A self-balancing thermistor a.c. potentiometer with d.c. output

The rate of flow of liquid through an electromagnetic flow tube is given by the ratio of the output e.m.f. of the tube to a reference e.m.f. derived from the current flowing in its field winding. In practice both e.m.f.s are alternating and neither is constant; their ratio is usually measured by an electro-mechanical a.c. servosystem whose output shaft is coupled to a pen or pointer. If a d.c. output signal is required, e.g. to facilitate long-distance transmission, displacement-to-d.c. transducer may be coupled to the output shaft. The paper describes an analogous arrangement which contains no moving parts and should prove very reliable. A transistor demodulator replaces the servo-motor, and drives two indirectly heated thermistors: these are connected in series to form a variable potential divider which simulates a slidewire. A.C. and d.c. reference signals are applied simultaneously to the potential divider, whose output provides the feedback signal for the a.c. servo loop and, in addition, d.c. signal which is proportional to the product of the flow velocity and the d.c. reference. The latter may be either constant or caused to vary with, for example, another physical variable by which it is desired to multiply the flow velocity.