An analysis of the behaviour of the single-core, auto-self-excited, transductor element is made for the condition when the control circuit has finite resistance. Consideration is given to control by direct-voltage signal as an introduction to the performance with an alternating-voltage signal of the same frequency as the supply. Factors influencing the output characteristic are examined, and methods of improvement, such as phase shift and special bias arrangements, are discussed.The a.c. control of full-wave transductors and some push-pull circuits with half-wave and full-wave outputs are referred to. The behaviour of a typical push-pull 4-element design is examined experimentally, and its performance when the control is by alternating-voltage signal is compared with that for direct-voltage control.