Design of log-scale D-C meters

IN MANY quantities, such as money, equal increments are of equal value regardless of the total amount. In other quantities, the increment is evaluated in relation to the total amount. Physical sensation is one example. According to the Weber-Fechner law, a stimulus which increases in geometric progression produces a sensation which increases in arithmetic progression. This means that the magnitude of the stimulus determines the importance of its increment. For instance, an increment of one foot-candle added to illumination of five foot-candles would noticeably increase visibility. Yet, one foot-candle added to 100 foot-candles would not be perceptible. The eye is equally sensitive to equal percentage differences. A light meter should, therefore, give an equal per cent of reading accuracy at each scale point. An instrument having this distribution has a logarithmic scale. Sound is evaluated on a logarithmic basis expressed in the familiar decibel units.