Sweep-balance recorders

SWEEP-BALANCE RECORDERS are a new kind of automatic potentiometers different from step-by-step recorders and continuous-balance recorders. A Wheatstone bridge circuit (Figure 4) is combined with a strip chart recorder in such a way that the slide contact is co-ordinated with a stylus traveling across a chart, which consists of voltage-sensitive paper such as “Teledeltos,” the medium used in Western Union facsimile recorders. The conventional null galvanometer or null motor in the output of the bridge has been replaced with a high-speed null relay which may be either of the electromagnetic or electronic type. The paper is marked only when a discharge from a capacitor is passed from the stylus to the paper and the grounded drum under the chart. The connections are such that the discharge is released when the output voltage of the bridge becomes zero or the phase of the output voltage reverses with respect to the input voltage (bridge balance).