Test Instruments & Measuring Devices AMMETERS By Kirchhoff’s current law, Figure 8    Ammeter with Shunt I_SH I_T I_m Since the voltage across the shunt must be equal to the voltage across the  ammeter,  shunt  resistance  is calculated as follows: I_SHR_SH I_mR_m R_SH I_mR_m I_SH R_SH I_mR_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 . R¹_m  in  place  of  R_m (14-8) R¹_m R_mR_SH R_m R_SH Equation (14-9) is a mathematical representation of the relationship between input voltage and current to the ammeter and the value of input resistance. (14-9) R¹_m V_in I_in I_mR_m I_T Example: An ammeter, with a 100 W meter resistance and a full scale deflection current of 4 mA, is to be shunted to measure currents from 1 to 20 mA. Find:    1. R_SH 2. R¹_m Rev. 0 Page 13 ES-14