GB559240A - Improvements in and relating to electrically operated indicating or measuring instruments - Google Patents

Abstract

559,240. Determining physical qualities; voltage and impedance measurements; compression resistances. HARTSHORN, L., and WILSON, W. June 23, 1942, No. 8585. [Class 37] The response of a measuring instrument is dependent on both the conductance and susceptance of an object, the relative proportions of conductance and susceptance producing the response being predetermined by the instrument Such variations in a physical condition or dimension of an object as produce corresponding changes in the conductance and susceptance of an object are measured by passing an alternating current through the object and a fixed resistor in series with it and measuring a selected component of the potential difference across the resistor. In the arrangement shown in Fig. 2, the secondary winding 26 of a mains-excited transformer supplies a phase adjuster consisting of a choke 27, resistor 28 and condenser 29. A variable tapping 30 on the resistor 28 is connected to one electrode of the cell 31 containing the material under examination, the other electrode of the cell being connected through a fixed resistor 32 to the common return lead 16. A second secondary winding 14 of the transformer supplies polarizing voltage to the anodes 17, 18 of two rectifying triodes, its mid point being connected to the common return lead 16. The voltage to be measured, across the resistance 32, is applied to the grids 19, 20 in parallel. The cathodes 21, 22 are connected together through a resistance R, a variable tapping 23 on which is connected to the common return lead 16 and provides a zero connection. The cathodes are also bridged by a condenser 24 and by a micro-ammeter M in series with a choke 25. A cell for use in measuring the moisture content of seeds, grain, or other granular or pulverulent materials is shown in Fig. 4. A rectangular box 34 of insulating material is provided with electrodes 35, 36 of sheet metal on the inside of two opposite walls, connection being made thereto by contact pieces 37 on the outside of the box. Alternatively the electrodes 35, 36 may be on the outside of the box, conductive connection then being made through two condensers in series with the material under test. The material to be tested is poured into the opening at the top of the cell until the electrodes are completely covered. To avoid the effect of edge capacitance and conductance a guard ring is formed by separating a strip 35a along the upper edge of the electrode 35 and causing the current passing to this strip to byepass the indicating instrument. In another form of cell, Fig. 6, the electrodes consist of coaxial metal cylinders 38, 40 closed at the lower end by an insulating disc 39. The inner cylinder 40 is closed at the upper end by a conical cap 41 which distributes the powder evenly when it is poured into the space between the cylinders 38, 40, the level of the powder being brought up to the point at which it just covers the apex of the cone. An insulating ring 49 guides the material during pouring. The cell is removable from the rest of the instrument, for example it fits in an aperture in the instrument deck 43, the ring 49 abutting against another insulating ring 43a on the deck 43. Connection is made to the inner electrode 40 by means of a plug 44 carried thereby engaging a socket 45 carried by an insulating disc 47, which is supported by a cylinder 48 forming a dust-proof closure. Spring tongues 46 make contact with the outer electrode 38. A cell for testing fibrous material such as tow or straw comprises a base 50, Fig. 8, carrying two cheeks 51 of wood or other insulating material, leaving the cell open at the top and both ends. Electrodes 52 consisting of metal plates are attached to the cheeks. The space between the cheeks is nearly filled with straw 53 and pressure is applied thereto by a plate 56 and lever 54. A cell for measuring the moisture content of air or other gases has electrodes which are open structures, such as sheets of perforated metal or gauze, which allow easy passage of gas. Water from the gas is absorbed or adsorbed by a material such as a textile fabric of open mesh, or very thin films of a plastic material such as cellulose acetate or nitrate, or goldbeaters skin.