Increasing the magnetic field around a wire

[EddieP]

I am interested in calculating the magnetic field around a wire. Using the formula found here http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcur.html#c3
I can calculate that B = (μ0*I)/(2*pi*r) Where I equal current in Amperes, and r equals distance from the wire in meters.
This gives me the magnetic field a certain distance from the current carrying wire, but what if there is a ferrous object near the wire? How can I use the above formula to calculate the field around a wire, if the wire is near a piece of iron with a relative permeability of say 100?

 

[EM_Guy]

$B = \frac{\mu I}{2 \pi r}$

$\mu = \mu_r \mu_0$