The vibrator is a very sensitive galvanometer so constructed that the mirror follows faithfully all changes of sound currents over a very wide range of frequencies. It consists of a loop of flat wire 0.0005 inches thick and 0.005 inches wide, stretched over two ivory bridges, each similar to a violin bridge except in size, mounted 7/16 of an inch apart. This loop is put under tension by means of a spring attached to a tiny pulley at the closed end of the loop. The two ends of the loop are connected to binding posts 0.01 of an inch apart. A very tiny glass mirror is cemented across the loop half-way between the two ivory bridges. The entire mechanism is immersed in oil to dampen the movement. The assembly is placed between the poles of a permanent magnet in such a way that the wires lie across the plane of the magnetic lines of force as shown in Figure 48. When a current flows through the loop a turning force is created in the same manner as described in the discussion of motors in section 21. When the current through the loop is in one direction the loop twists one way, and if the current flows in the other direction the loop twists the other way. Sound currents are alternating currents, the frequency of which vary over a wide range. Since the sound currents are alternating currents of the same frequency as the sound, and of a magnitude proportional to the loudness of the sound, they will vibrate the mirror at the frequency and in proportion to the magnitude of the sound waves recorded.

A beam of light from an incandescent lamp is focussed on the mirror. The vibration of the mirror sends the light across the exposed moving film through a suitable optical system containing lenses and a narrow slit. The mirror needs to move only a little to sweep the light beam comparatively large distances at the film. This beam, therefore, acts as a high ratio lever multiplying the effect of the moving mirror many times. This can be illustrated, and often has been by mischievious school boys, by placing a small mirror in a beam of light. A very small movement of the mirror changes the position of the reflected beam on a nearby surface through considerable space, making it difficult for the mischief maker to focus the beam on the eye of the person whom he wishes to annoy.

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