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# The Ampere, Redefined

4/11/2014 00:01 AM EDT
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Re: Ampere's long arm
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.

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Re: Ampere's long arm
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.....

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Re: Ampere's long arm
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!!!

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Re: Detour over voltage and resistance
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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Re: Detour over voltage and resistance
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.

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Re: Ampere's long arm
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 :-)

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Detour over voltage and resistance
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.

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Re: Ampere's long arm
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!!

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Re: Ampere's long arm
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..... :-(

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Re: Ampere's long arm
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.)

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