  A two dimensional disordered system of non-interacting fermions in a
homogeneous magnetic field is investigated numerically. By introducing a new
magnetic gauge, we explore the renormalization group (RG) flow of the
longitudinal and Hall conductances with higher precision than previously
studied, and find that the flow is consistent with the predictions of Pruisken
and Khmelnitskii. The extracted critical exponents agree with the results
obtained by using transfer matrix methods. The necessity of a second parameter
is also reflected in the level curvature distribution. Near the critical point
the distribution slightly differs from the prediciton of random matrix theory,
in agreement with previous works. Close to the quantum Hall fixed points the
distribution is lognormal since here states are strongly localized.
