What is the Basic Standard Of The Metric System?

[wacojoe]

My wife asked me, "Why do we have a Metric System, which is so confusing? I don't know what they are talking about when they use it in recipes."

I tried to explain that the problem is us Americans and not the system, which is all based on rational relationships, while Emperial measurement is a hodge-podge of who-knows-what. I tried to remember what the basic standard was, but could not other than the meter was a specific of the measurement of the speed of light somehow, which never changes, thus is a reproducable standard. Everything else in the metric system is derived from that. Looked it up and found the history is quite interesting. Thought I would share in case your wife asks too —

The origins of the meter go back to at least the 18th century. At that time, there were two competing approaches to the definition of a standard unit of length. Some suggested defining the meter as the length of a pendulum having a half-period of one second; others suggested defining the meter as one ten-millionth of the length of the earth's meridian along a quadrant (one fourth the circumference of the earth). In 1791, soon after the French Revolution, the French Academy of Sciences chose the meridian definition over the pendulum definition because the force of gravity varies slightly over the surface of the earth, affecting the period of the pendulum.

Thus, the meter was intended to equal 10-7 or one ten-millionth of the length of the meridian through Paris from pole to the equator. However, the first prototype was short by 0.2 millimeters because researchers miscalculated the flattening of the earth due to its rotation. Still this length became the standard. (The engraving at the right shows the casting of the platinum-iridium alloy called the "1874 Alloy.") In 1889, a new international prototype was made of an alloy of platinum with 10 percent iridium, to within 0.0001, that was to be measured at the melting point of ice. In 1927, the meter was more precisely defined as the distance, at 0°, between the axes of the two central lines marked on the bar of platinum-iridium kept at the BIPM, and declared Prototype of the meter by the 1st CGPM, this bar being subject to standard atmospheric pressure and supported on two cylinders of at least one centimeter diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other.

The 1889 definition of the meter, based upon the artifact international prototype of platinum-iridium, was replaced by the CGPM in 1960 using a definition based upon a wavelength of krypton-86 radiation. This definition was adopted in order to reduce the uncertainty with which the meter may be realized. In turn, to further reduce the uncertainty, in 1983 the CGPM replaced this latter definition by the following definition:

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

Note that the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m·s-1. The original international prototype of the meter, which was sanctioned by the 1st CGPM in 1889, is still kept at the BIPM under the conditions specified in 1889.
http://physics.nist.gov/cuu/Units/meter.html

Incidentally, I suffer from the same malady of ignorance in everyday life with measurements as does my wife. Growing up here, the metric system just does not compute without laborious translation. When someone tells me it is 15 degrees Celcius outside, I have no idea what to wear.

[Dave Grubb]

Interesting

I live in both worlds---and think in both worlds. I have compartmentalized areas of my life to be either Imperial or Metric---some days I can really confuse myself

[Phillbo]

When someone tells me it is 15 degrees Celcius outside, I have no idea what to wear.

Isn't 0* Celcuis freezing? If so then it would be 47* - dress warm

[FredK]

....... When someone tells me it is 15 degrees Celcius outside, I have no idea what to wear.

I moved to AZ where it's T shirts and shorts year around. Not sure how that helps with the metric system though.

[TriGuy]

Isn't 0* Celcuis freezing? If so then it would be 47* - dress warm

,try 59°F

[Truckman]

Winter came on a Thursday afternoon here last year...It was gone by lunch the next Saturday...I remember there being dust on my jacket collar when I got it out of the closet......Ben

[wacojoe]

Scientists are about to change what a kilogram is. That’s massive.

https://www.washingtonpost.com/news/...=.5710393bcb32

[Dave Grubb]

Frankly my good man---I don't give a damn----I'm keeping mine at 2.2 pounds---my slide rule does not allow me to work at 34 decimal places of accuracy

All kidding aside---I'm so anal I find the very interesting

[Sandman]

Because of habit.. prices in most Canadian groceries will officially list an item by metric units .. Price per kilogram or liter.. and then as a side note price per pound or gallon.. The baby boomers grew up before metric..

At some Canadian delis if you order a pound of something you will receive half of a kilo (500 grams.. 1.1 lb)

[Billy_Rightwing]

Poke here.

[Bernhardt]

Silly me, and here I thought a kg was based on 1000cc of pure water at STP(standard temperature and pressure). Not by it's weight, but by the mass that fits into that volume.

[TriGuy]

I have always preferred using the metric system. It is so much simpler to use than the US system when inter-converting things like weight per volume etc.

[wacojoe]

Silly me, and here I thought a kg was based on 1000cc of pure water at STP(standard temperature and pressure). Not by it's weight, but by the mass that fits into that volume.

Silly me, and here I thought a kg was based on 1000cc of pure water at STP(standard temperature and pressure). Not by it's weight, but by the mass that fits into that volume.

I always was taught the metric system devolved from one basic immutable standard too, and that was the recognized length of a meter as discussed in the original post. But then the question is, "What is a standard meter?" From that comes 1/100 of the meter — a centimeter, then a cubic centimeter (from which all volumes are derived), then the mass (weight) of pure water at a standard temperature contained in a cc from which all weights & mass are measured. Quite elegant and rational, but the primary standard of a meter must be maintained, agreed and measured.

That is the reason for this thread, and why I find the subject interesting. The newest article in #7 above does not say it, but I am guessing that the 1889 platinum and iridium cylinder maintained as the standard kilogram is a stand-in for the theoretical pure water measure of that mass because it is more stable to maintain, but it is not explained in the article. Plank's Constant blew right past me though.

[Dave Grubb]

Because of habit.. prices in most Canadian groceries will officially list an item by metric units .. Price per kilogram or liter.. and then as a side note price per pound or gallon.. The baby boomers grew up before metric..

At some Canadian delis if you order a pound of something you will receive half of a kilo (500 grams.. 1.1 lb)

Canada is officially bimensuric, meaning there are aspects of both Imperial and SI (metric) systems.

They are not alone in that. For example, don't go into a British Pub and order a half liter of beer.

I go both ways In somethings I think in metric and have to mentally convert to Imperial when speaking to someone rooted (and stuck) in Imperial units. In somethings I still think in Imperial units and have to mentally convert to SI as needed.

My personal preference is to be completely metric, it makes far more sense.

[Bernhardt]

I'm sure the block is a stand in. What they're looking for with plank's constant is something that is even less variable than the pure water block in case the conditions are off a little bit when measuring. They did the same with the second by basing it on being defined by the number radiation periods of cesium 133.

The numbers won't be nice and round in the new definitions since the definitions are being "reverse" engineered to meet what the old definitions, which were pulled out of thin air, say things are. They're not trying to change what a second, meter, or kg are - just the basis of the legal definition.