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The Ampere, Redefined

4/11/2014 00:01 AM EDT
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MeasurementBlues
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Ampere's long arm
MeasurementBlues   4/11/2014 10:05:47 AM
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The Ampere is one of just seven basic units of measurement. Others are the kilogram, meter, Kelvin, second, Mole, Candela. But, the ampere is used to derive many other units of measurement such as the Farad, Ohm, Henry and yes, even the volt.

 

MeasurementBlues
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Re: Ampere's long arm
MeasurementBlues   4/11/2014 10:07:07 AM
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Here's a link to NIST about the SI units.

http://physics.nist.gov/cuu/Units/units.html

 

Sheepdoll
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Re: Ampere's long arm
Sheepdoll   4/11/2014 1:58:37 PM
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I have a number of old science text books. Some dating as far back as the 18th century. Most of this relates to static electricity.  What I found interesting was how the quantities were measured.  Franklin's spark gap was used.  The length of the spark was what we call voltge.  The diameter of the spark was the current.  To measure the current a card was placed into the gap.  The spark would then burn a hole through the card.

 

MeasurementBlues
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Re: Ampere's long arm
MeasurementBlues   4/11/2014 2:29:19 PM
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@Sheepdol,

Franklin flew his kite on a Philadelphia night;
He saw that lightning was electricity.
Coulomb could tell that like charges repel
By the inverse square of their distance.
Orsted saw magnetic fields make a compass needle yield
When current passed through a nearby wire.


From my song, Electrical Heroes


MeasurementBlues
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Re: Ampere's long arm
MeasurementBlues   4/11/2014 2:30:39 PM
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@Sheepdoll,

I recommend reading The Story of Electrical and Magnetic Measurements by Joseph F. Keithley.

pseudoid
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How Odd...
pseudoid   4/11/2014 3:50:48 PM
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OffTopic >> Thank you for the link but I find it quite odd that NIST (old NBS) calls out the unit of mass as being the Kilogram (kg) rather than a gram (g).

Wikipedia provides the following historical perspective:

In 1921 the Convention of the Metre was revised and the mandate of the CGPM (Conférence générale des poids et mesures) was extended to provide standards for all units of measure, not just mass and length. In the ensuing years the CGPM took on responsibility for providing standards of electric current (1946), luminosity (1946), temperature (1948), time (1956) and molar mass (1971).  Those are not too far away in the rear view mirror!

MeasurementBlues
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Re: How Odd...
MeasurementBlues   4/11/2014 4:03:48 PM
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I would trust NIST over Wikipedia any day.

MeasurementBlues
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Re: How Odd...
MeasurementBlues   4/11/2014 4:07:37 PM
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From the University of North Carolina, Again I would go with this over Wikipedia.

http://www.unc.edu/~rowlett/units/sifundam.html

 

MeasurementBlues
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Re: How Odd...
MeasurementBlues   4/11/2014 4:08:30 PM
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And from the source itself

http://www.bipm.org/en/si/history-si/name_kg.html

The reason why "kilogram" is the name of a base unit of the SI is an artefact of history.

Louis XVI charged a group of savants to develop a new system of measurement. Their work laid the foundation for the "decimal metric system", which has evolved into the modern SI. The original idea of the king's commission (which included such notables as Lavoisier) was to create a unit of mass that would be known as the "grave". By definition it would be the mass of a litre of water at the ice point (i.e. essentially1 kg). The definition was to be embodied in an artefact mass standard.

After the Revolution, the new Republican government took over the idea of the metric system but made some significant changes. For example, since many mass measurements of the time concerned masses much smaller than the kilogram, they decided that the unit of mass should be the "gramme". However, since a one-gramme standard would have been difficult to use as well as to establish, they also decided that the new definition should be embodied in a one-kilogramme artefact. This artefact became known as the "kilogram of the archives". By 1875 the unit of mass had been redefined as the "kilogram", embodied by a new artefact whose mass was essentially the same as the kilogram of the archives.

The decision of the Republican government may have been politically motivated; after all, these were the same people who condemned Lavoisier to the guillotine. In any case, we are now stuck with the infelicity of a base unit whose name has a "prefix".

pseudoid
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Re: How Odd...
pseudoid   4/11/2014 4:41:28 PM
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Oh forget it! 

I am just gonna stick with the old Triplett VOM to measure Amps.  It can't be that inaccurate of a reading!  I know all about parallax and how to compensate for it! 

How many digits of accuracy would you like with your french fries, Sir?  LOL!

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