Patent GB-580626-A (GB)
Title: Improvements in electrical measuring devices
IPC: G01K7/02
Filed: 1944-07-12
Abstract: 580,626. Thermionic valve voltmeters; potentiometers. PHILIPS LAMPS, Ltd. July 12, 1944, No. 13346. Convention date, May 11, 1943. [Class 37] In a temperature-responsive device comprising a source of variable potential, a thermal voltage generator is connected in series with a source of polarizing voltage and in shunt with a variable voltage source so as to produce a resulting voltage opposing the voltage of the variable voltage source, and a pulsating voltage proportional to the difference between these opposing voltages is applied to a rectifier, the rectified voltage controlling means for varying the voltage of the variable voltage source. As shown in Fig. 1 a control valve 35 has its anode 38 connected to a source of potential 41 through a temperature-calibrated meter 39. The cathode 37 is connected to earth through a resistor 16 across which a voltage is developed proportional to the current through the valve 35 and controlled by potentials applied to the control electrode 36. A battery 40 connected between the cathode 37 and the control grid 36 through resistors 33, 34 applies a bias of such value as to operate the valve 35 on the straight-line portion of its characteristic. The steady-state current is balanced out of the meter 39 by a battery 42 and adjustable resistor 43. In shunt with the resistor 16 is a thermocouple 10 in series with an interrupter 13, the primary winding 14 of a transformer 15, and a battery 17 which opposes the voltage developed across the resistor 16. The secondary winding 24 of the transformer 15 is connected to the control grid 21 of an amplifier valve 20, the anode 22 of which is connected to the battery 41 through a loadresistor 25. Bias is derived from a resistor 26 shunted by a by-pass condenser 27. By means of a condenser 28 the anode 22 is coupled to the anode 31 of a rectifier valve 30, the cathode 32 of which is connected to the junction of the resistors 33, 34. The voltage derived from the rectifier 30 opposes the bias voltage applied to the valve 35 by the battery 40. A resistor 29 serves as a direct current return for the rectifier 30. By reason of the current flowing through the valve 35 the resistor 16 generates a voltage having a fixed component determined by the voltages of the batteries 40, 41 and a variable component determined by the voltage derived from the rectifier 30. With increasing temperature the thermal E.M.F. generated by the thermocouple 10 adds to the voltage of the battery 17 and the sum of these voltages opposes the voltage across the resistor 16. The battery 17 ensures that the voltage derived from the thermocouple circuit maintains a constant polarity throughout the temperature range to be covered. The resulting voltage of the battery and thermocouple at the lowest temperature to be indicated is equal to the steady-state voltage of the resistor 16. At all temperatures above this a current flows in the thermocouple circuit and induces in the secondary winding 24 a proportional alternating voltage which is amplified in the valve 20, rectified in the valve 30, and applied to the grid 36 of the control valve 35 in such sense as to oppose the voltage of the battery 40. The increase in anode current correspondingly deflects the meter 39 and simultaneously increases the voltage across the resistor 16. In the arrangement shown in Fig. 2 the source of direct current comprises a transformer 50, rectifier 54, and smoothing circuit 55, 56, 57, 58. Shunting the output is a voltage divider comprising gas-filled regulator tubes 59, 60 connected in series with their junction earthed. A control valve 70 and rectifier valve 88 correspond to the valves 35, 30, Fig. 1. A resistor 72 corresponds to the resistor 16, Fig. 1, and a meter 71 corresponds to the meter 39. The sensitivity is adjustable by resistors 73, 74, 75 selected by a switch 76. The steady state current of the valve 70 is balanced out of the meter 71 by a counter-voltage derived from a variable resistor 77. In shunt with the resistor 72 is a thermocouple circuit consisting of a resistor 78, thermocouple 80, interrupter 81, and primary winding of a transformer 82. The resistor 78 replaces the battery 17, Fig. 1, and is connected to the positive side of the D.C. source through one of three resistors 83, 84, 85, which are of different values so as to enable the lower temperature limit to be varied. The resistor 78 has a temperature coefficient such as to counterbalance changes in the voltage of the thermocouple due to changes in the temperature of the reference junction. The voltage induced in the secondary of the transformer 82 is amplified by a two-stage amplifier 86, 87, the output of which is coupled through a condenser 89 to the anode of the rectifier 88. The thermocouple may be replaced by a thermally responsive resistance.