A galvanometer is a sensitive instrument used to detect small electric currents, but it cannot measure voltage directly. To measure voltage, the galvanometer must be converted into a voltmeter by connecting a high resistance (series resistor) in series with it.
Understanding this conversion process is essential for students studying electrical circuits and instrumentation. This topic explores key concepts, important questions, and numerical problems related to the conversion of a galvanometer into a voltmeter.
What is a Galvanometer?
A galvanometer is an instrument that detects and measures small currents in a circuit. It operates on the principle of electromagnetic deflection, where a coil in a magnetic field rotates when current passes through it.
However, a galvanometer:
✔ Has high sensitivity and can detect very small currents.
✔ Cannot measure voltage directly because it is designed for current detection.
✔ Has low resistance, which can disturb the circuit when connected directly across a voltage source.
To use it as a voltmeter, it must be modified to measure voltage without affecting the circuit.
Why Convert a Galvanometer into a Voltmeter?
A voltmeter is used to measure voltage across circuit elements. Since a galvanometer is highly sensitive to small currents, it must be converted into a voltmeter by adding a high resistance (series resistor) to limit current flow.
Key Reasons for Conversion:
✔ To extend the range of voltage measurement.
✔ To prevent excessive current from flowing through the galvanometer.
✔ To ensure minimal impact on circuit operation.
A voltmeter should have high resistance so that it does not draw significant current, which could alter the actual voltage being measured.
How to Convert a Galvanometer into a Voltmeter?
The conversion process involves connecting a high-value series resistance (Rₛ) with the galvanometer. This allows the galvanometer to measure voltage instead of current.
Formula for Series Resistance (Rₛ)
The value of the series resistance is calculated using the formula:
Where:
✔ V = Desired voltage range of the voltmeter
✔ I₉ = Full-scale deflection current of the galvanometer
✔ G = Galvanometer resistance
✔ Rₛ = Series resistance
The high resistance ensures that only a small current passes through the galvanometer, preventing damage while allowing voltage measurement.
Important Questions on Conversion of Galvanometer into Voltmeter
1. Concept-Based Questions
✔ What is a galvanometer?
✔ Why can’t a galvanometer measure voltage directly?
✔ What modifications are required to convert a galvanometer into a voltmeter?
✔ What is the role of the series resistance in a voltmeter?
✔ Why should a voltmeter have high resistance?
✔ What happens if the resistance used for conversion is too low?
2. Multiple-Choice Questions (MCQs)
Q1: Why is a high resistance connected in series with a galvanometer to convert it into a voltmeter?
- To allow more current to pass through
- To prevent the galvanometer from getting damaged
- To decrease the voltage range
- To increase the current through the circuit
✔ Answer: B) To prevent the galvanometer from getting damaged
Q2: The resistance of an ideal voltmeter should be:
- Very high
- Very low
- Equal to the resistance of the circuit
- Zero
✔ Answer: A) Very high
Q3: If a galvanometer has a resistance of 50Ω and a full-scale deflection current of 1mA, what happens if it is connected directly to a 10V source?
- The galvanometer will function normally
- The galvanometer will burn out
- The resistance of the galvanometer will increase
- The needle will not move
✔ Answer: B) The galvanometer will burn out
3. True or False Questions
✔ A galvanometer can measure voltage without modification. (False)
✔ A series resistor is used to protect the galvanometer in a voltmeter. (True)
✔ A voltmeter should have very low resistance. (False)
✔ The formula for the series resistance is Rₛ = (V/I₉) – G. (True)
4. Fill in the Blanks
✔ A galvanometer is used to detect ____________. (small currents)
✔ The resistance of an ideal voltmeter is ____________. (infinite)
✔ A ____________ is connected in series with a galvanometer to convert it into a voltmeter. (high resistance)
✔ A voltmeter should be connected in ____________ across the circuit component. (parallel)
5. Numericals on Conversion of Galvanometer into Voltmeter
Problem 1
Given:
✔ Galvanometer resistance (G) = 100Ω
✔ Full-scale deflection current (I₉) = 2mA
✔ Desired voltage range (V) = 10V
Find: Series resistance (Rₛ)
Solution:
Using the formula:
Thus, a series resistance of 4900Ω is required.
Problem 2
Given:
✔ Galvanometer resistance (G) = 50Ω
✔ Full-scale deflection current (I₉) = 1mA
✔ Desired voltage range (V) = 5V
Find: Series resistance (Rₛ)
Solution:
Thus, a series resistance of 4950Ω is required.
Practical Applications of Voltmeter Conversion
✔ Used in laboratories for voltage measurement.
✔ Essential for electrical circuit analysis and troubleshooting.
✔ Helps in calibrating and testing voltmeters for accuracy.
✔ Used in electronic devices and home appliances for voltage monitoring.
The conversion of a galvanometer into a voltmeter is an important process that allows accurate voltage measurement. This is done by connecting a high-value series resistance, ensuring minimal current flow through the galvanometer while allowing precise voltage readings.
Students should practice conceptual questions, numerical problems, and MCQs to strengthen their understanding. By mastering this concept, they can confidently handle real-world applications and exam questions related to electrical circuits.