Voltmeter (pak-physicsnotes.blogspot.com)

VOLTMETER

The electrical instrument which is used to measure the potential difference between two points in volts is called voltmeter.Voltmeter is always connected in parallal in the circuit for measuring the potential difference.

CONVERSION

Voltmeter is the modified form of galvanometer. If we connect a high resistance with the galvanometer in series, the galvanometer is coverted into such electrical instrument which is called voltmeter.

Expression

As high resistance R_h  is connected in  series with the galvanometer, hence voltage in R_h and R_g is different. While same current I_g will flow through them. A small current I_g­ will required for the metermovement (coil) of galvanometer, So that the full scale deflection is obtain and rest of the current should through the component of the curcuit , Whose potential is required to measure.

                                 Applying OHM’s law

                                           V= IR

                                           V= I_gR_{eq    _________________ (1)}

Where

                                          R_eq= R_h + R_g

                                          Put in eq no. (1)

                                           V = I_g(R_h + R_g)

                                            V = I_gR_h + I_gR_g

                                              I_gR_h =  V - I_gR_g

                                               R_h = V  - I_gR_g

                                                              I_g­

                                       
                              (www.pak-physicsnotes.blogspot.com)

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