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PRINCIPLES OF CINEMATOGRAPHY
FIELD
coin
TO FILM AND LENS
HOLLCW
., .„,. >~
SPRING CONSTANT
ENSION LIGHT BEAM
DEVCE @
Fig. 88.— Principles of the Light Valve.
Now let us consider a single loop of wire as shown in Figure 88 at 'A'. If the sides of this loop are held very close to each other, and connection is made to a D.C. current, it is obvious that two magnetic fields will be set up, one around each side of the loop. These fields will tend to repel each other and to force the loop apart. If the steady D.C. current is now replaced by a alternating current, such as would be obtained from the amplified signals from a microphone, the strength of the magnetic fields will be continually varying and, in consequence, the wires will be parted or drawn together in sympathy with these changes in flux. To increase the strength of these reactions the wires are not circular but rectangular in section with the longer sides of the rectangles facing each other. Once more it is necessary to place the wires between electro-magnets to ensure that the flux changes set up by the speech currents have a permanent force against which to thrust. It is also necessary to place the wires under a mechanical tension to guard against their touching each other if the amplitude of the signals should become too large and 'over-load' the valve.
The actual arrangement of a simple light valve based upon these principles is seen at 'B', Figure 88. A loop of Duralumin ribbon is passed round a spring tension pulley and so constrained, by four adjustable guides, that it forms a narrow slit over an area covered by the field magnet. The two ends of the loop are attached to adjustable clamps by which the ribbon may be brought under tension. Leads from the amplifier are attached to these clamps and supply the varying speech currents to the valve. A crosssectional view of the valve is seen at 'C and illustrates the relative positions of the pole pieces and the Duralumin ribbon. The upper