Test Instruments & Measuring Devices AMMETERS Equation (14-4) is the mathematical representation of the current with the meter installed in the circuit. (14-4) I_w V R_o R_m The accuracy of the ammeter K_A is the ratio of the current when the meter is in the circuit, I_w, to  the  current  with  the  meter  out  of  the  circuit,  I_o.    Equation  (14-5)  is  the  mathematical representation for solving for the accuracy of the ammeter (K_A). (14-5) K_A I_w I_o By  substitution  laws,  Equation  (14-6)  is  a  mathematical  representation  of  the  accuracy  using circuit resistance. (14-6) K_A V R_o R_m R_o V R_o R_o R_m The percent loading error is that percent of error due to loading effects that result from the added resistance of the meter.   Equation (14-7) is a mathematical representation of the percent loading error. % loading error = (1 - K_A)(100 %) (14-7) A second error which occurs in an Figure 7    Ammeter Accuracy ammeter     is     calibration     error. Calibration  error  is  an  error  that occurs due to inaccurately marked meter  faces. Typical  values  of calibration  error  in  terms  of  full scale current are about 3 percent. Example: An ammeter, with a 10   mA   full   scale deflection   and   an internal    resistance of 400 W, is placed in  a  circuit  with  a 20  V  power  source and a 2 KW resistor (Figure 7). Rev. 0 Page 11 ES-14