Patent GB-677318-A (GB)
Title: Self-balancing systems for voltage measurement
IPC: G01R1/28
Filed: 1951-01-29
Abstract: 677,318. Voltage-measurements. LEEDS & NORTHRUP CO. Jan. 29, 1951 [Feb. 7, 1950], No. 2208/51. Class 37. In self-balancing systems for measuring voltage across a high impedance, as in pH measurements using a glass electrode, errors due to stray capacitance are avoided. A high resistance voltage source 10, Fig. 2, is connected at its upper end by a lead 15 to the upper fixed contact of a vibrator 13, the operating coil 13a of which is connected to an A.C. supply 14. The lower fixed contact is connected by a lead 16 to the chassis 26 of an amplifier 18 and also through a resistor 12 to the lower end of the source 10. The vibrator 13 alternately connects its upper and lower contacts to the grid of the first valve 18a of the amplifier 18, the output of which is applied to a rectifier 21. This feeds back a current which changes with change in the unbalance voltage applied to the input of the amplifier and of a magnitude which will develop across the resistor 12 a voltage drop equal and opposite to that of the source 10. A meter 24 is calibrated in terms of voltage of the source 10. The conductor 15 is provided with a shield 25 connected to the chassis 26, so that a capacitance C is introduced between the conductor 15 and the chassis and application of any change in voltage of the source 10 to the valve 18a is delayed on account of the time constant of the capacitance C and high resistance 10a of the source 10. Since the feed-back resistor 12 is in series with the source 10 and the capacitance C, the same delay is applied to the feed-back voltage and a correct reading on the meter 24 is obtained with very small delay in spite of an increase in the resistance 10a of the source 10. Resistances 30a, 30b, Fig. 2A, may be introduced into the lead 15 so as to divide the capacitance C into two parts Ca, Cb. The rectifier 21 may be of the synchronous type operating synchronously with the vibrator 13. Fig. 3 shows a modification employing an indicating and recording high-speed instrument 35 for recording change in pH value of a solution within a container 36. The input conductor 15 is connected to the measuring electrode 37, the other side of the input circuit being connected to the reference electrode 38. The difference between the voltage developed between the electrodes 37, 38 and the voltage across a portion of a slide-wire 41 is applied to the input circuit of the amplifier, a battery 42 and variable resistor 43 being connected across the slide-wire 41. The output of the amplifier is applied to the control winding of a motor 46, which has a power winding supplied from A.C. mains 14. The motor 46 rotates in one direction or the other according to the polarity of the input signal to the amplifier and drives the slidewire 41 in a direction to decrease the unbalance voltage applied to the input of the amplifier. For high-speed operation and to prevent under or over damping a small generator 47 is driven by the motor 46 and connected to resistors 48, 49. The connection to the resistor 48 is such as to produce in the input circuit of the amplifier a voltage which is in the direction to decrease the unbalance voltage and is proportional to the speed of rotation of the motor 46. The resistor 49 and capacitor 50 introduce into the input circuit a component of voltage proportional to the acceleration of the motor 46. This assists rapid rebalancing without over or under adjustment of the slidewire 41.