Patent GB-620834-A (GB)
Title: Improved method of weighing and apparatus therefor
IPC: G01G7/04
Filed: 1944-09-01
Abstract: 620,834. Automatic control of alignment, force and current strength. THOMAS, A. R. Sept. 1, 1944, Nos. 16711/44, 3724/45 and 3725/45. [Class 38 (iv)] [Also in Groups XXXVIII and XL (b)] An apparatus for controlling the current flow in a circuit comprises a solenoid connected in the said circuit, an armature freely supported in a predetermined position by the solenoid without contact with the solenoid or any other part of the apparatus, and means responsive to movements of the armature to adjust the solenoid current to the strength necessary to maintain the armature in equilibrium, whilst upon the application of forces to the armature (such as the addition or subtraction of weights) the consequent movement of the armature results in such a current change as tends to return the armature to its predetermined position. The apparatus may be used as a constant current generator, or for providing an unvarying source of electromotive force by passing the solenoid current through a known resistance. A range of currents or e.m.f.'s may be obtained by armatures of different weights. To centralize the armature in the bore of the solenoid, the former may carry at its upper end a light aluminium tube terminating in a small bar magnet which is coaxial with a fixed radially magnetized ring magnet having the polarity of its inner surface the same as that of the adjacent portion of the bar magnet. As applied to a micro balance, the apparatus may comprise an armature 5 supporting the balance pan and freely suspended by the magnetic field of a solenoid L arranged in the anode circuit of a small power valve V4 in series with a variable resistor R13 and meter M2. The armature carries a diaphragm 3 having a slot 4 controlling the illumination of a photo-electric cell P from a lamp 1, the voltage output of the cell, after amplification by a valve V3, being applied as grid bias to the power valve V4 and automatically controlling the solenoid current to the strength necessary to support the armature plus any additional load on the balance pan. A variable impedance valve V6 in the feed line to the solenoid and controlled by anode voltage changes of V4 over the differentiating network C2, C3, R14, R15, R17, and phase reversing amplifier V5, is provided to stabilize movement of the armature. The load on the scale pan is measured by adjusting the position of the slider of a variable potentiometer R11 connected across the solenoid and valve V4, the slider voltage being balanced, on a null indicator M1, against a reference voltage supplied through a potentiometer R1 from apparatus 8 which is generally similar to that shown to the right of potentiometer R11 but having an armature of fixed weight. To increase the sensitivity of weight indication, the standing current through solenoid 5 in the absence of load on the balance pan may be reduced to a small value by arranging for the armature to be supported mainly by either a permanent magnet, or a further coaxial solenoid carrying a current derived from the reference solenoid circuit. The power valve V4 may be shunted by an adjustable resistor whereby the current in solenoid 5 may be increased when weighing heavy articles requiring a current in excess of that which can be passed safely by the valve.