A circuit consists of an ideal battery of constant emf \(\varepsilon\), a resistor of resistance \(R\), an inductor of inductance \(L\), and an open switch connected in series. At time \(t = 0\), the switch is closed. At the instant when the current in the circuit reaches half of its maximum steady-state value, what fraction of the total energy delivered by the battery up to that time is stored in the magnetic field of the inductor?