
 --- MAGNETIC FIELD : define a non-uniform magnetic field 
     with the following behaviour:
        Bx = 0, 
        By = 0,  
        Bz = Bz(x) 
     where the function Bz(x) is:

               0                                               if x <= 0 
      Bz(x) = 
               increases linearly, to up to a maximum of       if x >  0  
               4 Tesla, with a slope of 1 Tesla / 10 cm.     

