Conversion of Galvanometer into Ammeter

AIM
To convert the given galvanometer (of known resistance and figure of
merit) into (i) an ammeter of a desired range (say 0 to 30 mA) and to verify the same.

Specifications:

This Apparatus has been designed to calculate the value of the shunt connected in parallel to the galvanometer to convert it into an ammeter.

This Instrument comprises of:

• A DC-regulated power supply that can provide a voltage range of 0-5V DC
• A high-quality galvanometer and milliammeter are mounted on the front panel
• Three types of shunt resistances that can handle current ranges of 150mA, 100mA, and 50mA
• Built-in variable load resistance of 25Ω/3 Watt on the front panel
• A shunt that protects the instrument from damage due to excess current passing through in its absence
• An ammeter that is capable of measuring large currents when the shunt is connected in parallel with the galvanometer

What is a galvanometer?

A galvanometer is a device that can detect the presence of small currents and measure them; since large currents will damage the coil. A needle shows deflection on a scale graduated in ampere  (unit of current). The maximum current that can cause full scale deflection in a galvanometer is called “ maximum deflection current  The value of  is of the order of 

                  Galvanometer

What is an ammeter?

An ammeter is an equipment used to measure large currents. An ideal ammeter has zero resistance. Lesser the resistance of the ammeter, more accurate its reading will be. Ammeter is always connected in series in a circuit.

Converting a galvanometer into an ammeter

A galvanometer can be converted into an ammeter by connecting a low resistance called shunt in parallel with it; so that the majority of the current will be allowed to pass through the shunt while only a small portion of the current will pass through the galvanometer.

Let  be the current in the circuit and  be the full scale deflection of the galvanometer. 

is the current that will pass through the shunt. The combination of galvanometer and shunt will behave as an ammeter. Let  be the value of shunt resistance and  indicate the galvanometer resistance. Since they are connected in parallel, their voltage drop would be the same, meaning the product of their respective currents and resistances are equal.

I.e,  

The above equation gives the shunt resistance value, which needs to be connected in parallel with the galvanometer to measure current.

The effective resistance of the ammeter so formed is given by .