Two Useful Electromagnetic Gauges
Edward B Brown · Journal of Scientific Instruments · 1948
In both gauges the displacement to be measured moves an iron armature modifying the e.m.f. of two differentially connected transformers. In the first described, which is direct reading, the resultant output of the transformers actuates the moving coil of the indicator which is a separately excited ironcored dynamometer instrument graduated to read the displacement directly. The analytical theory of the direct-reading gauge is given. The second gauge described uses an identical gauge head and dynamometer instrument, but the latter is used only as a null indicator, the displacement being indicated by a pointer attached to a coil which can slide along an iron rod near the ends of which are fixed two coils of opposite polarity connected in series with the gauge primary circuits, the sliding coil being connected in series with the gauge secondaries and moving coil of the indicator. If the gauge armature be moved from its central position, an e.m.f. is set up displacing the indicator from zero. This e.m.f. may be balanced by the e.m.f. in the sliding coil caused by moving it from its central position so that the indicator is restored to zero. The pointer attached to the moving coil then indicates the displacement of the gauge armature on a very enlarged scale, which is closely linear. The sliding coil principle of this instrument has been adapted to make a recorder in which the sliding coil replaces the ordinary slide wire with the advantage of having no moving contacts.