Measuring Voltage in Series Circuits
Measuring the voltage in a circuit is like measuring the drop in pressure across two points.
 Figure 1. Measuring across R1 |
 Figure 2. Measuring across R2 |
Figure 1 shows a measurement of the voltage drop across R1. When set to read voltage, the DMM will read the voltage VR1. Compare that with Figure 2: a measurement of the voltage drop across R2, that is, VR2.
Notice that the circuit is active when making voltage measurements. Because the DMM in voltage mode has a very high resistance, it usually has little effect on the circuit when connected across components like this.
 Figure 3. Measuring across R1, R2, R3 |
Now consider Figure 3. In this case, the probes measure the voltage drop VRTOT across the total series resistance of RTOT = R1 + R2 + R3.
Can you see how this works out?
VRTOT = VR1 + VR2 + VR3