A galvanometer, G, ?is a device used to measure small electric currents. An AMMETER, used to measure larger currents, is constructed using a galvanometer, a large resistance, rg’ ?to reduce the current through the galvanometer, and a small resistance, Rs, ?for most of the current to go through; by careful calibration, the reading of the galvanometer can be used to deduce the current going through the whole setup. Which one of the figures shown is the most appropriate circuit diagram for an ammeter? Take the resistance of G ?as zero.
(A) 3
(B) 4
(C) 2
(D) 5
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The correct answer and explanation is:
The correct answer is: (C) 2
Explanation:
An ammeter is designed to measure the current in a circuit, and its construction involves certain key features to ensure minimal disruption to the circuit it is measuring. To build an ammeter using a galvanometer, the following principles must be considered:
- Shunt Resistance (Rs):
A small resistance, RsR_s, is connected in parallel with the galvanometer. This is called a shunt resistor. Its purpose is to allow most of the current to bypass the galvanometer. Since the galvanometer is designed to measure small currents, it can only handle a limited amount of current without being damaged. The shunt resistor protects the galvanometer and allows the device to measure larger currents indirectly. - Calibration:
The galvanometer is calibrated such that the deflection of its needle corresponds to the total current passing through the circuit. This calibration takes into account the current through both the shunt resistor (RsR_s) and the galvanometer. - Low Effective Resistance:
For an ammeter to function effectively, it must have very low resistance so that it does not alter the circuit’s current. The parallel combination of RsR_s and the internal resistance of the galvanometer (rgr_g, assumed to be negligible here) ensures the overall resistance of the ammeter is very low.
Circuit Diagram (Option 2):
- Galvanometer (G): Measures a fraction of the current.
- Shunt Resistor (Rs): Connected in parallel with the galvanometer to allow most of the current to bypass it.
In other options, the placement of RsR_s or other components is inconsistent with the principles of ammeter design. Specifically:
- Option (3) and (5): These arrangements either lack a proper shunt resistor or do not ensure current division correctly.
- Option (4): Misplaces or misuses RsR_s.
Thus, Option (2) represents the most appropriate circuit diagram for an ammeter.