When a subscriber called (by turning the handle of his magneto- 'generator), the shutter of the annunciator associated with his line dropped.
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.
The addition of a foot-bridge greatly facilitates the raising and lowering of these shutter weirs, and also aids the regulation of the discharge; but it renders this form of weir much more costly than the ordinary frame weir, and where large quantities of drift come down with sudden floods, the frames of the bridge are liable to be carried away, and therefore boats must be relied on for working the weir.
Sometimes it is a loosely-fitting shutter or windowframe, a hanging drawer-handle, or a lamp-shade which will rattle; the timbers in a roof may creak, or a group of wine-glasses with their rims in contact may chatter.
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.
- 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.
The sun shining, he fixed a round glass speculum (orbem e vitro) in a window-shutter, and then closing it the images of outside objects would be seen transmitted through the aperture on to the opposite wall, or better, a white paper screen suitably placed.
2) represent a :small hole in the shutter of a darkened room, and OS a narrow beam of sunlight which is allowed to fall on a white screen so as to form an image of the sun at S.
- Shutter Weir with Foot-bridge, Port a l'Anglais, Upper Seine.
In a much larger /!w/ - Shutter '60' inch Reflecting Telescope Dome and Building, Solar Observatory Mount Wilson,Calif.
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.'
The rays which stream through the shutter will be no longer remembered when the shutter is wholly removed.
- There are three main types of movable weirs, namely frame weirs, shutter weirs and drum weirs, which, however, present several variations in their arrangements.