The Twilight of the “Big K”


The “Big K,” more properly known as the “International Prototype Kilogram” is
the shiny metal cylinder under the three glass domes.

How important is it that when you purchase an ounce of gold you get exactly one ounce of gold for your money? The “Big K” is essential to make sure that can happen.  Or how important is it that when you purchase a gallon of gasoline you get exactly one gallon of gasoline for your money? Or if you purchase a pound of bread that you get exactly one pound of bread for your money? There are “standards” to make sure fair measures happen.  These matters are so essential to the trust needed to live together in harmony that they are mentioned repeatedly in the Bible and also are mentioned in the U.S. Constitution, as well as in the equivalent documents of most countries in the world.

The congress shall have the power to. . . coin Money, regulate the Value thereof, and of foreign Coin, and fix the Standard of Weights and Measures.” (From the U.S. Constitution, Article 1 Section 8.  Emphasis added.)

Leviticus 19:35-36 “Do not use dishonest standards when measuring length, weight or quantity. Use honest scales and honest weights, an honest ephah and an honest hin.”

Deuteronomy 25:13-15 “Do not have two differing weights in your bag—one heavy, one light. Do not have two differing measures in your house—one large, one small. You must have accurate and honest weights and measures, so that you may live long in the land the LORD your God is giving you.”

Proverbs 11:1 “The LORD abhors dishonest scales, but accurate weights are his delight.”

Proverbs 16:11 “Honest scales and balances are from the LORD; all the weights in the bag are of his making.”

Proverbs 20:10 “Differing weights and differing measures – the LORD detests them both.”

Proverbs 20:23 “The LORD detests differing weights, and dishonest scales do not please him.”

Ezekiel 45:10-12 “You are to use accurate scales, an accurate ephah and an accurate bath. The ephah and the bath are to be the same size, the bath containing a tenth of a homer and the ephah a tenth of a homer; the homer is to be the standard measure for both. The shekel is to consist of twenty gerahs. Twenty shekels plus twenty-five shekels plus fifteen shekels equal one mina.”

Amos 8:4-7 “Hear this, you who trample the needy and do away with the poor of the land, saying, ‘When will the New Moon be over that we may sell grain, and the Sabbath be ended that we may market wheat?’—skimping the measure, boosting the price and cheating with dishonest scales, buying the poor with silver and the needy for a pair of sandals, selling even the sweepings with the wheat. The LORD has sworn by the Pride of Jacob: ‘I will never forget anything they have done.’”

Micah 6:10-11 “Am I still to forget, O wicked house, your ill-gotten treasures and the short ephah, which is accursed? Shall I acquit a man with dishonest scales, with a bag of false weights?”

Everyone needs accurate weights and measures, engineers included.   The U.S. Congress has delegated the job of defining weights and measures to the National Institute of Standards and Technology (NIST), formerly called the National Bureau of standards (NBS).   In order to achieve standards that are internationally accepted, NIST relies on the work of the General Conference on Weights and Measures (GCWM) which is an international body that meets every six years to consider new developments in science as they might relate to achieving accurate weights and measures.  NIST also relies on the work of the International Committee on Weights and Measures, and on the work of the International Buearu of Weights and Measures.  NIST interprets the work of these groups and creates regulations so that U.S. law is consistent with the work of these international agencies.  The process of defining weights and measures depends similarly on these three international agencies in most countries in the world, so these three international agencies have a very important role.  In a very real way they can delight the Lord.

The three international agencies concern themselves mostly with just seven so-called “SI basic units.” (SI stands for “International Scientific” after the french abbreviation.) They are the second (time), the kilogram (mass), the kelvin (temperature), the meter (length), the ampere (electric current), the mole (number of particles in a substance), and the candela (luminous intensity).  It may seem amazing, but all other units we use in commerce and law are defined in terms of these seven SI basic units.  For example, an hour is exactly 3600 seconds.  An inch is exactly 0.0254 meters.  A foot is exactly 12 inches.  A mile is exactly 5280 feet.  And hence, a mile-per-hour is exactly defined in terms of seconds and meters, two SI basic units.  Similarly, ounces (mass), fluid ounces (volume of a liquid), volts (electricty), pounds-per-square-inch (pressure), carats (mass of a gem), and hundreds of other units are defined in terms of the seven SI basic units.  All these other units are said to be derived units, whereas the second, kilogram, kelvin, meter, ampere, mole and the candela are said to be SI basic units.  It should now be obvious that the definitions of the seven SI basic units are a very significant matter for worldwide trade, commerce, science, and life in general.  A change in any one of these definitions would have repercussions far and wide.

