27.9
Galvanometer
Common devices, including car instrument panels, battery chargers, and inexpensive electrical instruments, measure potential difference (voltage), cur…
A galvanometer is an electrical device that detects electric current, ranging from micro to milli amperes.
It consists of two concave-shaped permanent magnets, providing a uniform radial magnetic field in the center.
A coil of fine copper wire wound on a soft iron core is placed between the permanent magnets.
A needle and spring assembly is attached to the axis of this coil.
If no current flows through the coil, the needle points at the center of the scale.
When there is a current flow, the magnetic field exerts a torque on the coil; as a result, the coil turns.
The magnitude of the torque is proportional to the current passed through the coil. As the coil turns, the spring exerts an opposing restoring torque.
Under equilibrium conditions, the angular deflection of the coil is obtained, which is directly proportional to the coil current.
The full-scale deflection is typically on the order of milli amperes.
The potential difference required for full deflection can be obtained using Ohm's law, where the coil resistance is typically ten to a thousand ohms.
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Q1: What is the basic structure of a galvanometer?
A galvanometer consists of two concave-shaped permanent magnets creating a uniform radial magnetic field. A pivoted coil of fine copper wire is placed in this field, with a spring and pointer needle attached to the coil. A calibrated scale below the pointer allows measurement readings when current flows through the coil.
Q2: How does a galvanometer detect and measure electric current?
When current flows through the coil, the magnetic field exerts a torque proportional to the current. The coil rotates, and the spring provides a restoring torque. At equilibrium, the angular deflection of the pointer is directly proportional to the coil current, allowing the device to measure currents ranging from microamperes to milliamperes.
Q3: What is full-scale deflection in a galvanometer?
Full-scale deflection is the maximum angular displacement of the pointer needle, typically occurring at milliampere current levels. The potential difference required for full deflection can be calculated using Ohm's law, where the coil resistance typically ranges from ten to a thousand ohms.
Q4: Why does the spring in a galvanometer play an important role?
The spring, similar to a watch hairspring, exerts a restoring torque proportional to the coil's angular displacement. This restoring force balances the magnetic torque, allowing the pointer to reach equilibrium at a position directly proportional to the current, ensuring accurate and stable measurements.
Q5: What are common applications of galvanometers in everyday devices?
Galvanometers, also called moving coil galvanometers or d'Arsonval galvanometers, are used in car instrument panels, battery chargers, and inexpensive electrical instruments. They measure potential difference, current, or resistance by converting electrical signals into mechanical pointer deflections on a calibrated scale.
Q6: How is the galvanometer's deflection related to voltage?
Since the coil obeys Ohm's law, the current through it is proportional to the potential difference across the coil terminals. Because meter deflection is proportional to current, the deflection is also proportional to the applied voltage, allowing the galvanometer to measure voltage when properly calibrated.
Q7: What are the key electrical characteristics that define a galvanometer's performance?
The essential electrical characteristics are the current required for full-scale deflection and the coil's resistance. These parameters determine the meter's sensitivity and range. Understanding these characteristics is critical when using galvanometers in circuit analysis and when calculating power dissipated in a circuit problem solving scenarios.