Sentence:  The <WSD>magnetic field</WSD> "B" bears the same relation to a hypothetical magnetic charge "m" as the electric field bears to the electric charge "q", so the development of the previous chapter can be taken over completely. The stress tensor for a <WSD>magnetic field</WSD> "B<sub>i</sub>" is identical in form to that for the electric field. The energy density is obviously "B"2/8π and the <WSD>magnetic field</WSD> is a physical reality just like the electric field. Electric and <WSD>magnetic fields</WSD> superpose linearly.... This is perhaps a convenient place to remark that a <WSD>magnetic field</WSD> exerts no force whatever on an electric charge. 
Meaning of the word marked with <WSD> tags:  A condition in the space around a magnet or electric current in which there is a detectable magnetic force and two magnetic poles are present.