A theory for the dynamic balancing of the double thin‐film arms Wheatstone bridge, when both of its sensor arms are growing in thickness in vacuum, has been developed. Rate of growth of the imbalancing signal, bridge sensitivity at the null balance, and its decay with the growth of the sensor arm thickness are derived and discussed. It is shown that as long as the bridge remains dynamically balanced, the ratio of the deposition rates of the films on the sensor armsRandSis proportional to the external resistance ratioP/Q. Hence, if the bridge is kept balanced by the feedback system, the composition of the film codeposited on a common substrate will remain constant which can, however, be varied by changing the ratioP/Q. An experiment was performed to test the validity of this theory by evaporating Cu and Cr separately on the two sensor arms of the bridge in the same vacuum chamber. The vapor pressures of both the metals were recorded simultaneously by a RGA scanning 50–70‐amu range. It was found that as long as the bridge remains balanced, the ratiopCu/pCrremains constant. In contrast, when the bridge is imbalanced with either polarity,pCu/pCrvaried in either direction from this constant value. Further, for a ±10% deviation in thepCu/pCrfrom the constant ratio, the signal grows at a rate of ∼10 &mgr;V s−1. This signal is found to be capable of electronic feedback control of the evaporation system.