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KilogramThe kilogram (symbol: kg) is the SI base unit of mass. A gram (grain in now obsolete Portugese spelling, in Latin granum) is defined as one thousandth of a kilogram. See 1 E 0 kg for comparisons with other masses.
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The kilogram is the only one of the SI units which is defined in relation to an artifact rather than to physical properties.
The kilogram was originally defined as the mass of one liter of pure water at a temperature of 4 degrees Celsius and standard atmospheric pressure. This definition was hard to realize accurately, partially because the density of water depends ever-so-slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependency in the definition of the kilogram.
To avoid these problems, the kilogram was redefined as precisely the mass of a particular standard mass created to approximate the original definition. Since 1889, the SI system defines the unit to be equal to the mass of the international prototype of the kilogram, which is made from an alloy of platinum and iridium of 39 mm height and diameter, and kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). Official copies of the prototype kilogram are made available as national prototypes, which are compared to the Paris prototype roughly every 10 years. The international prototype kilogram was made in the 1880s.
By definition, the error in the repeatability of the current definition is exactly zero; however, in the usual sense of the word, it can be regarded as of the order of 2 micrograms. This is found by comparing the official standard with its official copies, which are made of roughly the same materials and kept under the same conditions. There is no reason to believe that the official standard is any more or less stable than its official copies, thus giving a way to estimate its stability. This procedure is performed roughly once every forty years.
It seems, that the original kilogram has lost about 50 micrograms in the last 100 years. The reason is still unknown (reported in Der Spiegel in 26/2003. Therefore the search for alternatives is intesified:
There is an ongoing effort to introduce a definition by way of fundamental or atomic constants. The proposals being worked on are:
crossed the little plateau between the smouldering camp-fires
and slowly entered the dim charcoal-burner's track. Not a word was spoken
the sergeant in the rear, as he crossed the little breastwork
half turned and glanced at the dying fires, the low, newly made mounds
slowly, in a low voice to himself,
"well.html">Well, by go/god.html">God!"
passing through "the square". I had seen him once or twice on the street,
so that I was surprised to hear what he said about the war.html">war.
but on the other side -- a gentleman with the loyal-legion button in his coat,
of a charge.html">charge in some battle.html">battle or skirmish in which, he declared, his company,
told off to hold the horses; and that that day he had had the ill luck
in the charge -- did the finest work he ever saw, and really (so he claimed)
for I had been accustomed all my life to hear the war talked of;
But the speakers, although they generally boasted of their commands,
had been in the active force, and thus tacitly shared in the credit.
to any of the praise, declaring that he was safe behind the crest.html">crest of the hill.html">hill
for the rest of the command.
"It happened just as I have told you here," he said, in closing.
and sent Binford Terrell to the colonel to ask him to let him go over there
at a walk either, I tell you; it wasn't any promenade: they went clipping.
then they played the devil with 'em: but the boys were up there right in 'em
down the hill (oh, our boys could handle the tubes then as well as
and we formed a line of battle right there on that crest, and held it
and with my foolishness went over there. That evening, you know,
.
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