a = $\frac{\mu B_J}{IJ}$
From the experiment, we can calculate this. This a is namely a constant for as far as I know. Both atoms have the same electron cloud, meaning J stays the same and since we use the same setup B_J is also the same.
We get the equation:
$\frac{\mu_1}{I_1} = \frac{\mu_2}{I_2}$ which is easely solvable to $\mu_2$