A high-sensitivity ratio instrument for industrial resistance thermometers

To make practical the extensive use of resistance thermometers for industrial temperature measurement and control, an instrument having high sensitivity, sturdy construction, and simplicity is needed. To fill this need the ratio instrument described in this paper was developed. Conventional ratio instrument mechanisms proved to be inherently unstable at the high sensitivities required. The mechanism of the new instrument employs a crossed-coil moving system and a novel permanent magnet pole-face geometry resulting in higher sensitivities than have been previously obtained with ratio mechanisms. A resistance thermometer using the new instrument, while sturdy enough to withstand the shocks and vibration encountered in industrial applications, has a sensitivity high enough to make practical scale spans as small as 50 degrees centigrade.