Orbital contribution to a magnetic moment

It is convenient to express fj. in units of Bohr magnetons, jS, and so we write Combining the last two equations we find that 

Notice that, defined in this way, is a pure number, the Bohr magneton number, which refers to a single molecule. Although often met, it is not strictly correct to call it the effective magnetic moment, and it certainly is incorrect to express it in units of Bohr magnetons, although this is a usage met all too often. 

We now turn to the quantum mechanical approach to the phenomenon of paramagnetism and discuss first the orbital contribution to a magnetic moment. It is convenient at this point to make a few general statements which will be explained by the subsequent discussion. 

That is, the magnetic moment is an integer times where is the Bohr magneton. It transpires that P is also the fundamental quantity in the modern quantum mechanical treatment. 

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