  We determine the $u$ and $d$ quark contributions to the proton magnetic form
factor at finite momentum transfer by applying chiral corrections to quenched
lattice data. Heavy baryon chiral perturbation theory is applied at next to
leading order in the quenched, and full QCD cases for the valence sector using
finite range regularization. Under the assumption of charge symmetry these
values can be combined with the experimental values of the proton and neutron
magnetic form factors to deduce a relatively accurate value for the strange
magnetic form factor at $Q^2=0.23$ GeV$^2$, namely $G_M^s=-0.034 \pm 0.021$
$\mu_N$.
