Improvements relating to graph-plotting apparatus

688,549. Electric indicating systems. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Dec. 11, 1950 [March 22, 1950], No. 7184/50. Class 40 (i) [Also in Group XXXVI] Apparatus for plotting a graph connecting two variables comprises two self-balancing potentiometer units wherein two representative voltages are respectively compared with reference voltages derived therefrom and the difference voltages each deflect a galvanometer to initiate automatic potentiometer adjustment to rebalance the corresponding voltages, which adjustments respectively displace at the same time a recording pen and a chart table in mutually perpendicular directions to graph the unknown variables. In Fig. 1 a motor 1 drives a shaft 3 to intermittently operate through cams and gears a mechanical detector 4 of a self-balancing potentiometer unit 5 wherein the unbalance between one unknown voltage to be measured and a reference voltage from a fixed contact engaging a rotary potentiometer slidewire (not shown) deflects a galvanometer 6, whose pointer is cyclically contacted by a feeler mechanism operated. by the rotating shaft to initiate rotation of the slidewire in the direction of balance, the cycle being repeated until balance is obtained. A second similar self-balancing potentiometer unit 5' is operated by shaft 3 through coupling 7 in synchronism with unit 5 to balance a second unknown voltage. The rebalancing displacement of the slidewire of unit 5 operates a spring and pulley drive to deflect a pen 8 along the Y axis proportionately to the first unknown, and the displacement of the slidwire of unit 5' similarly deflects a chart table 9 along the X axis proportionately to the second unknown so that a graph relating to the two quantities is drawn. A standardizing unit 10 is driven by shaft 3 to cyclically compare the voltage. drop across a fixed resistance in the potentiometer energization circuit with the voltage of a standard cell and thus automatically readjust the current to a standard value. To graph either of the unknowns against time, a roll chart driven through gearing from the shaft 3 is traversed by a recording pen 81 which may be selectively linked mechanically with the deflections of the pen 8 or the chart table 9 Fig. 2 (not shown). Nonlinearity between the quantity to be measured and the corresponding voltage applied to the apparatus is compensated by a cam arrangement. Fig. 3 (not shown) as described in Specification 688,609, for mechanically shifting the appropriate poten