Solid State RMS Recording Ammeter

The ammeter described in this paper is essentially a resistor with a variable cross section and a reversible temperature indicator. It has a foil cut into the shape of a double taper with the narrow ends joined at the center of the foil. Current flow through the foil causes a temperature gradient with the higher temperatures in the center. Liquid crystal material coated on the foil substrate changes color reversibly over a discrete range. A narrow color band forms and moves on the taper. The location of the color band is shown to be linearly proportional to the rms current passing through the foil. With the use of irreversible temperature sensing materials, this device will record the peak rms current. The analysis, experimental verification, and design data are presented. In addition, the advantages and disadvantages of this ammeter are described.