Ammeter and Voltmeter – definition & connection

Last updated on October 16th, 2023 at 03:34 pm

Ammeters measure the current of a circuit, and voltmeters measure the voltage drop across a resistor. Ammeters and voltmeters are cleverly designed because it is important in the design to make sure that the use of these meters doesn’t change the circuit in such a way as to influence the readings. While both types of meters are technically resistors, they are specifically designed to make their readings without changing the circuit itself.

Ammeter

What is an ammeter and how is it connected to the circuit?

An ammeter measures the current traveling through the circuit. They are designed to be connected to the circuit in series and have extremely low resistance. In the circuit sketched above, the ammeter is m2.

Why is an ammeter connected in series and not in parallel?

The ammeter has extremely low resistance. If an ammeter were connected in parallel, all of the currents would go through the ammeter and very little through any other resistor. As such, it is necessary for the ammeter to be connected in series with the resistors. This allows the ammeter to accurately measure the current flow without causing any disruptions.

Voltmeter

What is a voltmeter and how is it connected to the circuit?

A voltmeter measures the voltage drop across a resistor. A voltmeter is designed to be connected to a circuit in parallel and has a very high resistance. In the sketch, the voltmeter is m1.

Why is a voltmeter connected in parallel and not in series?

A voltmeter measures the voltage drop across a resistor and does not need to have the current travel through it to do so. When a voltmeter is placed in parallel with a resistor, all the current continues to travel through the resistor, avoiding the very high resistance of the voltmeter.
However, we know that the voltage drop across all resistors in parallel is the same, so connecting a voltmeter in parallel allows it to accurately measure the voltage drop.

Summary

• Ammeters measure the current through a resistor.
• Ammeters have low resistances and are placed in the circuit in series.
• Voltmeters measure the voltage drop across a resistor.
• Voltmeters have high resistances and are placed in the circuit in parallel.