620,156. Automatic control systems for R.F. heating. FERRANTI, Ltd., and WOOD, H. June 11, 1946, No. 17338. [Class 38 (iv)] Frequency, impedance control. Fig. 1 shows an oscillatory system for R.F. heating materials wherein a high-frequency supply circuit 10 of fixed frequency is inductively coupled to a tuned load circuit 11 having a condenser 12, between the plates of which may be placed the material to be heated. Within the coil 13 across the condenser a copper tuning rod 14 is axially oscillated by an electric motor 16, about a mean position controllable by a split field motor 29, to cyclically vary the resonant frequency of the load circuit about a mean value. The varying R.F. voltage across the condenser 12 is rectified by the diode 18 and the output is fed to the differentiating circuit 21, 22, the resulting voltage pulses corresponding to the decreasing half-cycles of the load circuit frequency variation being selected directly or through a relay by the commutator 25 rotated by the motor 16 in synchronism with the oscillations of the rod 14 and integrated by the circuit 23, 24, the resulting voltage applied to the grid of valve controlling the field excitation of the motor 29, which is stationary when the grid voltage is zero. When the mean load circuit tuned frequency is equal to the supply frequency, the rectified voltage at P varies symmetrically about a maximum corresponding to the mean load circuit frequency so that the differentiated voltage pulses selected by the commutator are symmetrical about zero and the integrated voltage on the grid of the motor control valve is zero so that the motor 29 remains at rest and the mean load circuit tuned frequency is undeviated. An increase in this frequency due, for example, to a change in the dielectric constant of the heated material causes the instantaneous load circuit tuned frequency to approach and recede from the supply frequency with cyclic oscillations of the tuning rod, and the rectified voltage at P accordingly increases and decreases with the instantaneous load circuit frequency. The differentiating circuit produces alternate positive and negative pulses, the positive being selected by the commutator, integrated and applied as a positive voltage to the grid of the control valve which causes the motor 29 to rotate, shifting the mean position of the tuning rod so that the mean tuned frequency of the load circuit again coincides with the supply frequency. A fall in the load circuit mean tuned frequency develops a negative voltage on the grid of the control valve by the reverse action and the motor 29 rotates in the opposite direction, again restoring the load circuit mean tuned frequency to coincidence with the supply frequency. Small deviations of the mean load frequency falling within the range of frequency variation due to the oscillations of the tuning rod set up a similar control action in which the selected differentiated pulses are partly positive and partly negative, the predominant integrated polarity determining the direction of rotation of motor 29. In modifications the motor 16 may be clockwork, or the control voltage may be derived from the differentiated pulses corresponding to increasing half-cycles of the load frequency variation; or the system may alternately select the pulses corresponding to both half-cycles which are separately integrated and applied to a pair of valves controlling the excitation of the split motor field. The system may be used to maintain the natural frequency of an aerial equal to that of a transmitter, while the load circuit frequency may be cyclically varied by a condenser, or by a rotating eccentric slide. Fig. 6 shows a modification in which the impedance of a tuned load circuit is automatically matched to the impedance of a supply circuit, using a similar system to that of Fig. 1, in which motor 16 cyclically varies the coupling between coils 55 and 56 about a mean position controllable by motor 29, the voltage across condenser 12 being rectified by diode 18 to control motor 29 which adjusts the mean coupling between the supply and load circuits in a similar manner to that described above, as both over and under coupling result in reduced energy transference by comparison with correct coupling. The system may be combined with that controlling frequency.