Neal Sheeran

Rants, Raves, and Geekery

How Long Is One Meter?


One area I have always found interesting is the standardization of measurements; that locked in a vault somewhere there was some piece of metal–probably some rare alloy in a vacuum or at some specific temperature–that was exactly twelve inches long or weighed precisely one pound, and these were the standard foot and pound by which all others were measured (and for once that phrase would not be a cliche). I also find the fact that the definitions for these measurements have changed over the years as science and technology evolve to higher levels of precision (What?! A yard isn’t a yard anymore?). I stumbled upon an article titled This Kilogram Has a Weight Problem and followed it up with this Wikipedia entry and came away with some interesting facts, in a dorky ‘umm… sounds great, gotta go’ kind of way:

Originally, the kilogram was defined as equal to the mass of one cubic decimeter, or liter, or water. In 1799, this was refined from water at 0° to 4°—the temperature where water reaches its most stable and maximum density.

In 1879, the International Prototype Kilogram was constructed, a cylinder of 90% platinum and 10% iridium. It was ratified as the kilogram in 1889…and remains so to this day. The IPK and six of its siblings reside under bell jars inside a vault that requires three keys and is maintained in an underground facility outside Paris, France by the International Bureau of Weights and Measures. The kilogram is the only unit that is still defined by an actual, physical artifact.

Copies of the IPK have been produced and distributed around the world to other nations as their version of the “standard”. The United States owns two of these, named K4 and K20, and they come from a batch of 40 delivered in 1884. The IPK replicas are compared to the original every 50 or so years…and many of the copies have actually gained mass, albeit a near-infinitesimal amount, over the years. Or the original IPK has lost mass, but since it is the standard, it doesn’t lose mass—it is always correct.

Work has been ongoing for many years to redefine the kilogram in terms of fundamental concepts of nature, and not in relation to a physical piece of metal locked in a safe. This new definitions include using the number of carbon-12 atoms, a sphere of silicon, or a new-fangled machine called a watt balance.

The Meter

According to Wikipedia, in 1791 the meter was defined as one ten-milionth of the length of a meridian from the Equator to the North Pole (that happened to pass through Paris). In 1889, the meter was defined much like the IPK—the distance between two marks on a bar made of platinum and iridium, measured at the melting point of ice.

This was refined in 1927 to:

the distance, at 0°C, between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard atmosphere of pressure and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other

Not 570 millimeters. 571. In 1960 the meter definition was changed and became:

equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.

The current definition of the meter was determined in 1983:

The metre is the length of the path travelled by light in vacuum during a time interval of 1⁄299 792 458 of a second.

The Second

Our current definition of the meter now begs the question; how long is one second? Again, Wikipedia says that the second first became measurable in 1670 with the development of a seconds pendulum for pendulum clocks [geeky sidenote: early proposed definitions of the meter related to the length of this pendulum].

In 1960, the definition of the second became:

the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.

Don’t ask me what ephemeris time is. With the advent of atomic clocks, the definition changed again in 1967:

the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.

This had to be “tweaked” in 1977 due to atomic clocks being affected by altitude and in 1997 it was further clarified to include the following:

This definition refers to a cesium atom at rest at a temperature of 0 K.

And that is how a second is defined today. The interesting thing to me in all of this is that as definitions for these measurements have evolved to become more precise, they have also become more esoteric. I like it that an actual, master kilogram exists somewhere.

What About the Foot and the Pound?

I almost forgot. Is there an extremely precise piece of metal that is exactly the mass of one pound or a foot or a yard long locked away somewhere or are they defined in terms of atoms or light? Ironically enough, since July 1959, US units are defined in terms of the metric system:

1 yard = 0.9144 meter

1 pound = 0.45359237 kilogram