Re: The case of Sal Manela
Author: Commenter
Date: 09-26-2017 - 17:03
The meter (or metre) was supposed to be one ten-millionth of the distance from the north pole to the equator on the meridian running through Paris. The first measurement was difficult as the surveyors had to be protected by guards. The time of the French Revolution wasn't exactly peaceful in the countryside.
So having that measurement in hand an iron bar with two marks on it became the standard. Later a platinum-iridium bar of one meter in length became the standard. But progress marches on and in 1960 the meter was redefined as 1,650,763.73 wavelengths of orange-red light, in a vacuum, produced by burning the element krypton (Kr-86). More recently (1984), the Geneva Conference on Weights and Measures has defined the meter as the distance light travels, in a vacuum, in 1/299,792,458 seconds with time measured by a cesium-133 atomic clock which emits pulses of radiation at very rapid, regular intervals.
None of the definitions changed the length of the meter, but merely allowed this length to be duplicated more precisely.
Our English foot has not been so constant. The U. S. Congress legalized the use of the metric system in 1866 on the basis that one meter is exactly equal to 39.37 inches. In 1959 a number of English-speaking countries agreed that an inch is exactly equal to 2.54 centimeters so that the International foot is exactly equal to 0.3048 meters. The United States retained the old 1866 equivalency and called it the U. S. Survey foot so that 1 U. S. Survey foot equals 1.000002 International feet.
This confusion over the length of the foot has caused several oopses in land surveys working from the opposite coasts and using the international foot, which is the usual length of foot. The problem is that earlier surveys had used the survey foot. The error is about 10 yards in 3,000 miles.
Sometimes metric is just easier. If one works in electronics everything is metric: picofarads, microvolts, kiloamps, whatever.
The only metric unit (and thereby that of all other systems) that is still based on a physical object is the kilogram. This should change in 2018. In 2011, the CIPM formally agreed to express the kilogram in terms of Planck’s constant, which relates a particle’s energy to its frequency, and, through E = mc2, to its mass. Measuring Planck's constant isn't easy. Three different groups have been working on the problem. Fortunately, as the different measurements have gotten more accurate they have converged, much to everyone's relief.
That should be enough techieness for a while.