# Speed of a propagating electromagnetic field?

1. Apr 12, 2012

### Exiguus

Hi, I'm wondering if there has been any actual experiments done to measure the speed at which magnetic fields propagate?

2. Apr 12, 2012

### sophiecentaur

When you say "propagate" you must be referring to the effect of switching on or changing a magnetic field. You must 'mark it' in some way if you want to find its speed. Once you start to change a magnetic field, an Electric field is formed and then you have an Electromagnetic Wave. The speed of that has been measured with increasing accuracy for several hundred years.

3. Apr 12, 2012

### Exiguus

Yes, when turning on or altering a magnetic field created by a current. It seems to me when I read that it is more or less assumed it travels with the speed of light, but I cannot find any reference to accurately measuring that using 2 synchronised atomic clocks for example.

4. Apr 12, 2012

### sophiecentaur

But it's just a propagating em wave, isn't it? As I said, once you change the H field, an E field results. You can't have one without the other.
You don't need atomic clocks to measure the phase difference in the magnetic field which two search coils would pick up at different distances from a loop with AC passing through it. All you need is a reasonably good measure of the frequency of your AC and look at the phase with a suitable 'scope or timing mechanism. This sort of experiment has been done implicitly or explicitly on many occasions and in many different guises.
It always boils down to measuring the speed of em waves, which always proves to be independent of their frequency. You can either use an electric pickup ( a dipole) or a magnetic pickup (a loop). You get the same answer. I think you can't find references to what you say because it's sort of taken for granted, bearing in mind the theory and past evidence.

5. Apr 12, 2012

### Bob S

It is easy to measure the velocity of a TEM (transverse electric magnetic) wave in an air-filled coaxial cable. It is $v=\left[ \frac{1}{\epsilon_o \mu_o} \right]^{1/2}$

6. Apr 12, 2012

### sophiecentaur

Once you have accepted that the magnetic field is part of an EM wave, that is.