What is an ammeter? Conversion of galvanometer into ammeter?

An ammeter is an instrument used to measure electric current in an electric circuit.

A ammeter is a modified form of a galvanometer for measuring current in the circuit we require a device of a very low or practically zero resistance.

Conversion of Galvanometer into Ammeter :

"A Galvanometer can be converted into an ammeter by connecting a low resistance ( called shunt) parallel to the galvanometer." 

A small resistance S (shunt resistance) connected in parallel to galvanometer (G) is shown in figure. 

Let G and S be the resistance of a galvanometer and shunt respectively. Let I be the total current to be measured by an ammeter in the circuit. 

Let $I_{g}$ be the current flowing through the galvanometer. The remaining current $(I-I_{g})$ flows through the shunt resistance. 

Since G and S are parallel, the potential difference across them is same. 

$$(I-I_{g}) S=I_{g} G$$

$$S = \left (\frac{I_{g} }{I-I_{g} } \right )G$$

This is the required value of shunt resistance to convert a galvanometer into an ammeter of range 0-I ampere. 

Effective resistance of ammeter : 

The total effective resistance $R_{eff}$ of an ammeter is given by

 $$\frac{1}{R_{eff}} = \frac{1}{G} + \frac{1}{S}$$

$$\frac{1}{R_{eff}} = \frac{G+S}{GS}$$

$$R_{eff} = \frac{GS}{G+S}$$

$$Since \ G>>S, $$

$$so \ (G+S) \simeq G$$

Hence, 

$$R_{eff} = \frac{GS}{G}$$

$$R_{eff}  = S$$

Thus, an ammeter is a low resistance device. Resistance of an ideal ammeter is Zero.  

Note : an ammeter is always connected in series in the circuit in which current is to be measured.