A system is disclosed, comprising a base and at least a first moveable entity, the first moveable entity being moveable with respect to the base and positionable in at least a first position with respect to the base. The base comprises a first base electrode and a second base electrode, and the moveable entity comprises a first moveable entity electrode and a second moveable entity electrode. The electrodes are arranged such that when the moveable entity is in the first position the first base electrode and the first moveable entity electrode align to form a first capacitor and the second base electrode and second moveable entity electrode align to form a second capacitor. The first moveable entity further comprises a first resistor connecting the first moveable entity electrode to the second moveable entity electrode, and the base further comprises: signal supply means arranged to supply a time-varying electrical signal to the first base electrode; and signal detection means arranged to detect an electrical signal from the second base electrode.