# Experimental Determination of Fundamental EM Constants

1. Oct 4, 2004

### Reality_Patrol

Can anyone provide a good reference (preferably on-line), or a decent description, of how the fundamental constants of electromagnetism are experimentally determined?

I'm talking about constants like the permitivity and permeability of free space. I'm also curious how the unit of charge (coulomb) is defined experimentally. Perhaps more how the "standard of measurement" for the coulomb is defined -not how the fundamental charge on an electron was determined (I've got that one!).

(Also, I've seen in other posts use inset math typesets, which I would like to do here but I don't know how! Can anyone provide some guidance for this as well?)

2. Oct 4, 2004

### ZapperZ

Staff Emeritus
For permitivity and permeability, you should look at something like CRC Handbook, available at most libraries. For the accepted standards of fundamental constants like "e", "h", etc., you need to look at the CODATA standards. Refer to P.J. Mohr and B.N. Taylor, Rev. Mod. Phys. v.72, p.351 (2000). "e" is defined from a superconductivity measurement while "h" is from a quantum hall effect measurement. The CODATA paper explains the experiment and how these values are obtained.

Zz.

3. Oct 4, 2004

### Integral

Staff Emeritus

4. Oct 4, 2004

### Gonzolo

Anyone know which of permittivity, permeability, and c are used as standards? Which 2 of the 3 values can be measured more precisely? Is the third standard from a third experiment or from their relation?

5. Oct 4, 2004

### Integral

Staff Emeritus
6. Oct 4, 2004

### pervect

Staff Emeritus
Try

http://physics.nist.gov/cgi-bin/cuu/Category?view=html&Universal.x=84&Universal.y=11

when you look under "electric constant" and "magnetic constant", you'll see that $$\epsilon_0$$ and $$\mu_0$$ are exact. This means they aren't experimentally determined nowadays, they have defined standard values.

This shouldn't come as a surprise, the speed of light has been used to define the meter for some time now, so c is constant, and the magnetic permeability has also been fixed for some time. For the basic fundamental defintions of the SI base units like distance and time see

http://physics.nist.gov/cuu/Units/current.html

For more on mangetic permability, you can try also

http://scienceworld.wolfram.com/physics/PermeabilityofFreeSpace.html

Last edited: Oct 4, 2004
7. Oct 5, 2004

### Reality_Patrol

Well, thanks to all for the many fine answers. The on-line references spell it all out clearly. Still, seeing the values of these fundamental quantities so cleverly defined, mostly through thought experiment, leaves one without a warm fuzzy. Ahh, maybe I'm old fashioned but a good experiment is still worth the effort.

I guess I was being naive in hoping there was still a standard in use for determining their values based on the use of capacitors & inductors.

Again, many thanks for the answers!

8. Oct 5, 2004

### pervect

Staff Emeritus
Probably what you want is the history of how these quantities used to be defined, back in the old days when the meter was defined by two marks on a bar of metal kept in Paris.

Unfortunately that's a bit harder to find out, though it is certainly possible to easily come up with some ad-hoc measurements for at least some of them.

For instance, one might start off with measuring the capacitance of two parallel plates to estimate the permittivity of free space, of course this measurement would be off because of "fringing" effects.

We really have only gained by moving away from the old meter standard, though - the new standards are much more accurate and repeatable, and don't have the problem of "drift" that physical standards do.