The problems involved in obtaining a satisfactory visual null indicator are considerably more exacting than those in obtaining a suitable acoustical indicator. For the acoustical method of balance, the ear can tolerate the presence of considerable harmonic distortion and extraneous electrical noise without materially reducing the accuracy to which the balance can be obtained. In this bridge, an electron-ray tube (the so-called magic eye) is used as a detector. W ith this type of visual indicator, however, the presence of harmonics or electrical noise causes the "eye" at balance to appear fuzzy, ami unless these noises and harmonics are filtered out, a sharp, accurate balance is impossible.
By using a high-gain amplifier and a sharply tuned filter circuit, a visual null indicator having the sensitivity of the amplifier-earphone combination has been obtained. The schematic diagram, Figure 2, shows the connections of the detector circuit. The sensitivity potentiometer controls the gain of the amplifier and hence controls the sensitivity of the visual indicator. This sensitivity control is extremely useful when the full sensitivity of the bridge is not desired or when the bridge is being used as a limit indicator.
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