OHM METERS Test Instruments & Measuring Devices Equation (14-10) is the mathematical representation for determining full-scale deflection meter current. (14-10) I_m V R_o When  the  unknown  resistance  R_x  is  connected  across  the  ohm  meter  terminals,  the  current  is measured by calculating the total series resistance and applying Equation (14-10).    Equation (14-11) is the mathematical representation of this concept. (14-11) I V R_o R_x An easy way to determine ohm meter deflection is by use of a deflection factor (D).  Deflection factor  is  the  ratio  of  circuit  current  to  meter  current.    Equation  (14-12)  is  the  mathematical representation of the deflection factor. (14-12) D I I_m V R_o R_x V R_o R_o R_o R_x The  current  through  the  circuit  can  be  determined  by  solving  for  I.    Equation  (14-13)  is  the mathematical representation of this relationship. (14-13) I DI_m To  solve  for  R_x  using  Equations  (14-10)  through  (14-13),  the  relationship  between  deflection factor and the meter resistance to the unknown resistance can be shown.  Equation (14-14) is the mathematical representation of this relationship. (14-14) R_x 1 D D R_o If half-scale deflection occurs, then R_x  = R_o, so that the value of R_o  is marked at mid-scale on the ohm meter face. ES-14 Page 16 Rev. 0