An ammeter is a measuring instrument used to measure the electric current in a circuit. The name is derived from Ampere-meter, as the standard unit of electric current is the Ampere (A).
If you accidentally connect an ammeter in parallel across a battery (like you would a voltmeter), its ultra-low resistance will cause a massive surge of current (a short circuit) that will instantly blow the ammeter's internal fuse or destroy the device.
An ammeter is used exclusively to measure electric current — the rate of flow of electric charge (electrons) through a wire or component.
An ammeter must ALWAYS be connected in SERIES with the component or circuit whose current you want to measure.
Why in Series? In a series circuit, the current remains the same at all points. By placing the ammeter in series, all the current flowing through the circuit is forced to flow directly through the ammeter, allowing it to count the full flow of electrons.
To ensure it does not alter the current it is trying to measure, a good ammeter must have very low electrical resistance.
| Feature | Ammeter | Voltmeter |
|---|---|---|
| Measures | Electric Current (Flow) | Potential Difference (Voltage/Pressure) |
| Unit | Amperes (A) | Volts (V) |
| Connection | Always in Series | Always in Parallel |
| Resistance | Very Low (Ideal = 0) | Very High (Ideal = Infinity) |
| What happens if connected wrong? | If placed in parallel, it causes a short circuit and may burn out | If placed in series, it stops the current from flowing |
A galvanometer is converted into an ammeter by connecting a very low resistance wire, called a **shunt**, in **parallel** across the galvanometer. This allows the heavy main current to bypass the delicate galvanometer coil, preventing it from burning out.
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