I seem to recall that the ampere was (or perhaps is going to be) replaced by the volt because we can produce a voltage with an extremely high degree of accuracy and precision using Josephson junctions in the presence of a specific microwave radiation, which would give it a resolution comparable to the definition we already have for length and time.

The idea is to base the ampere on quantum metrology instead of a force measurement. Volts using "Josephson junctions", Ohms using the "Quantum-Hall" effect and current with a single-electron current standard (charge on a electron).

https://www.euramet.org/fileadmin/d...011/SI_JRPs/SIB07_Publishable_JRP_Summary.pdf

 

It would seem like you can't do all three otherwise you have an overdefined system.

If we define the volt, then because a volt is 1 kg-m/s^2-C and since, presumably, the kg, m, and s are elsewhere defined that as soon as you define the volt you have defined the coulomb and the ampere as derived units. Similar situation if you define one of the other others. Right now we define the ampere and so the coulomb and volt are derived units.

You can use quantum devices to build references for the derived units, which is what we do now (or at least were doing when I was at NIST two decades ago) for the volt. But it is only a very, very good reference and not a standard or a definition.

My guess is that this project is essentially aimed at saying, "Hey, we are at a point where we could choose any one of these three units to be a quantum-based standard, so let's examine all three so that we can make the best choice of which single one to adopt."

 

http://www.french-metrology.com/en/si/units-measurement.asp

seems like the ampere is currently defined in terms of Newton-meters




Interesting page, short and to the point.

 

http://www.french-metrology.com/en/si/units-measurement.asp

seems like the ampere is currently defined in terms of Newton-meters




Interesting page, short and to the point.

It's defined in terms of the newton and the meter, not the newton-meter. The newton-meter is a unit of energy (or torque). In a current balance, it's newtons/meter that is important (the ratio and not the product).

 

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


I'm ready to learn something new. A force of 'X' newtons per meter is not referred to as 'X newton meters'? I've used that contraction before thinking it was correct.

So I should use Joule as the unit of force and not newton meter. And newton PER meter would still be a measure of a force wouldn't it?

All new names for everything and all new formula to memorize to go along with the new names.

 

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


I'm ready to learn something new. A force of 'X' newtons per meter is not referred to as 'X newton meters'? I've used that contraction before thinking it was correct.

Nope. Is the mileage of a car measured in mile-gallons? No, it's measured in miles-per-gallon. You speed is in miles-per-hour, not mile-hours.

But the light-second is a measure of distance because it is a speed (the 'light', which is the speed of light) multiplied by a time (the second). Hence

1 light-second ≈ (186,000 miles/sec)(1 sec) = 186,000 miles

So I should use Joule as the unit of force and not newton meter. And newton PER meter would still be a measure of a force wouldn't it?

All new names for everything and all new formula to memorize to go along with the new names.

The newton is a unit of force. The newton-meter (meaning the product of one newton and one meter) is a unit of energy (or torque), not force. The newton-per-meter doesn't have a specific name. It is NOT a force, it is a force per length. These are not the same.

 

I'll try to remember that...


but I ain't making any promises

 

The units are based on the meter, kilogram and second. ( MKS units )

The meter was originally defined as 1.0 x 10e-7 x the distance from the north pole to the equator, measure along a path that passes through Paris.

The kilogram was defined as 1000 grams, one gram being the mass in one cubic centimeter of distilled water at 4 degrees Celsius. ( = greatest density at that temp )

The second already had a definition rooted in astronomy.

These units together defined the Newton as a unit of physical force and the Joule as a unit of "work". ( = energy = force x distance )

The Watt expresses a rate of energy flow equal to one Joule per second.

The Ampere was later defined in terms of the physical force between two parallel conductors carrying a current, as one of the previous posts has indicated.

From the Ampere and the Watt, the Volt can be derived and defined, as well as the Ohm and the Coulomb.

The Farad is defined as the capacitance that draws a one Ampere charging current if the applied voltage is increasing at a rate of one Volt per second.

The Henry is defined as the inductance that will sustain a one Volt drop across its terminals if the current flowing through it is increasing at a rate of one Ampere per second.

...PERIOD...

 

I have some recollection that an AMPERE was long ago defined as that amount of current which in a particular solution of electrolyte would deposit X amount of material on an electrode in a certain time. Electro plating.
I dont recall the exact numbers but I am sure they are around somewhere. Will publish if I find them.

 

I have some recollection that an AMPERE was long ago defined as that amount of current which in a particular solution of electrolyte would deposit X amount of material on an electrode in a certain time. Electro plating.
I dont recall the exact numbers but I am sure they are around somewhere. Will publish if I find them.

What you are thinking of is a realization of the ampere, not it's definition. The definition has always been directly tied to the force per unit length between two current carrying wires located a specific distance apart. Originally it was defined in the MKS system and then later in the SI system, but essentially the same definition in both.

It was "realized" as the current that would plate 1.118000 mg of silver per second from a silver nitrate solution (though this turned out to have an error of about 0.01%, which is significant in standards work).

Today is it usually realized using Ohm's Law since we can use Josephson junctions to produce very accurate voltages and use the quantum Hall effect to create very accurate resistances. Errors today are on the 100 ppb scale.

 

"Practical International Unit of Current is the AMPERE and is that unvarying current, which when passed through a solution of silver nitrate in water, will deposit silver at the rate of 1.118 milligrammes per second. It should be noted that definitions of electrical units are determined by international agreement. At the time of writing the definition of the AMPERE is under review to presenting it in a more practical terms"

"Practical International Unit of Resistance is the OHM and is the resistance offered to an unvarying electric by a column of mercury 106.3 centimetres long of constant cross-sectional area, having a mass of 14.4521 grams and at a temperature of 0 deg Centigrade"

"Practical International Unit of Electromotive Force is the VOLT and is that electric pressure which when applied steadily to a conductor having a resistance of 1 OHM will produce a current of 1 AMPERE."

Source:Sydney Technical College Electrical Trades Course , Applied Electricity 1 (Circa 1960s)

 

WBahn is right... profbuxton is offering an alternative way of expressing the Ampere, NOT the actual ( internationally recognized MKS ) "definition" of the Ampere, nor is it the original definition of the Ampere. The thread was begun with a question asking where the units of measure came from... their historical origin and definitions... profbuxton is answering a question that was never asked.

One horsepower equals 746 Watts also, but we don't define one Watt as 1.34 x 10e-3 horsepower, do we ?

 

The "international ampere" was an early realization of the ampere, defined as the current that would deposit 0.001118000 grams of silver per second from a silver nitrate solution.[12] Later, more accurate measurements revealed that this current is 0.99985 A. Source: Wikipedia
Note the word "defined". I don't know by whom or when but it seems pretty clear to me that its a definition.
While we can now argue about "forces between wires" etc I would see this as a real starting point and easily repeatable measure of an AMPERE which does not need any esoteric units of magnetic force(which I suspect would have been difficult to measure with the technology in the past.
I understood the Op to be asking the "derivation" of units, not their current definition.
As an aside, do you know how a "horsepower" was derived?