# Units of H

1. Nov 22, 2006

### ultimateguy

What are the units of the electric field strength H? I know that H = NI/L for a solenoid and I'm wondering if there is any condensed unit or if it is just ampere turns per meter.

2. Nov 22, 2006

### Hootenanny

Staff Emeritus
$\mathbbs{H}$ is the symbol used for magnetic field strength and has SI units of teslas.

3. Nov 22, 2006

### marcusl

The magnetic field $\mathbbs{H}$ has units of amp/meter in the SI system, with no special name attached. In the older cgs units it was called an Oersted (abbreviated Oe). The mention of teslas in the previous post refers to magnetic induction $\mathbbs{B}$. $\mathbbs{B}$ is now commonly called magnetic field, leading to confusion.

4. Nov 22, 2006

### ultimateguy

Thanks for the info. I was wondering because I'm plotting a magnetic hysteresis curve and have B vs. H and I didn't think they both had the same units.

5. Nov 25, 2006

### Meir Achuz

That is one of the virtues of the "old" gaussian system.
There B and H can have the same units so the hysteresis curve has a simple meaning. The gaussian unit for H is called "Oersted" for historical reasons, but it really is the same as the unit "gauss".

6. Nov 25, 2006

### ObsessiveMathsFreak

More like a vice! I always wondered why people seemed so flippant about interchanging the magnetic flux and field. More often than not, you'll find B being referred to as the field! I guess this was the reason.

7. Nov 27, 2006

### marcusl

Agreed, this is a nice feature of Gaussian units.

You mean magnetic induction and field (flux is yet another quantity). Yes, many writers call B the magnetic field without explanation or comment. Mel Schwartz, in "Principles of Electrodynamics" (1972), is one of the few who are up front in addressing this:
At this point we interject a small bit of philosophy. It is customary to call B the magnetic induction and H the magnetic field strength. We reject this custom inasmuch as B is the truly fundamental field and H is a subsidiary artifact. We shall call B the magnetic field and leave the reader to deal with H as he pleases.

8. Nov 27, 2006

### Meir Achuz

Mel deserves his Nobel prize for that sentence alone.

9. Nov 27, 2006

### marcusl

Edit: For that and more. I was friends with a couple of Mel's grad students working on the pi-mu atom, and had the chance to speak with him a few times. He was not only sharp, but was very nice as well. His accent (Bronx?) and mannerisms reminded me a little of Feynman. I was thrilled when I heard he had won the prize.

Last edited: Nov 27, 2006