Patent GB-516379-A (GB)
Title: Improvements relating to the electrical estimation of the moisture content of materials
IPC: G01N27/04
Filed: 1938-06-24
Abstract: 516,379. Determining physical qualities of materials; resistance measurements. BRITISH COTTON INDUSTRY RESEARCH ASSOCIATION, and JONES, E. H. June 24, 1938. No. 18749. [Class 37] In order to estimate the moisture content of samples of materials such as cotton, paper &c. by measuring the resistance between electrodes pressed on or into the material, the electrodes are connected between terminals 2, Fig. 1, in series with a known resistance 1 across one section DE of a potential divider through which passes rectified current from the secondary winding of a transformer the primary winding of which is connected to A.C. supply mains. The potential across the standard resistance 1 is a function of the resistance between the terminals 2, and is measured by applying a fraction thereof between points D, 3 to the grid 4 of a triode 5. The anode 7 is connected through an adjustable resistance 8 to an ammeter 9 and a resistance 10. A resistance AB is arranged so that a current tends to flow through the ammeter 9 opposite to that in the anode circuit so that at zero indication there is sufficient anode current to ensure that the valve is operating on the linear part of its characteristic curve. The variable resistances E, 8 allow adjustments to be made for valves of different characteristics and for variation with age. A multi-point switch enables the resistance 1 to be shunted by resistances 11 of different values for measuring different ranges of moisture content and also enables alternative ammeters of different ranges to be used. The switch also has positions in which the gap 2 is short-circuited, disconnected, and bridged by a known resistance, for adjusting the zero, maximum, and midscale readings. Fig. 4 shows one construction of electrodes for applying to the material under test. Two flat metal plates 41, 42 are separated by a wedge 46 of insulating material such as ebonite, the edges 43, 44 that make contact with the material being flush with the surface 45 of the ebonite. The plate 42 is fixed to or integral with a metal cone 49, the other plate 41 being connected to split pin 47. Two or more such pairs of electrodes may be connected in parallel so that the current traverses the material in different directions. In a modification, Fig. 6, one electrode consists of a rod 61 passing axially through a truncated cone 62 of ebonite. The other electrode 63 is conical, the edges 64, 65 being flush with the end surface of the ebonite. The electrode 61 is formed at its outer end as a split pin, and may be formed as a truncated cone instead of cylindrical. Alternatively, the inner electrode may be of polyhedral section with a conforming surrounding sheet. Either form of electrode is adapted to fit a holder 89, Fig. 8, of insulating material carrying outer and inner metal sleeves 81, 82. The outer casing 83 of an armoured lead by which the electrodes are connected in circuit passes into a hole in the sleeve 81 and makes electrical contact with it. The inner wire 84 enters the inner sleeve 82 and is gripped by an insulated screw 85. A screwed cap 87 engages the outer cone 49 or 63 of the electrode element the inner pin 47 or 61 plugging into the sleeve 82. For use with material of high specific resistance the electrodes are formed of two separate wires 91, 92, Fig. 9, wound round a flat ebonite plate 93 and connected to a split pin 99 and a cone 98 for fitting to the holder 89 by means of a screwed cap 87. A press for applying pressure to the electrodes comprises a stand 111, Fig. 11, carrying a pillar 114 to which may be adjustably clamped a member 115 carrying the cylindrical core 117 of a compressor spring 118. At its lower end the core 117 is bored to receive the electrodeholder 89. A stop 116 limits the downward motion of the core. The core and electrodeholder may be raised and allowed to descend on the sample to be tested, such as a cop of cotton 113, by means of a lever 122 fulcrumed at one end of a link 123.