A current \( I \) is uniformly distributed across a long straight non-magnetic wire (\( \mu_r = 1 \)) of circular cross-section with radius \( a \). Two points \( P \) and \( Q \) are at distances \( \frac{a}{3} \) and \( 9a \), respectively, from the axis of the wire. The ratio of the magnetic fields at points \( P \) and \( Q \) is ................