Measure full scale voltage:

A 2 mA meter movement along with internal resistance of 25 ohms is to be constructed as a voltmeter.

 What  value  have to  the  series  resistance  be  to  measure  full  scale  voltage  of  100 volts?

Solution:

R_s = (V/I_m) -R_m

Since R_m   is negligibly low, then:

R_s = V/I_m

=100 /2x10^-3

R_s   =   50k?

When  a  voltmeter  is  connected  in  a  circuit,  the  voltmeter  will  draw  current  from  in which  circuit. That current causes a voltage drop across the resistance of the meter that is subtracted from the voltage being measured through the meter. This reduction in voltage is known as the loading effect and could have a serious effect on measurement accuracy, especially for low current circuits. The accuracy of a voltmeter (K_v ) is declared as the ratio of measured voltage whenever the meter is in the circuit (V_w) to the voltage measured along with the meter out of the circuit. Equation (14-2) is a mathematical representation of the accuracy of a voltmeter or true voltage (V_o).

K_v =V_w/V_o                                                                                     (14-2)

Meter  accuracy  could  also  be  determined  through  comparing  the  relationship  among  the  input  and circuit resistances using Ohm's Law as display below.

K_v =V_w/V_o   V_w = I_mR_m

=I_mR_m/V_o    I_m=V_o/R_o +R_in

=(V_o .R_in/R_o +R_in)/V_o

K_v = R_in/ R_o + R_in

where

I_m     =     meter current

V_o    =     true voltage

R_o     =     circuit resistance

R_in   =     input resistance of the voltmeter

K_w    =     indicated voltage

K_v    =     meter accuracy