Patent GB-598743-A (GB)
Title: Electrical null-indicating instrument
IPC: G01R27/06
Filed: 1945-09-07
Abstract: 598,743. Thermionic valve voltmeters. STANDARD TELEPHONES & GABLES, Ltd. Sept. 7, 1945, No. 23146. Convention date, Sept. 14, 1944. [Class 37] A null-indicating instrument comprises four conducting arms arranged as a Wheatstone bridge circuit, a bridge branch including a D.C. indicating device, one of the arms including a vacuum tube detector and a vacuum tube amplifier, two of the arms containing fixed resistances, the fourth arm containing an adjustable resistance, a tunable probe circuit having terminals arranged to be connected to any selected points along an energized cable which it is desired to test, the probe circuit being inductively coupled to the grid circuit of the detector whereby any current in the probe circuit will cause current to flow in the arm containing the detector and amplifier tubes, thereby unbalancing the bridge and actuating the indicating device. A detector V1 has a plate 10, grid 11, and a filament 12, which is fed from a battery BA1 through an inductance L2 and is bridged by a condenser C6. A pentode amplifier V2 has a control grid 14, screen grids 13, 15, filament 16 and plate 17. The detector plate 10 is connected to the positive pole of the amplifier filament battery BA2 through an inductance L3 and resistance R1. Condensers C4, C5 form with the inductance L3 a circuit tuned to the frequency of the standing waves in the cable being tested. The detector plate 10 is connected to the amplifier screen grid 15 through the inductance L3, resistance R1 and battery BA2, and also to'the control grid 14 through a resistance R2. The equivalent bridge circuit is shown in Fig. 3, the points A, B, C, D being similarly indicated in Fig. 1. The arm AD is formed by a fixed resistance R6 and part of a variable resistance R5, the other part of which forms the arm DC. The arm BC is formed by a resistance R3. A D.C. meter M1 in series with a resistance R4 is connected across the diagonal BD and is shunted by a resistor having a negative coefficient of resistance, such as a selenium rectifier CR2, which at normal voltages has a high resistance falling rapidly at high voltages and thereby protecting the meter. The arm AB is formed by the plate circuit of the detector valve V1 and the battery BA1 is connected across the other diagonal CA. The grid 11 of the detector triode is connected to its plate circuit through a curved conductor 31 and an inductance L1. The conductor 31 is electrostatically shielded by a split metallic shield 32, earthed at 33, and its ends are connected to earth through a fixed condenser C3 and a variable condenser C2 by which the circuit may be tuned to resonance. A probe circuit comprises a curved portion 40, which is brought into inductive relation with the curved conductor 31 and to the ends of which are connected a pair of contacts 41, 41a. These are bridged by a variable condenser C1, which serves to tune the probe circuit to synchronism with the high-frequency energy of the cable under test. A selenium rectifier CR1 is connected in parallel with the resistances R1, R2 and decreases in resistance when the current through R1 increases, so that the potential on the grid 14 rises and at the same time the apparatus is protected against injury due to an abnormal increase in energy. A cable to be examined is energized at one end by a determined high-frequency so as to develop a series of standing waves. The contacts 41, 41a are adjusted along the cable until a position reached at which the meter M1 gives a null indication. The contacts are then positioned at a nodal point of a standing wave. The detector grid 11 is biassed from the plate circuit of the detector through the inductance L1 and the tuned circuit L3, C4, C5 which blocks alternating currents, so that any drop in voltage on the plate 10 correspondingly alters the grid bias and decreases the plate current, thus protecting the instrument against unusually strong signals. Also the grid 14 of the amplifier V2 is biassed from the detector plate circuit because of the connection of CR1, R2 across the resistance R2.