Dimmer socket.
Plugs into mains, then item to be controlled (e.g. incandescent bulb, etc) plugs into the front)
The knob then adjusts the brightness.
This might work with dimmable LED bulbs as well - but untested with those.
Collection from IP4 (Sidegate area)
Dimmer socket (IP4)
This offer has been gifted to someone and is no longer available.