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Measuring voltage is a little easier than measuring current because to measure voltage, you don’t have to insert the meter into the circuit. Instead, all you have to do is touch the leads of the multimeter to any two points in the circuit. When you do, the multimeter displays the voltage that exists between those two points.

For example, Figure 8-7 shows how you can insert a voltmeter into the LED circuit so that you can measure voltage. In this case, the voltage is measured across the battery. It should read in the vicinity of 9.3 V. (9 V batteries generally provide a bit more than a full 9 V unless you’ve placed a load on the circuit.)

FIGURE 8-7: Using a voltmeter to measure voltage in the LED circuit.

To measure voltages in the LED circuit, first put the circuit back together (assuming you took it apart to measure currents). Then spin the multimeter dial to a range whose maximum is at least 10 V. Now just touch the leads to different spots in the circuit. To measure the voltage across the entire circuit as shown in Figure 8-7, touch the black lead to the LED lead that’s inserted into the negative bus strip, and touch the red lead to the resistor lead that’s inserted into the positive bus strip.

If you connect the leads backward when measuring voltage, you’ll read a negative voltage. On a digital voltmeter, the meter will display a negative number. On an analog meter, the needle will move in the wrong direction. (Unless the voltage is large, this won’t damage your meter.)

Here’s an interesting exercise. Write down the following three voltage measurements:

 Across the battery: Connect the red meter lead to the resistor lead that’s inserted into the positive bus strip and the black meter lead to the LED lead that’s inserted into the negative bus strip.

 Across the resistor: Connect the red meter lead to the resistor lead that’s inserted into the positive bus and the black meter lead to the other resistor lead.

 Across the LED: Connect the black meter lead to the LED lead that’s inserted into the negative bus and the red meter lead to the other LED lead.

What do you notice about these three measurements? (It’s a little bit of a puzzle, so I won’t give the answer here. But you find it in Book 2, Chapter 2.)