Re: Where did the symbol B that stands for magnetic field come from?

I do not yet have a definitive answer, but am working on it.  I can say 
that according to The Dictionary of Scientific Biography, Voume IX, 
Charles Scribner's and Sons, New York (1974), page 206, that James Clerk 
Maxwell who was a pioneer in developing the theory of electromagnetism 
introduced the symbol B (and a related symbol H) in developing the 
equations describing his theory.  To quote the passage " . . . Maxwell 
distinguished two vectors, the magnetic induction and the magnetic force, 
to which he later attached the symbols B and H."

The question you are asking (which is an excellent one) in effect tries to 
understand why or how a scientist chooses or decides to choose one or 
another symbol to use in equations.

I will continue looking for the answer.

Here are some other references you might try to access:

Proceedings of the Cambridge Philosophical Society, Mathematical and 
Physical Sciences, volume 32 (1936) pages 695-750

E.T.Whittaker "A History of the Theories of Aether and Electricity," 2 
volumes (1951-1953)

Jed Z. Buchwald "From Maxwell to Microphysics: Aspects of Electromagnetic 
Theory in the Last Quarter of the Nineteenth Century."

{Admin note: In this case there might not be a good and satisfying reason for 'B' and 'G'. 
Electromagnetic theory involves lots of equations and lots of different quantities. It is not unlikely that 
Maxwell chose these two letters simply because they were the first two letters in the alphabet that were 
not already being used for other quantities in the equations he was working with.  John C}