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.
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!
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".
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.
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.
@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.)
"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.
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.
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.
"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.
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.