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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

 

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!

Measurement.Blues
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Re: How Odd...
Measurement.Blues   4/11/2014 4:46:24 PM
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Unless your VOM has nanoamp resolution, you won't see the difference.

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.

Sanjib.A
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Re: Ampere's long arm
Sanjib.A   4/12/2014 11:04:53 AM
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@Sheepdoll: Wow! That is a very interesting!! Do you have the title of the book and author's name on it? Could you please share those details if you have?

@MeasurementBlues: Thank you for recommending the book. That would definitely be my next one on the list! 

Sheepdoll
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Re: Ampere's long arm
Sheepdoll   4/12/2014 1:35:45 PM
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@Sanjib.A:  Do you have the title of the book and author's name on it? Could you please share those details if you have?

The text books are by James Ferguson (1710-1776) He was a prolific author and lecture, who must have had large print runs.  His lectures were aimed at the general public. Somewhat obscure now, he was  known as "The wheelwright of the heavens" His influence on people like William Hershel and Benjamin Franklin had great effect on popular science.  A Carl Sagan of his era.

The books Select Mechanical Exercises, Ferguson's Lectures, On electricity are out of print and can be downloaded from google e-books. His autobography is short to the point and well worth a read.

One good recent biography on Franklin that covers the electrical experiments is called Draw the lightning down by Michael Brian Schiffer.

Wheelwright of the Heavens is by Millburn and King.

In the near future I will be joining the  bloggers here at EETimes.  (What happens when you have a beer in the bar with Max and Karen.)

 

 

David Ashton
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Re: Ampere's long arm
David Ashton   4/12/2014 8:11:47 PM
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@Sheepdoll.... 

> In the near future I will be joining the  bloggers here at EETimes. 

Glad to hear it..I hope you will be telling us more about pipe organs, having whetted our appetite with your comments in another column.....

> (What happens when you have a beer in the bar with Max and Karen.)

You're lucky.  They got me in without the beer..... :-(

 

kfield
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Re: Ampere's long arm
kfield   6/30/2014 8:27:04 PM
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@David Ashton "They got me in without the beer."

 

David, you don't dirve a very hard bargain!!! 

David Ashton
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Re: Ampere's long arm
David Ashton   6/30/2014 9:17:12 PM
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@Kfield...."David, you don't dirve a very hard bargain!!! "

It must be the allure of seeing my name in lights, even if they are only pixels on a screen.... :-)

Then again, it depends who bought the beer.....

Sanjib.A
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Re: Ampere's long arm
Sanjib.A   4/13/2014 1:53:10 AM
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"In the near future I will be joining the  bloggers here at EETimes.  (What happens when you have a beer in the bar with Max and Karen.)"

I am glad to hear that!! Surely you enjoyed the beer with Max & Karen :)

Thank you for sharing the details about James Ferguson and his books. I was trying to find "Select Mechanical Exercises, Ferguson's Lectures, On electricity" in Google books, I am getting a catalogue mentioning about this book among many others, but the search result is showing mostly the other one "Ferguson's Lectures on Select Subjects in Mechanics, Hydrostatics...". Anyways I will try to search it on Google.

Thanks!!

Measurement.Blues
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Re: Ampere's long arm
Measurement.Blues   4/15/2014 8:48:59 AM
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@Sheepdoll, what topics will you be blogging about? You are welcome to contribute to Test & Measurement.

"What happens when you have a beer in the bar with Max and Karen."

I never, ever do that :-)

MeasurementBlues
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Re: Ampere's long arm
MeasurementBlues   7/8/2014 4:37:49 PM
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"What happens when you have a beer in the bar with Max and Karen."

I don't know. I didn't live to tell my story.

pseudoid
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The more accurate "Second"
pseudoid   4/11/2014 6:59:10 PM
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Our college days have taught us that there is a difference between 'accurate' and 'precise'.

OffTopic#2 [But not way off since the following relates to both one of the 7 units of measure and NIST]:

The US Department of Commerce's National Institute of Standards and Technology (NIST) [has] launched the new atomic clock, called NIST-F2, to serve as a new US civilian time and frequency standard, along with the current NIST-F1 standard. NIST-F2 would neither gain nor lose one second in about 300 million years, making it about three times as accurate as NIST-F1, which has served as the standard since 1999, NIST said.  NIST-F2 is now the world's most accurate time standard, NIST said in a statement. For now, NIST plans to simultaneously operate both NIST-F1 and NIST-F2.

 "If we've learned anything in the last 60 years of building atomic clocks, we've learned that every time we build a better clock, somebody comes up with a use for it that you couldn't have foreseen," said NIST physicist Steven Jefferts, lead designer of NIST-F2.

Both clocks use a "fountain" of cesium atoms to determine the exact length of a second.  Both NIST-F1 and NIST-F2 measure the frequency of a particular transition in the cesium atom - which is 9,192,631,770 vibrations per second, and is used to define the second, the international (SI) unit of time.

The key operational difference is that F1 operates near room temperature (about 27 degrees Celsius) whereas the atoms in F2 are shielded within a much colder environment (at minus 193 degrees Celsius).  This cooling dramatically lowers the background radiation and thus reduces some of the very small measurement errors that must be corrected in NIST-F1.   Pasted from <http://www.business-standard.com/article/pti-stories/new-super-accurate-atomic-clock-developed-114040400488_1.html>

_hm
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Does this effect Volt and Ohm
_hm   4/12/2014 9:09:33 AM
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Since Amp is redefined, will it effect volt and ohm too?

 

Bert22306
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Detour over voltage and resistance
Bert22306   4/13/2014 6:36:11 PM
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I immediately got stuck on this comment in the article:

Though it is a very basic dimension, and, like volt and ohm, it is one of the central units of electricity, it hitherto was not possible to measure it directly. Instead, it was necessary to take a detour over voltage and resistance to measure the current.

While I won't argue with the main premise, that currect hasn't been able to be measured directly, like charge say, do you really need voltage and resistance to derive current? A galvanometer measures current based only of knowledge of the magnetic field used.

In fact, it was the galvanometer that permitted ohm's law to be defined, because it showed independently that voltage, current, and resistance were related in linear fashion. Had current not been able to be measure independent of voltage and resistance, ohm's law could not have been proven to be correct. The existence of current would have had to be postulated instead.

Measurement.Blues
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Re: Detour over voltage and resistance
Measurement.Blues   4/15/2014 8:53:45 AM
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The Ampere definition:

"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7newton per metre of length."

Yes, galvanometers can measure current from magnetic fields, but their uncertainty isn't nearly as good as other methods.

Measurement.Blues
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Re: Detour over voltage and resistance
Measurement.Blues   4/15/2014 8:58:22 AM
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Instead, it was necessary to take a detour over voltage and resistance to measure the current.

Here's why.

http://www.bipm.org/en/si/si_brochure/appendix2/electrical.html

It has a lot to do with the Josephson Junction Array, the world's most accurate voltage source. NIST has one, as does Fluke, Keithley, and Agilent. JJA's are based on physical constants and are thus used to realize the Ampere.

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