In physics, there are two different attributes of a magnetic field:
- The "magnetic field strength". Usually given the symbol H. Measured in amps per metre (A/m).
- The "magnetic flux density". Usually given the symbol B. Measured in teslas (or microteslas or gauss).
In electromagnetism theory, it is absolutely clear that these are different quantities and you need to be precise about which you are measuring.
But for many practical purposes, the two are related by a simple numerical relationship:
where μ0 is a constant - it's called the permeability of free space and has a value (in SI) 4π.10-7
To be more precise, we can write out the full relationship:
That second constant we've introduced, μr, is called the relative permeability. For non-magnetic materials - for air, most building materials, the human body, etc - it has a value of very nearly 1. Which is why it's usually sufficiently correct just to say B=μ0H. Only near iron or steel or other magnetic materials do we have to go a bit further into the physics, because that's when μr starts taking different values.
So, most of the time, it really doesn't matter whether too much whether we're measuring B, the magnetic flux density, in microteslas, or H, the magnetic field strength, in amps per metre. You can simply convert from one to the other, as we do in our table of units.
Sometimes, people will be very precise and talk about either the magnetic flux density or the magnetic field strength. But actually "magnetic field" is perfectly valid as a description of either - both the magnetic flux density and the magnetic field strength are examples of a magnetic field. And just using "magnetic field" is a lot simpler ... so that's what we do most of the time on this site.
If you think we're labouring this point a bit, it's because sometimes people have seen us use "magnetic field" and assumed we'd really meant "magnetic field strength", then realised that we were actually talking about the "magnetic flux density", so criticised us for getting the two quantities confused. But that's a bit of false pedantry - what we do is perfectly correct from the university physicist's point of view as well as from the point of view of trying to make our communications as simple as possible.
Confused? The bottom line is: most of the time, when you see "magnetic field", that's all you have to worry about.