Test Instruments & Measuring DevicesAMMETERSBy Kirchhoff’s current law,Figure 8 Ammeter with ShuntI_{SH}I_{T}I_{m}Since the voltage across the shuntmust be equal to the voltage acrossthe ammeter, shunt resistance iscalculated as follows:I_{SH}R_{SH}I_{m}R_{m}R_{SH}I_{m}R_{m}I_{SH}R_{SH}I_{m}R_{m}I_{T}I_{m}Therefore, the input resistance of a shunted ammeter is related to the meter and shunt resistance.Equation (14-8) is a mathematical representation of this relationship.NOTE: When computing accuracy for a shunted ammeter, use .R^{1}_{m} in place of R_{m}(14-8)R^{1}_{m}R_{m}R_{SH}R_{m}R_{SH}Equation (14-9) is a mathematical representation of the relationship between input voltage andcurrent to the ammeter and the value of input resistance.(14-9)R^{1}_{m}V_{in}I_{in}I_{m}R_{m}I_{T}Example: An ammeter, with a 100 W meter resistance and a full scale deflection current of4 mA, is to be shunted to measure currents from 1 to 20 mA.Find: 1. R_{SH}2. R^{1}_{m}Rev. 0 Page 13 ES-14

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