Patent GB-497057-A (GB)
Title: Improvements in arrangements for measuring differences of potential
IPC: G01R19/10
Filed: 1937-06-07
Abstract: 497,057. Current and voltage measurements; influence machines; discharge apparatus. GREIBACH, E. H. June 7, 1937, No. 15780. Convention date, June 10, 1936. [Class 37] [Also in Groups XXXV and XL] An arrangement for measuring differences of potential comprises condenser apparatus having two sets of inducing plates S, T connected to the potential source 10, and two sets of induced plates K, N connected to a measuring instrument 27 for inducing by rotational means causing periodical constant frequency variations in the capacitive coupling between the inducing and induced sheets, current alternations which actuate the measuring instrument, the instrument circuit including inductance 28 which at the frequency of the generated currents has an inductive impedance substantially equal to the capacitive impedance between the inducing and induced plates in their position of maximum capacity, the inducing plates being shunted by capacity 13 which is a multiple of the maximum capacity between the inducing and induced plates, and the input circuit including sufficient resistance 14 for substantially preventing flow of current between the inducing plates and the source. As shown in Fig. 1, the induced plates of the condenser are rotated by a constant speed motor 16 which also drives a rectifying commutator 22 connected between the rotor slip-rings 17 connected to the induced condenser plates and the measuring instrument. In a modification, coupling condensers with fixed and rotary plates and a dry full wave rectifier, replace the slip - rings and commutator. In this case, when measuring small differences of potential such as would be obtained from a pyrometer thermo-couple, a standard cell or other constant voltage source may be connected in series in the input circuit to increase the output current and thereby reduce the resistance of the rectifer, the measuring instrument being supplied with an opposing current or otherwise biassed to correct for this additional voltage. Fig. 12 shows a modified condenser in which both inducing plates S<1> ... T<2> and induced plates K<1> ... N<2> are stationary and between them rotate insulated coupling plates V<1> ...V<2> and interconnected shielding plates X<1> ... Y<2>. The output circuit is connected to the moving coils 170 of a Kelvin balance, the stationary coils 190 being supplied from a constant voltage generator 180 driven by the condenser driving motor 170<1> and giving the same frequency as the condenser. The phase of the generated current is adjusted by varying the angular position of the stator. A contact 200 carried by the bracket 194 of the balance co-operates with fixed contacts 201 in the circuit of a reversible motor 198 which acts on an arm 195 spring coupled to the balance bracket to move the bracket to its neutral position and to give a current reading. Fig. 13 shows an arrangement for measuring X-ray intensities. An ionization chamber 202 located in the path of the rays has an outer cylindrical electrode 205 and an inner electrode 207 spaced from the other by insulating pillars 208. The inner electrode is connected to the high potential end of a potentiometer resistance 231 and the outer electrode is connected to a circuit giving a constant direct current voltage, such circuit including an alternating current source 233, rectifiers 234, voltage doubling condensers 235 and series connected glow tubes 239 acting as voltage regulators. The current through the ionization chamber produces a voltage drop in the resistance 231 which is proportional to the intensity of the X-rays traversing the chamber. The terminals of the resistance 231 are connected to the inducing plates of a rotary condenser 270, the induced plates being connected to the coil 253 of a watt-hour meter 251 in series with the primary winding of a transformer 280 supplying a measuring instrument circuit 140. The other coil 254 of the meter motor is fed from a constant voltage generator 271 similar to the machine 180 described in connection with Fig. 12. The recording dial 258 of the meter has a cam 263 co-operating with contacts 264 to give an alarm or to disconnect the X-ray tube when the predetermined X-ray dosage has been given. The intensity of the rays may be indicated by a pivoted pointer 298 carrying a magnet 295 which is deflected in proportion to the speed of the disc 252. In order to reduce the value of the inductance required in the circuit, the capacity of the rotary condenser and the frequency of the output current should be as great as possible. The stator, Fig. 5, comprises twenty segments 75 carried by insulating end plates 77, 78, each segment having nineteen plates, and the rotor has similar segments 63 abutting against insulating rings 61, 62 carrying slip-rings 68, 70. The capacity between the rotor plates may be reduced by providing cavities 99 on opposite sides of the yokes 64 of the segments or by making the yokes of insulating material. The cylindrical sheet 93 enclosing the stator may comprise an insulating layer and an outer conducting layer and thus acts as the shunting capacity across the stator plates.