throughout: a region of space
is: a vector field which gives rise to a magnetic force on moving charged particles at each point in the region, provided they are not travelling parallel to the magnetic field at the point in question. [P3.1]
is defined: at any point specified by a position vector 
as the vector quantity 
whose direction is identical to that in which the north pole of a vanishingly small compass needle, free to rotate in three dimensions, would point, and whose magnitude 
is obtained from the magnitude, 
of the magnetic force that acts - by virtue of the Lorentz force law - on a particle of charge 
as it moves through the point 
in a direction at right angles to the magnetic field with a speed 
So 
[P4.2]
may be more simply defined: as the vector field that determines the magnetic force 
on a particle of charge travelling with velocity 
at the point through the relationship
[P4.3]
has as its SI unit: the tesla (T), where 
[P4.2]
also may be denoted: 
since 
[P4.2]
Copyright 1997, The Open University