GB274280A - An apparatus for measuring small variations in resistance in a bridge connection by means of alternating current - Google Patents

Abstract

274,280. Marks, E. C. R., (Akt.-Ges. Brown, Boveri, et Cie). July 24, 1926. Vacuum gauges. - An alternating-current wheatstone bridge for measuring small variations of resistance such as occur in resistance thermometers has two diagonal corners connected to the rotating coil of an iron-cored dynamometer, while the external circuit of the other diagonal contains a self-inductance or capacity. Fig. 1 shows a hot-wire vacuum meter in which the degree of vacuum is measured by the rate of cooling of two heated resistance wires a, c of high temperature coefficient forming two arms of a wheatstone bridge, of which the other two arms are formed by resistances b, d. Alternating current from a source f is supplied to the points B, D of the bridge, heating the wires a, c, but producing no potential difference across the points A, C when the bridge is balanced. The points A, C are connected to a moving coil h moving in the field of an alternating-current electromagnet g excited by a coil e connected to the source f either directly or through a transformer wtih or without the interposition of a series resistance, for example an iron wire in hydrogen. The rotating coil h consists of two coils connected in series and mechanically coupled at 90‹ or other fixed angle as shown in Fig. 5. One coil acts as a transformer and supplies current for heating the bridge, the other coil acting as a motor and producing, with the compensating current of the bridge, a torque which causes a deflection of the coil against the action of a restoring spring i. Variation of vacuum causes variation in the rate of cooling of the wires a, c, causing a variation of their resistance and thereby causing unbalance of the bridge. A choking-coil k, or a condenser, is introduced into the circuit connected to the points B, D in order to produce the necessary phase relationship between the current in the .coil h and the flux of the magnet g. Alternatively the exciting-coil e may be connected in series with the points B, D, serving as a choking-coil. The points B, D in this case are joined by a resistance, as the magnetizing current of the coil e is greater than that required for the bridge. In the arrangement shown in Fig. 5 the bridge is fed from the coil h only, the diagonal points B, D being joined by a choking-coil o.