass with gloved hands or padded tongs and feeling the weight of that mass pulling against bone and sinew and muscle of the hand and arm. Now it is ghostly. Now there is an ethereal determination of the pull of something that we cannot readily see as we would the weight.
The device now used to determine the mass of the test object measures electricity. For the test object, the question, “How much does this weigh?” is answered in a vacuum with the soft vibrations of electrical current and the resistance and magnetism of a conductor. There are two forces to be measured. The first is a measurement of value of an applied electrical current in a conductive coil in a Watt-Balance. The second is a measurement of induced voltage in the coil while moving through a stationary magnetic field.
Each of the measurements can be converted into the units of power (watts) by a known and proven mathematical formula, hence the name of the device, the Watt-Balance. The mass to be measured is placed in a pan which pulls down against the stiffness of a conductive, metal coil. A current is applied to the coil. This current stiffens the coil until the upward force in the coil balances the downward pull of the mass being measured. The value of the current required to balance the mass is recorded. The current is then turned off, the subject mass is removed, and the coil is now tested. The coil, a conductor of electric current, is passed through a magnetic field of known strength at carefully controlled, constant velocity. The value of the induced voltage in the coil is recorded. Based on the values of the measured current from the first stage and the value of the induced voltage from the second stage, two mathematical equations can be compared. Through this process the mass of the item being measured can be established with an extremely high degree of accuracy.
The Washington Post article on the redefined kilogram may be found at https://www.washingtonpost.com/science/a-massive-change-nations-will-vote-to-redefine-the-kilogram/2018/11/15/b5704b0a-e6c7-11e8-b8dc-66cca409c180_story.html
The National Institute of Standards and Technology (NIST) information on the Watt-Balance (Kibble-Balance) may be found at https://www.nist.gov/si-redefinition/kilogram-kibble-balance