Patent GB-727400-A (GB)
Title: Improvements in or relating to electrical self-balancing instruments
Inventor: KEALY WELLESLEY ASHE
IPC: G01R17/02
Filed: 1951-05-24
Abstract: 727,400. Measuring bridge circuits; electric analogue calculating systems. NATIONAL RESEARCH DEVELOPMENT CORPORATION. June 23, 1952 [May 24, 1951], No. 12138/51. Class 37. Apparatus for automatic control of or for recording or indicating quantities which may be measured in terms of electrical values comprises a self-balancing circuit including a variable resistance or impedance automatically readjusted to restore balance in correspondence with the input value which is indicated and/or recorded on a chart driven by a timing mechanism, one or more additional variable resistances or impedances, and means for coupling the additional variable resistances or impedances for actuation to the balancing mechanism or to the timing mechanism and means for applying an electric potential across the additional variable resistance or impedance. Fig. 2 shows apparatus for automatically recording the acidity of a solution used in continuous processing, wherein a sample of the solution is drawn from a supply line at a steady rate and alkali is supplied at a constant controlled rate to arrive at a constant conductivity correspond ing to the neutral point. Fig. 1 shows a neutralization curve for the conductivity cell of the apparatus in which the abscissµ are resistances in ohms and the ordinates are volumes of alkali and also a measure of time. The curve departs from linearity at the point X, the extrapolated point F being that which is required to determine the original acidity. The portion AB is substantially straight and the resistance values at the points A, B determine the rate of change of the resistance with time. A transformer 1, Fig. 2, has secondary windings 3, 4, 5, 6 of which the winding 3 is connected to a slide-wire resistance S5 and the winding 4 is connected to the field winding 9 of a galvanometer 8 and to the terminals 10, 11 of a bridge 30. This has two arms formed by resistances 12, 13 and the parts of a slide wire resistance S6. The other two arms consist of a resistance 14 and the conductivity cell which is connected across the terminals 15, 16. The galvanometer 8 is connectible to the diagonal of the bridge by relays on a panel 7. The windings 5, 6 are identical and may be a centre-tapped coil as shown. The winding 5 is connected to slidewire resistances S1, S2 and with a switch D3 in the position shown, across a slide wire resistance S4, the sliding contact 23 of which is connected to a slide wire resistance S3. The sliding contacts 29, 27 of the resistances S1, S2 can be coupled by magnetic clutches to a timing shaft rotating at constant speed. The sliding contacts 25, 23, 20, 18 of the resistances S3, S4, S5, S6 can be coupled to a shaft driven from the balancing mechanism. The recording pen and the pointer of the apparatus are driven from the sliding contact 20 of the resistance S5. In operation the apparatus, which is controlled by a time cycle unit, draws a standard sample of the acid solution and then adds standardized alkali at a controlled rate, the sliding contact 29 being connected to the timing shaft simultaneously with the start of the flow of alkali, and the sliding contact 18 being clutched to the balancing shaft, the galvanometer 8 being connected to the bridge by contacts of the timing mechanism. The sliding contact 18 has attached to it two contacts which open at points A, B, Fig. 1. At point A the first contact opens and the sliding contact 29 is de-clutched, the sliding contact 27 is simultaneously clutched to the timing shaft, and the sliding contact 25 is clutched to the balancing shaft. The resistance S1 thus stores a voltage proportional to t1, Fig. 1. When the second contact on sliding contact 18 opens at point B the sliding contact 27 is declutched, so that the resistance S2 stores a voltage proportional to t2-t1=#t, Fig. 1. When the titration is complete the sliding contacts 25, 18 are de-clutched by the timing mechanism and the conductivity cell is emptied, flushed and filled with another sample. S3 stores a voltage proportional to R - r1 R being the resistance value at neutrality. The resistance S3 is tapped at a point A at a value proportional to r2 - r1 = #r, Fig. 1. The galvanometer 8 is connected by the timing mechanism between the tap A and the sliding contact 27 and the sliding contact 23 is clutched by the timing mechanism to the balancing shaft to bring the voltage at the tap A equal to that of the sliding contact 27. As sliding contact 25 occupies a position corresponding to R - r1 on slide wire 24 the potential between it and the conductor O is proportional to #t/#r (R - r1). This is added to t1 stored in resistance S1 by moving switch D3 to the right so as to connect the resistance S4 to the secondary winding 6. The voltage between the sliding contacts 29, 25 is then the required sum, i.e. RF, Fig. 1 representing the original acidity. The sliding contact 20 attached to the recording pen, is connected by the timing switch via the galvanometer 8 to sliding contact 25, and the contact 20 is clutched to the balancing mechanism and the voltages at contacts 20, 25 are balanced to equality so that the pen and contact 20 take up a position proportional to RE, which is proportional to the sample acidity. The apparatus is also applicable to the recording of a rate of change of temperature, e.g. for determining change points in the thermal analysis of alloys.