Solid state regulator, negative earth, 6v or 12v to replace Lucas style voltage regulator (control box)for Lucas E3 type dynamo. Approximately 5cm long, 4.5 cm wide, 2.5 cm deep. Set to operate at 6v, cut the bridging wire to run at 12v, includes 2 x 4mm hex head fixing screws and instructions. Min 5Amp-hour battery required.