The GCWM is scheduled to meet in 2014, and it looks like the agenda will include the potential revision of the definitions of all seven of the SI basic units!  What’s with this?  I predict it will be just as disastrous as the Y2K debacle that happened on January 1, 2000.  In other words, you won’t notice it, but some scientists and engineers will have to spin their gears a bit to make sure you don’t notice it. In order to understand why the GCWM wants to make these changes, we need to review some history.

Back in the 1700’s it was common for each country to have prototype standard units against which to calibrate other measuring instruments.  Maybe you wanted to survey some land and needed to measure it in chains.  You would purchase a measuring chain, and if you wanted to know how good it was, you would compare it to your country’s prototype standard chain.  It does not take much imagination to see what kinds of problems such a system might have.  Maybe the prototype chain in country X is a slightly shorter than that in country Y for example.  Or maybe on a cold day the prototype chain is a little shorter than on a warm day.  Or maybe some wear builds up in the links of the prototype standard chain so that it stretches out to longer and longer lengths (slightly) over time.  Or maybe the metal of the chain is slightly elastic (all metals are) so that in use, the chain changes length based on how much tension you put on it.  OK–specify that it must be stretched to a specific tension and used at a specific temperature–but how will you standardize the measure of that tension and temperature!  It gets complicated really fast.

For all of the above reasons and more, scientists have long wanted to eliminate these prototype standard methods of defining units of measure.  In the early 1900’s scientists realized that standard units could be defined in terms of nature.  For example, the meter could be defined as one ten-millionth of the distance from the North Pole to the Equator along a meridian passing through the center of Paris, France.  (If you were french you would especially appreciate that definition!)  This would be great if everyone agrees that the earth is not changing in size.  And in 1793, everyone agreed.  Now each country could attempt to measure that distance and compare their prototype (formerly the standard) meter to the natural standard, and if everyone did a good job, and if the earth cooperated by not changing size from time-to-time, everyone would have virtually identical prototype meters.  Thus the prototype units lost their relevance as definitions of standard units, although they obviously retained their practical use.  (e.g. something to compare measuring instruments against)  By the way, scientists now allow that the earth might change size.  The meter is now re-defined in terms of the speed of light in a vacuum, which is (still) thought to be constant.

Over about a century of time, each of the seven basic SI units have been redefined in terms of natural constants, save for one lonely holdout.  Since 1889, and to this day (to be noticeably redundant), the kilogram is defined as “The mass of the International Prototype Kilogram,” more affectionately known as “Big K.”  “Big K” is kept in very carefully controlled environmental conditions in a vault in Sèvres on the outskirts of Paris.  But in 2014, it looks like the GCWM is going to retire this definition.  To accomplish this, they are proposing to define the numeric values of four well-known natural constants.  These are Planck’s constant, the elementary charge of an electron, Boltzman’s constant, and Avagadro’s constant.  Previously, to eliminate other prototype units from having definitional status, the GCWM has defined the ground state hyperfine splitting frequency of the caesium-133 atom as exactly 9 192 631 770 Hz (thus defining the second),  the speed of light as exactly 299 792 458 meters/second (thus defining the meter), and the luminous efficacy  of monochromatic radiation of frequency 540 x 1012 Hz is exactly 683 lumen-per-watt (thus defining the candela).

There are a few competing proceedures for exactly how the kilogram might be redefined.  The use of a watt-balance and in particular, settling on a exact numeric value for Plank’s constant is at the heart of the leading contender as of now.

If this proposal to define the numerical values of the four constants passes, and it looks like it will, then there will be seven such defined constants, from which the definitions of all seven SI base units will be derived.  The last prototype unit, the “Big K,” will loose its definitional status.  Pretty cool and it’s about time IMHO.

References

The Holy Bible (quotations are from the NIV)
Proposed redefinition of SI base units in Wikipedia
Redefining the kilogram at phys.org
World’s Roundest Object on You Tube

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Image of “Big K” from http://www.bipm.org/en/scientific/mass/pictures_mass/prototype.html

About Douglas De Boer

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