When there is no current the shutter covers the perforations and no light passes, but when a current traverses the wires they are depressed by electromagnetic action, carrying the shutter with them, and a quantity of light proportional to the current strength is admitted through the perforations.
The earliest form of shutter weir, known as a bear-trap, introduced in the United States in 1818, and subsequently erected across the Marne in France, consists of two wooden gates, each turning on a horizontal axis laid across the apron, inclined towards one another and abutting together at an angle in the centre when the weir is closed; the up-stream one serves as the weir, and the down-stream one forms its support, and both fall flat upon the apron for opening the weir.'
- Shutter Weir with Foot-bridge, Port a l'Anglais, Upper Seine.
When a subscriber called (by turning the handle of his magneto- 'generator), the shutter of the annunciator associated with his line dropped.
- The invention of this instrument has generally been ascribed, as in the ninth edition of this work, to the famous Neapolitan savant of the 16th century, Giovanni Battista della Porta, but as a matter of fact the principle of the simple camera obscura, or darkened chamber with a small aperture in a window or shutter, was well known and in practical use for observing eclipses long before his time.