Question

Ammeter should have very low resistance
A) For large deflection
B) For better stability
C) So that it may not burn out
D) So that it may not change the value of the main current

Answer 1

Hint: An ammeter is a device that is used to measure the current in a circuit. It is connected in series with the circuit element, the current passing through which, is to be measured. This is because in a series combination, the current remains the same.

Complete answer:
An ammeter is used to measure the current passing through a circuit element. It is connected in series with the circuit element as the current through elements in a series combination remains the same. Also, in a series combination the resistances of different elements get summed up.
If the ammeter has a large resistance, then it will get summed up with the resistance of the rest of the circuit and increase the total resistance of the circuit greatly. Since for a constant voltage, the current through a circuit is inversely proportional to its resistance, the effect that the ammeter’s resistance would have on the resistance of the total circuit would be translated into the effect on the current in the circuit. This would mean that due to the insertion of the current measuring device (ammeter), the current in the circuit would change and hence, it would not give the accurate value for the current that was actually passing through the circuit element in the absence of the ammeter.
If the resistance of the ammeter is very small, it would not have any considerable effect on the resistance and hence, the current in the circuit.
This is the reason why ammeters have very low resistance.

So, the correct answer is “Option D”.

Note:
Students must note that contrary to ammeters voltmeters have a very large resistance value. Voltmeters measure the voltage across a circuit element across which they are connected parallel to. If the voltmeter had a low value of resistance then it would change the equivalent resistance of the circuit and hence change the current through the circuit and ultimately change the voltage across the circuit element. However, having a high resistance in parallel means that in the equivalent resistance calculations, it gets effectively cancelled out leading to no change from the resistance value of the total circuit in the absence of the voltmeter.