GALVANOMETER TO AMMETER AND VOLTMETER CONVERSION

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Galvanometer to Ammeter conversion

Galvanometer can only measure a smaller current such as 0.02A . For a galvanometer to measure  a higher current such as 2A or 5A, a resistance needs to be connected in series.
 

From figure 1 above

Voltage across the galvanometer =Gr where G = Galvanometer current and r= galvanometer  internal resistance.

The voltage across the resistance  R =( A-G)R  where A-G = current across R

The two voltages are equal since they are connected in parallel.

Voltage across the galvanometer = voltage across the resistance R

Gr=(A-G)R     therefore               

Where A is  the current to be measured by the galvanometer

Example

What is the value of resistance needed to be used in fig 1 above  to measure current of value A= 2A?

Solution

G=0.02A

r=10Ω

A=2A

In general

To convert a Galvanometer to an Ammeter , you need an External parallel Resistance of value

Where R = the external parallel resistance needed

A = Total current to be measured

r= the resistance of the Galvanometer

G= galvanometer current rating

 Galvanometer to Voltmeter Conversion

Galvanometer  can be converted to a voltmeter  when a series resistance is connected to the galvanometer. For a galvanometer to be able to measure a voltage of about 0-5V  a series resistance is needed.
 

From figure 2 above

Voltage across the galvanometer =Gr =V1 where G = Galvanometer current and r= galvanometer  internal resistance.

The voltage across the resistance  R =GR  , where G = current across R = Galvanometer current

The two voltages are equal  to the total voltage V in the circuit  since they are in series

Voltage across the galvanometer + voltage across the resistance R = Total voltage V

V= V1 + V2= Gr + GR      , V-Gr=GR                    

Where V is  the Voltage to be measured by the galvanometer

Example

What is the value of resistance needed to be used in fig 2 above  to measure Voltage value V=5v ?

Solution

G=0.02A

r=10Ω

V=5v

In general

To convert a Galvanometer to a Voltmeter , you need an External series Resistance of value

                            Where R = the external Series resistance needed

V = Total voltage to be measured

r= the resistance of the Galvanometer

G= galvanometer current rating

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