Patent GB-695866-A (GB)
Title: Improvements in or relating to computing devices
IPC: G06G7/24
Filed: 1950-02-03
Abstract: 695,866. Electric indicating systems. NAAMLOOZE VENNOOTSCHAP PHILIPS' GLOEILAMPENFABRIEKEN. Feb. 3, 1950 [Feb. 8, 1949], No. 2850/50. Class 40 (i) [Also in Group XXXVI] A device for computing the dependent variable of an exponential function of temperature, occurring in chemical and physical problems such as the determination of equilibrium vapour pressures, partial pressures in solutions, absolute and relative humidities, chemical reaction velocities and equilibrium constants, comprises a non-linear electric resistance composed of material such that its resistance or conductance varies with temperature according to an exponential law analogous to the function to be evaluated; e.g. thallium sulphide, mixed lithium and nickel carbonates, zinc, titanium, cobalt, nickel, or cuprous oxides. The variable resistances may be connected in known resistance measuring circuits such that a voltage or current measuring instrument indicates the dependent variable for a given temperature, and plural variable resistances are connectible in series or parallel to give the appropriate exponential law. Inductive or capacitative reactances may be given a similar exponential characteristic by the use of suitable ferromagnetic or dielectric materials. Since the relative humidity of air is the ratio of the equilibrium vapour pressure at the dew point temperature to the equilibrium vapour pressure at the ambient temperature.Fig. 1 (not shown) and since by Clapeyron's equation, p = #o e -Q/RT where p = equilibrium V.P. at ambient temperature, TK. #o = equilibrium V.P. infinite temperature. R = gas constant in cals/mol/K. Q = latent heat of vaporisation of water, an electrical analogy comprising a temperature variable resistance following the law # = #o e<SP>-U</SP>/ RT  where # = conductance at ambient temperature TK. #o = conductance at infinite temperature. U = activation energy of the resistance material in cals/mol. K = a constant, may be constructed, in which resistances of thallium sulphide and titanium dioxide combined in series, or a sinteted composite resistance of nickel carbonate with admixed lithium carbonate in specified proportions, are used, whereby U is made to vary with temperature similarly to Q so that the conductance follows the same law as the vapour pressure. In Fig. 4 the variable resistance SC is connected in series with a comparatively small fixed resistance R across a battery, and the reading of a voltmeter V across R is proportional to the variations of # with temperature, so that if the. resistance SC is subjected successively to the ambient and dew point temperatures T 2 , T 1 , the relative humidity H = #1/#2 determined by the ratio of the voltmeter readings. The resistance SC may be connected in a bridge (Fig. 5) rebalanced by varying R, for conductance and R, for resistance measurement. Other measuring methods may be used (Figs. 6 to 9 not shown). In Fig. 9a the resistance SC may be connected to impose variable damping on a' resonant circuit LC controlling further circuits. Fig. 10 shows a device for determining the relative humidity of air from the known relation, P 2 H - P, + BC (T, - Ti) = 0 where T, = wet bulb temperature T 2  = dry bulb temperature P 1  = equilibrium vapour pressure at T, P 2  = equilibrium vapour pressure at T, B = barometric pressure H = relative humidity C = a constant. A secondary winding 2 of transformer 1 supplies A.C. to a temperature variable resistance 6 at ambient temperature T 2 , in series with a fixed resistance 7 the voltage across which is amplified and supplied to a potentiometer 14 whose sliding tap is connected to the input of amplifier 11 through resistance 10, and represents relative humidity by its position. Secondary winding 3 energizes the temperature variable resistance 15 at the wet bulb temperature T 1  in series with a fixed resistance 16, the voltage across which is connected through a resistance 17 directly to the amplifier input, which also receives through resistances 22, 27 the voltages from the ganged barometrically operated sliders of the potentiometers 21, 26, which are respectively energized by the amplified voltages across the linearly temperature variable resistance 19 at T 2  and the similarly variable resistance 24 at T,. The resistance-temperature curves of the exponential resistances and the A.C. excitation phasing are such that the combined voltage at the amplifier input is analogous to the L.H.S. of the above equation, and the amplifier output is applied to a phase sensitive amplifier 12, fed with a reference A.C., which controls a servomotor 13 to adjust the slider of potentiometer 14 until the combined input voltage = 0 and the slider position represents the prevailing relative humidity. In a modification Fig. 11 (not shown) T 2  # T, is determined by thermocouples in series opposition, whose output is amplified and applied to the input of amplifier 11 through potentiometer 32 whose slider is barometrically operated. In other modifications Figs. 12, 13 (not shown) using an approximation to the above equation, the exponential resistances varying with T 1  and T 2  are connected in bridge networks, unbalanced by the temperature difference and rebalanced by a variable resistance measuring the relative humidity. A mathematical analysis is given.