What is one volt?

If you charged a ball bearing with one coulomb of electrons, and suspended it midway between two conductive planes that are 1 meter apart and with a potential difference of 1 volt, the force on the ball bearing would be 1 newton.

Of course, that ball bearing would have to be charged to about a trillion volts. Maybe using one electron would be more practical.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

to me this is clear. What is your problem?

Of course one cannot build a real laboratory voltage reference from that. In olde times they used a Mercury thread of 1 mm square and 1 meter lenght to build 1 Ohm.

The Ampere definition is similar:: The basic unit of electric current, equal to one coulomb per second and equivalent to the current, flowing in two straight parallel wires of negligible cross section separated by a distance of one meter, that

length. The value of an ampere in the International System differs very slightly from that in the meter-kilogram-second-ampere system of units.

Equally unuseable to build a real laboratory device. The Ohm: It was first agreed that this theoretical ohm should be represented by the resistance offered to an unvarying current of electricity by a column of mercury one square millimeter in cross section and one hundred and six centimeters in length.

From there to "legal units" is a long way. Actually you buy a voltmeter and rely on the manufacturer.

w.

I'll probably get this wrong, But the volt is a derived unit. We measure resistance (from the quantized hall effect.) and time... from atomic transitions, and then define the charge of the electron (I think) and from these get current and the volt follows.

George H.

An electric field generates a force on a charge.

A coulomb is a defined number of charges.

A voltage is required to set up an electric field.

The longer a distance an electric field is set up over, the higher the voltage needed to create the field (this is why high-voltage parts have large spacing between conductors).

So they're saying that if you have a pair of infinite plates, one meter apart, with one volt between them, that the electric field in between the plates will have a strength of one newton per coulomb.

To actually DO the experiment you'd use some lesser charge (like an electron), some other arrangement of charged conductors (finite-sized plates spaced closer together), some other voltage (probably), and you'd do a bunch of math to work out the relationship.

--
www.wescottdesign.com

Most everything is a derived unit.

The volt is now defined in terms of frequency, through a superconducting Josephson junction.

Wiki says...

The kilogram is the only SI base unit with an SI prefix ("kilo", symbol "k") as part of its name. It is also the only SI unit that is still directly defined by an artifact rather than a fundamental physical property that can be reproduced in different laboratories. Three other base units (Cd, A, mol) and 17 derived units (N, Pa, J, W, C, V, F, O, S, Wb, T, H, kat, Gy, Sv, lm, lx) in the SI system are defined relative to the kilogram, so its stability is important. Only

8 other units do not require the kilogram in their definition:

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

They're working on that --

Inneresting stuff.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

f

Oh, but that is usable. It's basically the same as a two-coil d'Arsonval meter movement (with scale and geometry constant factors).

Of course. One uses an electron instead, and observes the trajectory in a cathode-ray tube with electrostatic deflection.

Yes, it's derived from newtons and meters and kilograms, but I can't figure out where the long digits come from. Wiki says the elementary charge, the charge of a proton (equivalently, the negative of the charge of an

be rounded off to exactly 1.6 x 10-18th for a standard value. Maybe a small change to the meter or kilogram would make the numbers work out with less digits? But I suppose that would screw up something else.

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All the units are tied to various other things, so if you went to change one thing then it'd change many, if not all, of the others.

There's a desire among physicists and metrologists to base everything on Planck units

-- that way we won't have units that are derived from, ultimately, anthropogenic quantities.

Unless, of course, it turns out that the Planck units aren't as universal as we think, and are just a product of our understanding at this time...

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com