When a subscriber called (by turning the handle of his magneto- 'generator), the shutter of the annunciator associated with his line dropped.
- Shutter Weir with Foot-bridge, Port a l'Anglais, Upper Seine.
The size of the discharge aperture can be varied by means of a flexible wooden shutter sliding in a groove in a cast iron plate, curved to the slope of the casing.
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.
This apparatus has two coils, one of which, connected across the line, is provided for the purpose of projecting the shutter, while the other is intended for its restoration and is joined in a local circuit arranged to be closed when a plug is inserted in any one of the associated jacks.