Patent GB-440563-A (GB)
Title: Improvements in or relating to electric measuring instruments
IPC: G08C19/18
Filed: 1934-06-28
Abstract: 440,563. Current and pressure meters ; frequency measurements. NALDER BROS. & THOMPSON, Ltd., 97, Dalston Lane, LIPMAN, C. L., Stamford Grove East, Stoke Newington, CLARKE, H. M., 2, Willow Avenue, Barnes, all in London, and JOHNSON, C. W. D., 20, Burlington Gardens, Chadwell Heath, Essex. June 28, 1934, No. 19064. [Class 37] [See also Groups XIX and XXXVIII] A condenser is so arranged that it is charged and discharged at a rate proportional to a value to be measured or is discharged upon the occurrence of a movement it is desired to indicate or to use to control the operation of a relay, the discharge circuit containing a discharge device of the type in which discharge ceases when the voltage falls below a certain value, the discharge current energizing a measuring instrument, relay, &c. As shown in Fig. 1, resistances R, Rl form a potentiometer across a direct current supply. A neon lamp or other discharge lamp L, in series with a shunted micro-ammeter A, is connected across the resistance R1, as also is a condenser C. Across the resistance R, or part of it, is connected a photo-electric cell P for detecting the effect to be recorded. A mirror attached to a shaft under observation reflects a beam of light on to the cell P once or more during each revolution. The resistance R1 is adjusted so that normally the voltage across the lamp L is insufficient to initiate a discharge. When light falls on the cell P the resistance R is shunted, causing the voltage to rise sufficiently to discharge the condenser C through the lamp L and micro-ammeter A, which has a long natural period of oscillation and is heavily damped. During the discharge the voltage of the condenser falls until the lamp can no longer maintain its glow, whereupon the glow ceases and the condenser recharges. The rheostat R2 is adjusted to compensate for variations in the characteristics of different lamps L. The condenser C may be tapped for measuring different ranges of speed; the instrument shunt may also be used for this purpose. The cell may alternatively be exposed to flashes of light produced by interrupting a beam with rotating vanes or a slotted or perforated disc, the light being of a colour chosen for a particular effect or according to the sensitivity of a particular cell. Alternatively the shaft may mechanically close contacts short-circuiting a part of the resistance R for a small part of the time of rotation. Contacts may be opened or closed for initiating the discharge by means of a bell mechanism, tuning fork, or a thermostatic device. The measuring instrument or relay may be in series with the condenser C, in which case it must be sensitive to alternating currents. Alternatively a metal oxide rectifier may be inserted in the condenser circuit feeding a direct-current indicating, recording, or integrating instrument at the receiving end. Long leads of small cross-section may be used between different parts of the circuit, as the current is very small. To compensate for voltage variations a constant current device such as a saturated diode may be connected in series with the resistance R. In the case of alternating current supply, a saturated autotransformer with differential primary windings may be employed to compensate for voltage variations. The condenser discharge may excite an amplifier in the output circuit of which is the current measuring instrument or the counting device. An amplifier may be used between the detector circuit RR1P and the condenser and lamp circuits to amplify the effect of the cell P or to neutralize losses in long leads. A modified circuit including a grid-controlled mercury discharge tube T is shown in Fig. 2. The condenser C is connected in series with a resistance R across a direct-current supply. The mercury discharge tube T is connected across the condenser C and has a current meter A in its anode circuit. The valve T is biassed by means of a potentiometer resistance R6 and battery Vb so that the anode voltage is just below the value at which it begins to glow. When the cell P is exposed to light the grid of the valve T becomes less negative and the voltage at which it begins to glow is decreased to the voltage of supply. The resulting anode current discharges the condenser C, the voltage of which falls until the glow can no longer be maintained. Gridcontrolled vacuum tubes may also be employed. The invention may be applied to the transmission of readings of electrical quantities, for example a mirror or contactor may be fitted to an energy meter shaft to control the charging and discharging of the condenser. The receiving-instrument may be calibrated in watts, volt-amperes, amperes, &c. Other applications are the indication of the instantaneous position of a movable member, the measurement of the frequency of alternations, vibrations, oscillations, pulsations, &c., and the counting of a number of regular and irregular occurrences. The values or relative positions of a number of phenomena or observed bodies may be compared by using a number of circuits as described above and using an instrument having a number of elements acting on a common moving system. For measuring the torsion of a shaft, a mirror mounted at one end of the shaft is illuminated by a lamp and arranged to reflect the light on the photo-electric cell P. A mirror carried by the other end of the shaft is arranged to be illuminated by the flashes of the discharge device. An optical viewing device is placed so that light reflected from the discharge device is observed on cross wires, the angular position of the lamp or the discharge device being adjusted about the shaft as centre and the amount of adjusting movement observed. The mirrors may be fitted on different shafts so that the relative angular position of the shafts may be observed.