Patent GB-634342-A (GB)
Title: Improvements relating to electrical measuring instruments
IPC: G01R15/00
Filed: 1947-08-18
Abstract: 634,342. Ammeters and voltmeters resistance and capacitance measurements. PETTY, J. D. PAXTON-. Aug. 18, 1947, No. 22926. [Class 37] A multiple-range instrument includes one or more multipliers for the voltage-measuring ranges, and a bridge network, the components of which are contained in a single case, in which one or more of the multipliers are used as comparison resistances in one arm of the bridge. A moving-coil meter movement 1, Fig. 1, giving full-scale deflection for a current of 300 microamperes, is used in series with any one of four resistances 8 to give four ranges of voltage measurements, the required multiplier resistance being selected by a switch 2 in one of its positions I, II, III, IV. In positions V, VI one of two condensers 9 is selected as a standard condenser when measuring capacitance. In position VII a resistance 10 is connected as a shunt to the movement 1 for current measurements, reducing its deflection for a given current to one hundredth. The switch 2 is turned to position VIII for high and low resistance measuring ranges. Switches 3, 4, 5 select the voltage, current, resistance and capacitance ranges. A potentiometer 6 is used both to adjust the resistance measuring circuits before use and as a means of balancing the bridge. A dry battery 7 is employed in the resistance measuring circuits. For measuring high resistances, as shown in Fig. 2, the resistance to be measured is connected to sockets 12, 14 and is connected in series with a multiplier 8, the dry battery 7, and the instrument 1, which is shunted by the resistance 6. The sockets 12, 14 are first short-circuited and the resistance 6 is adjusted until the instrument 1 gives fullscale deflection. The resistance to be measured is then substituted and its value read from the instrument deflection. For low resistance measurements, sockets 13, 14 are employed, the connections being as shown in Fig. 3. The resistance 6 is adjusted so that the instrument 1 gives full-scale deflection with no external connection to the sockets 13, 14. The resistance to be measured is then connected across these sockets and its value read from the appropriate scale of the instrument 1. Fig. 4 shows the bridge connection for resistance measurement, the comparison arm being formed by one of the multiplier resistances 8 in series with a resistance 17, which replaces the meter movement 1 and has the same resistance. The resistance to be measured is connected across sockets 13, 14 and a source of E.M.F. is connected across terminals 16, the balance indicator being connected across terminals 15. The bridge is balanced by adjusting the potentiometer 6, which forms the ratio arms. The ratio of the arms is read on a scale. Fig. 5 shows the connections for capacitance measurement. The comparison arm is formed by one of the standard condensers 9, the capacitance to be measured being connected across terminals 13, 14. A source of alternating E.M.F. is connected across terminals 16, an oscillator or buzzer being included in the instrument for this purpose. A balance indicator is connected across the terminals 15.