A Widget is a small, hollow device designed to release gas into beer upon the opening of a can or bottle. The widget, usually a plastic disk, has a tiny circular aperture of 0.02 to 0.25 cm in its center. Because it is constructed to be heavier than beer, it does not float and sits at the bottom of the container. The operating principle is very simple: beer is usually kept under pressure and saturated with gas in solution—carbon dioxide, nitrogen, or a mixture of the two—before it reaches the filling line. Once the beer is bottled or canned, the gas builds up pressure inside the container, and, if a widget is present, beer also enters the hollow of the widget until the gaseous pressures in the container and inside the widget are at equilibrium. As soon as the container is opened, the pressure in the container drops, but the pressure inside the widget cannot escape so quickly. The widget shoots a jet of beer and gas into the surrounding beer, causing more gas to break out of solution and a sturdy, thick head to form. The effect of the widget, therefore, is to help canned or bottled beer to imitate the appearance and mouthfeel of a draught beer dispensed from a tap under normal or nitrogen dispense.
The widget was invented after years of development by two brewers, Tony Carey and Sammy Hildebrand, of Guinness in Dublin, in 1968. It was granted a patent in the UK in 1972 and in the United States 1989. It was introduced commercially by Guinness in cans in 1988 and in bottles in 1999. The widget won the UK’s Queen’s Award for Technological Achievement in 1991.