Magnetic Field Strength, B

[shaun_04]

Why is the symbol "B"?
Lots of theories but I need a definite answer, so does anyone know?

[theprof]

B is the symbol used to represent magnetic flux density not magnetic field strength. They are not the same thing. If I remember correctly, magnetic field strength was allocated the symbol H.

Magnetic flux density is now measured in Teslas or Webers per square metre in the SI system or I'm old enough to remember that magical CGS unit, the Gauss.

[shaun_04]

True, but B is now the accepted symbol for M-F-S. One of the theories is that it was named after Biot who done all the flux density stuff before the symbols (B & H) were swapped around.

[theprof]

Hmm, I thought my memory might be playing me up over this, so I've just done a quick check through some science databases.

Newnes Dictionary of Electronics (2000), The Penguin Dictionary of Physics (2000), Hargrave�s Communications Dictionary (2001) and The Standard Handbook for Electrical Engineers by Fink and Beaty are all in agreement that B is the symbol for magnetic flux density. Furthermore they also all agree that H represents magnetic field strength.

I've also accessed a number of textbooks online in the Knovel Library. I'll quote from two. The first, "Handbook of Small Motors" by Yeadon and Yeadon states:

"Flux Density B and Coercivity H. Two important properties of permanent magnet materials are the flux density (also called the magnetic induction) B and the coercivity
(also somewhat ambiguously referred to as the magnetic field strength) H. These two quantities are related, exist at every point in the magnet and its surroundings, and in general vary from one position to another. They are vectors�that is to say, each has a scalar (i.e., a number) value attached to it and also a direction. In free space the two have the same direction at a given point and are related by a simple constant called the permeability
of free space �0, but within a magnet the relationship
is more complicated. In some materials the two do not even have the same direction. These two quantities
are fundamentally different, the flux density playing a similar role in magnetic circuits as current (per-unit area)
in electrical circuits, and the coercivity of magnetic circuits resembling the electrical voltage (per-unit length)."

(continued)

[theprof]

The second work, called "Encyclopaedia of Scientific Units, Weights and Measures" by Cardarelli states:

"The advantage - or, for some physicists, the disadvantage - of this system lay in the fact that it made a clear
distinction between magnetic field strength H and magnetic flux density B and similarly between electric field strength E and electric flux density D. This distinction results from the expression for vacuum permittivity and magnetic permeability which is not equal to unity as in the cgs system"

I've also accessed my university servers from home this evening to check for any swapping around of the symbols as you assert. I can find no reference to any swap having taking place or any published or proposed papers on the subject using EBSCO, various electrical engineering annals and other resources. In essence, I've drawn a blank.

I'd be grateful if you could tell me the year when you believe these symbols were reversed. Can you provide a definitive source or text for your claim that B represents MFS?

It is difficult to proceed with all enquiries of this nature when initial concepts are flawed.

[shaun_04]

Thanks for your interest. I too have looked at several texts which seem to support your knowledge, but the A-level curricular that I am studying takes Magnetic Field Strength as B, and Flux Density as H.

[theprof]

I had to check this out myself as its been some years since I studied physics to this extent.

Now that we've established that B is not MFS, I think you need to go back to your teacher/lecturer and raise the matter with him/her because it looks like there's something seriously amiss here.

Using the wrong symbols or formulae in an externally marked exam could cost you dearly as far as marks are concerned.