Abstract: An apparatus for transferring power and communication data across an annular gap comprises an inductively-coupled primary coil winding and secondary coil winding rotatable therein. Two signal oscillators are electrically coupled to a transistor for controlling current flow through the primary coil winding, thereby inducing a signal with a respective frequency in the secondary coil winding. An electronic switch operates so as to apply one of the two signals to the transistor for charging a capacitor coupled to the secondary coil winding, and operates so as to apply the other of the two signals to the transistor for discharging the capacitor. Actuation of control devices operate so as to modulate the impedance of the secondary coil winding. The inductive coupling of the two coil windings results in this modulation being mirrored by a modulation of the voltage developed across the primary coil winding.