Patent GB-819354-A (GB)
Title: Improvements in or relating to electrical resistance classifying apparatus
IPC: G01N27/04
Filed: 1955-11-22
Abstract: 819,354. Resistance measurement; testing physical quantities.. IGRANIC ELECTRIC CO. Ltd. Nov. 22, 1955. [Dec. 9, 1954], No. 33419/55. Class 37. Apparatus for classifying electrical resistance values comprises an electron tube whose anode is energized from a voltage source over a current-responsive device, operable from a first to a second condition at one value of anode current and vice versa at another value thereof, such first and second conditions relating to distinct classes of resistance value; whose control electrode is energized with a control voltage which is a function of the resistance value to be classified, and with a variable bias voltage in opposition thereto; there being provided means for altering the magnitude of the bias voltage in response to operation of the currentresponsive device so as to vary the difference in control voltage necessary to operate the current-responsive device from one to other of its conditions, which may be indicated electrically in response to switch contacts operated by the device. The anode circuit of triode 10 (Fig. 1) is energized from the upper half of secondary 12b of A.C. fed transformer 12 in series with capacitance shunted relay coil 14a, and the grid is energized in antiphase with the anode from the lower half of the secondary over potentiometer 18 in series with, and resistance 16 shunted by, normally open contacts 14b of the relay. The secondary of transformer 22 energized from secondary 12c of the transformer is connected to the grid in series with the unknown resistance (which may be a workpiece 28 of e.g. plywood whose moisture content is to be determined as a function of resistance) and applies to the triode grid a voltage in antiphase to that from potentiometer 18, but in phase with that on the anode. Decrease of the wood resistance imposes a greater antiphased voltage on the grid which increases the anode current, and the voltage from potentiometer 18 is adjusted so that relay 14 operates at an initial resistance value to close contacts 14c to operate an indicator (not shown) and to shunt resistance 16 by closure of contacts 14b to increase the A.C. bias and decrease the anode current, so that the relay then hunts about its operative position when the wood resistance approximates a required value, or remains operated if the resistance is appreciably less. The operative threshold value is set by potentiometer 18 and the margin of anode current between operation and release of relay 14 is determined by the values of 16 and 18. Resistance value of the workpiece is directly measurable by replacing relay 14 by a milliommeter and calibrating the potentiometer 18 in resistance values for a given anode current of the triode. In a modification (Fig. 2), the bias voltage is variably derived from potentiometer 40 in series with a resistance 42 across the lower half of the secondary of transformer 12 and applied to the triode grid over the normally closed contacts 14b of relay 14, which also shunt resistance 44 connected in series with resistance 46 across the lower half of potentiometer 40. The grid is also connected to contact 47 of the workpiece, engaged by contacts 50, 52 energized in antiphase from transformer 58 excited from secondary 12c of transformer 12 over potentiometer 60. Contact 47 is positioned so that the grid voltage is in phase with the anode voltage, and potentiometer 40 is adjustable to set the level of relay operation, while the bias developed on operation of relay 14 opening contacts 14b is independent of the potentiometer 40 setting. The sensitivity level is also adjustable by potentiometer 60, and the potentiometers are replaceable by variable autotransformers.