In 1921 Francis Everard Lamplough (an engineer with AGA's rival firm in lighthouse provision: Chance Brothers) patented an alternative 'light valve' in the hope of breaking Dalén's effective monopoly. In subsequent years it was installed on several lighthouses and beacons, but because of its dependence on liquid it could only be used in static locations (unlike Dalén's valves, very many of which were installed on floating buoys). Lamplough's 'valve' was a form of rocker switch, on which were mounted two glass bulbs, one black, the other clear, part-filled with liquid ether and linked by a tube. During the day, heat from the sunlight would cause the air in the black bulb to expand, forcing the liquid into the clear bulb, the additional weight tipping the switch and cutting off the current to the lamp; at night, or at other times of insufficient daylight, the process was reversed, reconnecting the current.