Galvanometer
Galvanometer
A galvanometer is an electromechanical instrument used for detecting and measuring electric current. It is a type of ammeter—an instrument for measuring current—but is utilized for detecting and measuring small levels of current. It works on the principle of conversion of electrical energy into mechanical energy. When an electric current passes through a coil in a magnetic field, a torque is exerted on the coil, which rotates. The amount of rotation is proportional to the current that flows through the coil.
Principle of Operation
The galvanometer works on the principle of the magnetic effect of electric current. It consists of a coil of wire wound around a metallic frame. The coil is suspended between the poles of a magnet. When an electric current passes through the coil, it experiences a magnetic force that causes it to rotate. The deflection of the coil is proportional to the current passing through it.
The basic formula for the torque ( \tau ) experienced by a current-carrying coil in a magnetic field is:
[ \tau = nIBA\cos\theta ]
where:
- ( n ) = number of turns in the coil
- ( I ) = current through the coil
- ( B ) = magnetic field strength
- ( A ) = area of the coil
- ( \theta ) = angle between the normal to the coil and the magnetic field
Types of Galvanometers
There are mainly two types of galvanometers:
Moving Coil Galvanometer: It consists of a small coil of wire that rotates in a magnetic field. The deflection of the coil is proportional to the current passed through it.
Moving Magnet Galvanometer: It consists of a permanent magnet that moves in response to an electric current flowing through a stationary coil.
Key Components
- Coil: A rectangular coil of many turns of insulated copper wire.
- Magnet: A strong permanent magnet with concave poles.
- Suspension: A phosphor bronze strip that provides an axis for the coil to rotate and also conducts current into and out of the coil.
- Pointer: Attached to the coil, it moves over a scale to measure the deflection.
- Scale: Calibrated to read the value of the current.
Uses of Galvanometer
- Detection of Current: It can detect the presence of small currents in a circuit.
- Measuring Current: By calibrating the deflection with known currents, it can measure the amount of current.
- Null Detection: In bridge circuits like the Wheatstone bridge, it is used to detect the null point, indicating that no current is flowing through the galvanometer.
Conversion to Ammeter or Voltmeter
A galvanometer can be converted into an ammeter or voltmeter by adding certain components:
- To Ammeter: By connecting a low resistance (shunt) in parallel with the galvanometer, it can measure larger currents.
- To Voltmeter: By connecting a high resistance in series with the galvanometer, it can measure potential differences.
Differences between Galvanometer, Ammeter, and Voltmeter
| Feature | Galvanometer | Ammeter | Voltmeter |
|---|---|---|---|
| Function | Detects and measures small currents | Measures the flow of current | Measures the potential difference |
| Sensitivity | High (can detect very small currents) | Lower than galvanometer | Lower than galvanometer |
| Resistance | High | Very low (to minimize circuit impact) | Very high (to minimize current draw) |
| Connection | Not specific (used in various circuits) | Always in series with the load | Always in parallel with the load |
| Shunt/Resistance | None (for basic operation) | Low resistance shunt | High resistance series resistor |
Example: Using a Galvanometer as an Ammeter
To use a galvanometer as an ammeter, we need to connect a shunt resistor in parallel with it. The value of the shunt resistor ( R_{\text{shunt}} ) can be calculated using the formula:
[ R_{\text{shunt}} = \frac{R_{\text{g}}}{\frac{I}{I_{\text{g}}} - 1} ]
where:
- ( R_{\text{g}} ) = resistance of the galvanometer
- ( I ) = full-scale deflection current of the ammeter
- ( I_{\text{g}} ) = full-scale deflection current of the galvanometer
Example Calculation:
Suppose we have a galvanometer with a full-scale deflection of 1 mA and a resistance of 50 ohms. We want to measure currents up to 10 mA. The shunt resistor needed would be:
[ R_{\text{shunt}} = \frac{50\ \Omega}{\frac{10\ \text{mA}}{1\ \text{mA}} - 1} = \frac{50\ \Omega}{10 - 1} = \frac{50\ \Omega}{9} \approx 5.56\ \Omega ]
By adding this shunt resistor in parallel with the galvanometer, we can now measure currents up to 10 mA.
In conclusion, the galvanometer is a versatile instrument in the field of electronics and electrical engineering. Its ability to detect small currents and its conversion into other measuring devices make it an essential tool in various applications. Understanding its principles, operation, and uses is crucial for students and professionals alike.