Ammeter and Voltmeter — Definition

Definition

In the realm of electricity, measuring current and voltage are fundamental tasks for understanding how circuits behave. This is where ammeters and voltmeters come into play. At their core, both these instruments are derived from a sensitive device called a galvanometer.

A galvanometer is essentially a current-detecting device; it shows a deflection proportional to the current passing through it. However, a raw galvanometer is too delicate and has a specific resistance, making it unsuitable for directly measuring large currents or voltages without modification.

An ammeter is specifically designed to measure electric current. Imagine you have a river and you want to know how much water is flowing through it per second. You'd place a device directly in the path of the water.

Similarly, an ammeter is connected in *series* with the component whose current you want to measure. This means the entire current you're interested in must pass through the ammeter. For an ammeter to accurately measure current without significantly altering the circuit's original current, it must offer very little resistance to the flow of electricity.

Ideally, an ammeter should have zero resistance. To achieve this from a galvanometer, a very small resistance, called a 'shunt resistance,' is connected in *parallel* with the galvanometer. This shunt resistance diverts the majority of the current, allowing only a small, safe fraction to pass through the sensitive galvanometer coil, which then provides the reading.

The scale is calibrated to show the total current.

A voltmeter, on the other hand, is used to measure the potential difference, or voltage, between two points in a circuit. Think of voltage as the 'electrical pressure' driving the current. If you want to measure the pressure difference between two points in a water pipe, you'd connect a pressure gauge across those two points, not in line with the flow.

Likewise, a voltmeter is connected in *parallel* across the two points where you want to measure the voltage. For a voltmeter to accurately measure voltage without drawing significant current from the circuit (which would change the voltage it's trying to measure), it must offer extremely high resistance.

Ideally, a voltmeter should have infinite resistance. To convert a galvanometer into a voltmeter, a very large resistance is connected in *series* with the galvanometer coil. This large series resistance limits the current flowing through the galvanometer to a very small value, even when a large voltage is applied across the combination.

The deflection of the galvanometer, proportional to this small current, is then calibrated to indicate the voltage across the entire series combination. Understanding these fundamental principles of construction and connection is crucial for both theoretical comprehension and practical application in NEET physics.