17.  B-field:  the magnetic induction B.  This is defined, analogously to E, by the force it exerts.  The force on a point charge moving in a magnetic field is at right angles to the velocity vector of the charge and also at right angles to the magnetic field.  Actually, the H-field was originally named the magnetic field and the B-field was named the magnetic induction.  This is an annoyance because it is B which produces or involves the force, and hence should be named the field.  Other writers have noted this discrepancy, and Sommerfeld called H the magnetic excitation - which correctly describes its role - rather than the magnetic field.  In Kaluza-Klein unified field theory, all electromagnetic is in the 5th dimension, which is wrapped around each and every point in our normal 3-dimensional space.