Allen G. Stimson

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.

Instrumentation of 400-Cycle Aircraft Electric Systems

THE use of 400-cycle alternating current1 as a power source on large aircraft originates the problem of adequate instrumentation at the distribution panel. Load conditions on a d-c system are determined by the simple parameters of voltage and current, whereas those of the a-c system include frequency and active and reactive power, in addition to voltage and current. Specifically, the flight engineer will require the following information for intelligent operation of the system:

A new instrument mechanism

A NEW instrument mechanism has been developed which is remarkable for its small size and weight compared to conventional instruments. It is suitable for use in d-c electric indicating instruments in either portable or panel devices. The instrument is capable of moderately high sensitivity and is most readily adaptable to applications where expanded scale distribution is desirable.