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What is an Ammeter? - Definition & Function

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  • 0:01 What Is an Ammeter?
  • 0:35 Current vs. Voltage
  • 1:17 Using an Ammeter
  • 3:47 Lesson Summary
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Lesson Transcript
Instructor: Damien Howard

Damien has a master's degree in physics and has taught physics lab to college students.

In this lesson, we'll learn what an ammeter is and what it has to do with electric circuits. We'll also talk about the proper ways to connect an ammeter in series and parallel circuits.

What Is an Ammeter?

Among its many uses, electricity heats and lights our homes, makes our cars start up when we turn the key, and powers all our electronic devices. Sometimes we need to measure the electricity flowing through these devices. One of the instruments that can do this is the ammeter, which measures electric current. It gets its name from the standard unit of measurement for electric current, the ampere. Often you will see the word ampere shortened to amp. Nowadays, the job of an ammeter is often done with another, more versatile instrument called a multimeter, which can measure more than just current.

Current vs. Voltage

It is important to note that an ammeter only measures current, not voltage. Current and voltage are two separate quantities. Voltage can be defined as the electric potential difference per unit charge. It can be viewed as the energy contained within an electric circuit or field at a single point. Current, on the other hand, is the rate at which electric charge passes any given point on a circuit.

One common way to try and understand the difference between the two is to view electricity moving through a wire like water moving through a hose. In this analogy, voltage is like water pressure, and current is like the water flow rate. Changes to one can affect the other, but they are not the same thing.

Using an Ammeter

When using an ammeter, it is very important that the instrument is correctly connected to the circuit. To gain an understanding of how the ammeter must be set up, we'll use a simple circuit with a voltage source and three resistors.

Simple circuit with a voltage source and three resistors

This circuit has a combination of series and parallel elements. Resistor 1 and resistor 2 form a parallel circuit that is connected in series with resistor 3. The ammeter must be connected in series with the part of the circuit where we want to measure the current. Let's start by seeing how to measure the overall current flowing through the entire circuit.

On the right, the ammeter is connected incorrectly; the ammeter is connected correctly on the left

On the right of the image above, the ammeter is incorrectly connected to the circuit in parallel, which presents two problems. The first problem is that there are alternative paths through which current can flow, meaning it will not be measured by the ammeter. The second problem is that a short circuit has been created. Like a wire, ammeters have a very low resistance so they will not affect the current when correctly installed in a circuit. However, when incorrectly connected in parallel, this low resistance will allow a very high current to flow through the instrument, resulting in a blown fuse.

On the left, the ammeter is connected such that all current flowing through the circuit must pass through it; there are no alternative paths. This is the correct way to connect the ammeter to measure the total circuit current, but it's not the only way. There are actually several points in the circuit where the ammeter can be connected to take this measurement. In this image, each ammeter will also measure the total current of the circuit.

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