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.
"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.
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.
"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.
@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.)
January 2016 Cartoon Caption ContestBob's punishment for missing his deadline was to be tied to his chair tantalizingly close to a disconnected cable, with one hand superglued to his desk and another to his chin, while the pages from his wall calendar were slowly torn away.122 comments