Patent GB-656056-A (GB)
Title: Compensating circuit for electrical instruments
IPC: G01N21/27
Filed: 1947-04-08
Abstract: 656,056. Current and voltage measurements. GENERAL ANILINE & FILM CORPORATION. April 8, 1947, No. 9328. Convention date, May 7, 1946. [Class 37] The response of an instrument is varied at certain points of the scale by shunts connected by means of rectifiers to which are applied corresponding opposing voltages. As shown in Fig. 1 the output of a densitometer for photographic film is applied to input terminals 10, 11 across which are connected resistors 12, 13 forming a voltage divider. A meter 20 having a uniformly graduated scale is connected across the resistor 13 and directly indicates the density of the sample although the electrical values so indicated are derived from logarithmically varying quantities. The scale is approximately linear up to the point 23, Fig. 2, and then follows the curve 21. In order to compensate for this curvature and obtain a linear response 22 an additional resistance is connected in parallel with the resistance 13 at the instant when the meter indication corresponds to the point 23. A resistor 26 has one terminal connected to the point 15 and the other terminal connected, in series with a selenium or other rectifier 27, to an adjustable tap 28 on a battery 25. The tap 28 is adjusted to the voltage value 23, Fig. 2, so that when the voltage drop over the resistor 13 reaches this value current flows through the resistor 26 which is thus effectively in parallel with the resistor 13. A second resistor 36 in series with a rectifier 37 may be connected to a second tap 38 on the battery 25 so as to pass current when the voltage drop over the resistance 13 reaches the point 34. In the densitometer shown in Fig. 3, light from a source 41 is projected through a sample of film 43 on to the cathode 46 of a photomultiplier tube 45. A voltage divider 49 has taps connected to the dynodes 48 and its lower end connected to the negative terminal 47 of a potential source and to the cathode 46 of the tube 45. The upper end 51 of the potential divider 49 is connected, through a voltage stabilizing tube 50, to the cathode 52 of a control tube 55. The anode 53 of the photo-tube 45 is connected to the control grid 54 of the tube 55. A battery 56 is connected between the cathode 52 and anode 53 through a resistor 57. The screen grid 58 is connected to the battery 56. A milliammeter 65 is connected in the output circuit of the photo-tube 45 in series with resistors 68, 71. A tapped voltage divider 73 is connected across a source of potential 72 and has its centre point 75 earthed at 76, the potential of each half being maintained constant by means of stabilizing tubes 77, 78. The upper half of the voltage divider supplies through a tap 80 and resistor 79, the bucking current for zero setting of the meter, whereas the lower half supplies the biassing potential for the compensating shunts. A pair of resistors 81, 82 are each connected, in series with a rectifier 83, 84, to a tap 86, 87 on the voltage divider 73. When the voltage drop across the resistor 71 exceeds the potential of the tap 87, the resistor 82 carries current and is effectivoly in parallel with the meter 65. A similar action with respect to resistor 81 takes place when the voltage drop exceeds the potential of the tap 86. More accurate linearity of response of the meter 65 with respect to the density of the sample 43 is thereby produced. One or both of the rectifiers 86, 87 may be reversed in polarity to obtain expansion or contraction at different parts of the scale. Specifications 624,313 and 632,378 are referred to.