Ammeter range and shunt resistance Its said that for an ammeter to give good reading, the full current in the circuit must pass through it.  But if I am right, the ammeter is basically a galvanometer connected parallel to a very low resistance called a shunt.  I am aware that connecting a low resistance in parallel will reduce effective resistance to a value lesser than the least resistance.
But in an ammeter, if the shunt is a low resistance (lesser than galvanometer's resistance), then most of the current would pass through the shunt than the galvanometer.  Thus, the reading given by galvanometer would decrease (as its the component which gives deflection in an ammeter), which means that the reading of ammeter would decrease.
Is my interpretation correct?  If its wrong please explain me where I have gone wrong.
Also, how will range and sensitivity of a an ammeter change if we increase or decrease shunt resistance?
 A: In a practical ammeter there will be a number of fixed shunt resistances, selected by a switch.  The galvanometer is acting as a high-resistance voltmeter, measuring the voltage across the shunt, and has little effect on the current through the circuit was a whole.
If we know the value of the shunt resistor, then the voltage read by the galvanometer is proportional to the current passing.  I = V/R.
Changing the shunt resistor affects the reading greatly.  It's up to the user to select the correct shunt for the current being measured.
A: An Ammeter which  is a current measuring device has some limitations. If the instrument is designed as a moving coil galvanometer -it can measure small currents and the maximum limit may be about 30 micro amperes.
Therefore  one uses a shunt resistor to share  larger part of the current and limited(within range ) current is allowed to pass through the ammeter.
one can select a range of shunt resistance for utilizing   them in measuring current of different  ranges.
the ammeter can be calibrated  along with different shunts and the same instrument can have switching device for different  ranges.
Regarding sensitivity of an ammeter usually  its defined as amount of current required for a full scale deflection- and it varies from 10 micro-Ampere to 30 micro-Ampere .
