Theoretical Possibilities in an Internally Heated Bimetal Type of Thermal Watt-Demand Meter

There are two generally recognized methods of measuring demand. The block-interval type has been used extensively where great accuracy is desirable, a simple explanation necessary, and a quick testing method essential, but the ``logarithmic´´ type has received increased attention in recent years, and a greater dissemination of knowledge of its characteristics is desirable. A mathematical expression for ``logarithmic´´ average is given which can apply to a variable load as well as to a uniform load. It is shown that a meter with a simple exponential time-deflection characteristic will indicate this ``logarithmic´´ average, but that meters with characteristics obtainable in present commercial thermal meters will theoretically differ from it except when used with certain loads. The accuracy of commercial meters has increased in recent years. So besides indicating that the various types of characteristics available in commercial meters have apparently been satisfactory on normally encountered loads, it is shown that sturdiness and high torques are desirable features to sustain these accuracies. The internally heated thermal watt-demand principle is discussed, and the conclusion is reached that by its use high torque can be obtained using the time tested bimental type of meter. In addition, the rapid diffusion of heat in the bimetal where it is generated theoretically allows shorter ``heat-up´´ time when the meter is being tested and results in a time-deflection characteristic much closer to the ideal simple exponential.