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?
Best Answer
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.