Measuring current, voltage, and resistance

This is a simple electric circuit: a battery, a little lamp, and a switch, connected with wires. When you turn on the switch, you close the circuit. You can see the lamp light up as the electrons start flowing. The rate at which electrons flow is the current. It is measured in amperes, or amps.

How much current flows through the lamp in this circuit? You can measure it using an ammeter. Most ammeters have two colour-coded cables, test leads, each with a jack at one end and a probe at the other. One lead is positive, and usually red. The other is negative, and usually black.

Connect the leads’ jacks into corresponding ports on the ammeter: red to red and black to black. Make sure that the power source is disconnected, and that the ammeter is off. Then, disconnect the wire between the switch and the lamp to break the circuit. Connect the ammeter. The positive probe should touch the wire that comes out of the positive pole of the battery, and the negative — the wire that continues to the negative pole of the battery.

This way, the current will go directly through the ammeter. You’ve connected it in series! Now, choose whether you are measuring direct or alternating current. Direct current is usually marked with the letter A for amperes, and a straight and dotted line symbol. Alternating current, also marked with an “A”, has a wave symbol.

For a battery-powered circuit, use the direct current setting. Now, reconnect the power source and turn on the ammeter. On this analog ammeter, a pointer moves across a dial with a scale, indicating the measured current. A digital ammeter shows the measured amps on a numerical display instead. What else can be measured in an electric circuit?

Voltage, for example. It is the difference in electric potential between two points, measured in volts. To measure voltage, you will need a voltmeter. How do you connect the voltmeter? First, make sure the leads are plugged into the correct spots.

Then, choose the setting. Either alternating current, marked as ACV and a wave symbol, or direct current marked as DCV and a straight and dotted line. For measuring voltage across the terminals of a battery, choose DCV. Now, connect the positive probe to the wire at the positive terminal of the battery. Connect the negative probe to the wire at the negative terminal.

You’ve created two junction points, and connected the voltmeter in parallel. When the current arrives at a junction of the parallel connection, it will flow along the circuit’s wire, rather than through the voltmeter. Now, turn the voltmeter on. Read the result of the measurement. You can also measure how difficult it is for the current to pass through an object.

This is electrical resistance, measured in units called ohms. We measure resistance using an ohmmeter. To measure the resistance, start by plugging in the leads and disconnecting the power supply. Place the probes at the two points between which you want to measure the resistance: one at each side of the lamp. No need to turn on the power supply!

Ohmmeters measure resistance by sending a small current through the object, and comparing the input and output voltage. There is also a device that allows you to measure current, voltage, and resistance — although not all at once. It is called a multimeter. A multimeter has a selection knob, and often more than two sockets into which you can plug in the leads. This allows you to switch between different functions, so the multimeter can act as an ammeter, voltmeter, or ohmmeter.