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244 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
in damping due to the changes in fluid viscosity. The oil-damped galvanometer was widely used in the United States prior to 1933 for RCA equipment for variable area; it was most widely used in Germany after 1933.
{2) A dry galvanometer — an iron armature is located in a magnetic field and is caused to move by the application of the electrical signal to a coil wound on the field poles of the magnet (Fig. 54). The construction of the dry galvanometer is theoretically quite similar to that of electromagnetic phonograph pickups with high armature stiffness; one design (RCA) used the mechanical advantage of leverage to increase the motion of a relatively large mirror mechanically connected to the armature by thin flat metallic strips, other designs (Maurer and Western Electric) have a smaller mirror cemented directly to the armature. Although the early dry galvanometers
Fig. 55. Diagrammatic sketch showing Fig. 55A. Modern Western Elechow light valve string moves under influ trie light valve, ence of audio current.
were of poor fidelity, modern dry galvanometers are of good fidelity. There are distinctive design differences in the damping arrangements used by different manufacturers. The freedom from distortion of a particular light modulator depends far more upon the choice of suitable materials and upon the care taken in galvanometer manufacture than upon theoretical considerations. Dry galvanometers have displaced string oscillograph galvanometers almost entirely in the United States ; they are widely used for variable-area sound recording.
(3) A light valve — a pair of ' ' strings ' ' that are caused to move toward and away from one another when a signal currant is passed through them, the signal current reacting upon a magnetic field provided by a permanent magnet (Fig. 55). The strings of a light valve — also called ribbons — are flat metallic strips made of molybdenum or other metal of high tensile strength. The light valve is the modulator furnished by Western Electric ; it is usually associated with variable-density recording. The movement of the ribbons of a light valve changes the amount of light transmitted between them. Strings always have high tensile strength and low mass.