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 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.
There were many such schools in the pond, apparently improving the short season before winter would draw an icy shutter over their broad skylight, sometimes giving to the surface an appearance as if a slight breeze struck it, or a few rain-drops fell there.
When a subscriber called (by turning the handle of his magneto- 'generator), the shutter of the annunciator associated with his line dropped.
Before reaching the paper the light passes through perforations in two iron plates which are, in fact, the pole pieces of a strong electromagnet; between these is an aluminium shutter which is attached to two parallel wires or thin strips.
The conversation was finished either of the subscribers could release the shutter of the clearing-out drop by turning his generator handle, and the operator thus notified of the fact removed the plugs and discontinued the connexion.
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.
By means of chains worked by a winch .2 The shutter weir, introduced on the upper Seine about the middle of the 19th century and subsequently adopted for weirs across several rivers in France, Belgium and the United States, consists of a row of wooden or iron shutters turning on a horizontal axis a little above their centre of pressure, borne by an iron trestle at the back of each shutter, which is hinged to the apron of the weir, and supported when raised by an iron prop resting against an iron shoe fastened on the apron (fig.
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.