GB229374A - Dynamic model of a thermionic valve circuit - Google Patents

Abstract

229,374. British Thomson-Houston Co., Ltd., and Clinker, R. C. Nov. 16, 1923. Teaching electricity. - In dynamic models of thermionic valve circuits, an inductance is represented by an intermittently-rotating mass, a condenser by a spring, and the current by the motion of a continuous cord passing around pulleys, one of which is driven by a motor, the coupling between the. grid and plate circuits being represented by a governor controlling the driven pulley and controlled by a spring. A dead-blacked baseboard A may carry a painted circuit diagram comprising a valve V, a generator G which may be shunted by a condenser, a main inductance L and capacity C, a grid inductance L<1>, and a capacity C' coupling the plate and grid circuits. The continuous cord S<1> passes around a driven pulley 1 representing the generator, pulleys 2, 6, 7, M pivoted on the board A, and floating pulleys 3, 5, the pulley M being large and having weights W to represent the inductance L. The spring C<2> representing the capacity C is fixed at one end 13 and connected at its free end to a cord S<2>, which passes around pulleys 8, 9 pivoted to the board A and connects the centres of the pulleys 3, 5. The pulleys 3, 5, 8, 9, M have ball bearings to reduce friction, representing electric resistance. The valve has wooden relief representations of the plate, grid, and filament, a perforated metal plate being moved alternately over positive and negative signs on the grid, to indicate its change of potential, by a thread H connected to the spring C<2>. The coupling between the plate and grid circuits is represented by a cord 11 connecting an adjustable point 12 of the spring C<2> to a centrifugal governor of the gramophone type controlling a friction brake on the shaft of the pulley 1, downward motion of the spring C<2> accelerating the pulley 1. In demonstrating the generation of electric oscillations, the electric or other motor driving the pullev 1 clockwise is started, the pulley 5 is pulled to the right by the string S<1> owing to the inertia of the pulley M, the spring C<2> descends and accelerates the pulley 1, the pulley M acelerates, and the spring C<2> rises through its central position and stops the pulley 1, which is restarted when the spring C<2> falls. The spring S<1> thus moves forward intermittently with an oscillating motion superposed on a continuous motion to represent the current through the valve, the relative phases of the currents in the oscillatory and main circuits being represented by the relative motions of the string S' and the spring C<2>. In demonstrating valve transmission. the aerial is represented by a long spring B fixed at one end to the spring C<2> and painted black and white to represent the aerial current, and the earth connection by a painted metal strip e working up and down behind a metal piece E. By releasing a set-screw holding the pulley 1 to its shaft, the string S<1> may be moved by hand to demonstrate the passage of high-frequency current mainly through the condenser, and of low-frequency current mainly through the inductance in a circuit with a condenser and inductance in parallel, and to demonstrate resonance and the high impedance of such a circuit at resonance frequency if the resistance be low. A prior model in which inductance is represented by a linearly-moving mass and capacity by a cantilever spring is referred to. The Provisional Specification describes the demonstration of the negative resistance property of a coupled three-electrode valve, an adjustable stop being arranged to co-operate with a disc on the centrifugal governor.