OHM METERSTest Instruments & Measuring DevicesEquation (14-10) is the mathematical representation for determining full-scale deflection metercurrent.(14-10)ImVRoWhen the unknown resistance Rxis connected across the ohm meter terminals, the current ismeasured by calculating the total series resistance and applying Equation (14-10). Equation(14-11) is the mathematical representation of this concept.(14-11)IVRoRxAn easy way to determine ohm meter deflection is by use of a deflection factor (D). Deflectionfactor is the ratio of circuit current to meter current. Equation (14-12) is the mathematicalrepresentation of the deflection factor.(14-12)DIImVRoRxVRoRoRoRxThe current through the circuit can be determined by solving for I. Equation (14-13) is themathematical representation of this relationship.(14-13)IDImTo solve for Rxusing Equations (14-10) through (14-13), the relationship between deflectionfactor and the meter resistance to the unknown resistance can be shown. Equation (14-14) is themathematical representation of this relationship.(14-14)Rx1DDRoIf half-scale deflection occurs, then Rx = Ro, so that the value of Ro is marked at mid-scale onthe ohm meter face.ES-14 Page 16 Rev. 0
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