A long straight wire of radius a carries a steady current \(I\). The current is uniformly distributed over its cross-section. The ratio of the magnetic fields \(B\) and \(B'\) at radial distances \(\frac{a}{2}\) and \(2a\) respectively, from the axis of the wire, is:
1. \(\frac{1}{2}\)
2. \(1\)
3. \(4\)
4. \(\frac{1}{4}\)