Ammeter and voltmeter | Extension of the range of ammeter and voltmeter

AMMETER
Ammeter is a electrical Current Measuring device. it is connected always in series with circuit whose value is to be  measured or friction of it. Therefore it should have a low value resistance.hence the operating coil an ammeter should have few turns of thin wire. and deflection torque is produced by current to be measured.

Classification or Types of Ammeter
Depending on the constructing principle, there are many types of ammeter we get, they are mainly -

1. Permanent Magnet Moving Coil(PMMC) ammeter.
2. Moving Iron(MI) Ammeter.
3. Electrodynamometer type Ammeter.
4. Rectifier type Ammeter.

Depending on this types of measurement we do, we have-

1. DC Ammeter.
2. AC Ammeter.

Extension of the range of ammeter:
The range of ammeter can be extended using shuns resistance parallel with coil. Circuit is show in fig

                          Where               I  = Current to be Measured

                                                Im = Full Scale Deflection Current

                                                                  Is = Shunt Current

                                               Rm = Resistance of ammeter

                                                Rs =  Shunt Resistance

We have                                            I = Im + Is 

                                                                                                   Is  =  I  - Im

                                                    (I  - Im) Rs  =   Im  Rm

                                                                                                 Rs  =   Im  Rm

                                                                                                                 (I  - Im)

                                                      Rs  =     Rm     

                                                                                                              (  I  - 1  )

                                                                                                                    Im                                                                                                                                                                                                                                                        Rs  =     Rm     

                                                                                                             (  N - 1  ) 

                                                                                      

The ratio of total Current yo be measured to The full scale Deflection current is Called  As the multiplying power of the shunt. it may be denoted by N .

VOLTMETER
Voltmeter is a electrical voltage ( Potential Difference ) Measuring device. it is connected always in Parallel with circuit whose value is to be  measured. Therefore it should have a High value resistance.hence the operating coil an Voltmeter should have Large turns of thin wire but it is not sufficient, therefor  a high value resistance connected in series with operating coil and deflection torque is produced by current flow though operating coil which is proportion to the  voltage across the meter.

Extension of the range of Voltmeter:

The range of Voltmeter can be extended using Multiplier( a high resistance ) Series with coil. Circuit is show in fig

   Where              V = Voltage to be measured

                         v  = Voltage across the meter

                                 Im = Full scale deflection current

                        R  = Resistance in series with coil to extend the range.

                                          Rm = Voltmeter resistance.

                                                           V =   Im ( R + Rm )

                                                    V =  Im R + ImRm

                                                 Im R = V - ImRm

                                                     R = V - ImRm 

                                                                Im

                                                          R = V  - Rm         OR

                                                            Im

The ratio of total Voltage to be measured to the voltage across the voltmeter for which it is actually designed   is known as multiplying factor . it may be denoted by m .
                                                            m = V

                                                                   v    
                                                          V  =   Im ( R + Rm ) 

                                                          v              ImRm

                                                    m  =   R  + 1

                                                                     Rm

                                                                        R =   Rm (m - 1 )

Hence for the measurement of voltage  m times the voltage range of instrument , the series multiplying resistance R  should be  (m - 1 )  times the meter resistance Rm.