We have already mentionned the distorsion phenomenon in the core, due
to the non-linear induction curve B=f(H). The percent of harmonic
distorsion generated by a transformer of equivalent resistance :
and of primary inductance 
at
a frequency f is :
where 
is the amount of harmonic distorsion on the
output current in case of a zero impedance source. This value can be
found on the data-sheets of the core.