We can see the axon as a current-carrying wire. The magnetic field produced by a current-carrying wire is given by
[tex]B(r) = \frac{\mu_0 I}{2 \pi r} [/tex]
where
[tex]\mu_0 = 4 \pi \cdot 10^{-7} Tm/A[/tex] is the vacuum permeability
I is the current in the wire
r is the radial distance from the wire at which the field is calculated
The current in the axon is
[tex]I=0.040 \mu A=0.040 \cdot 10^{-6} A[/tex],
therefore the magnetic field strength at distance
[tex]r=1.2 mm=1.2 \cdot 10^{-3}m[/tex]
from the axon is
[tex]B= \frac{\mu_0 I}{2 \pi r}= \frac{(4 \pi \cdot 10^{-7} Tm/A)(0.040 \cdot 10^{-6} A)}{2 \pi (1.2 \cdot 10^{-3} m)}=6.67 \cdot 10^{-6} T = 6.67 \mu T [/tex]
