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We did an experiment using a very simple circuit shown below: enter image description here

The voltage drops over each resistors were: 4.9, 2.8, and 2.2 respectively. According to the rules the voltage drops across each resistor must add up to the voltage of the source, which in this case was 5 V, however this is not the case. Why is that?

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Take into account the resistance of your voltmeter dropping the voltage more... – creagtheleprechaun Mar 6 at 17:02
1  
Why is there an ??? resistor? – Daniel Tork Mar 6 at 19:11
    
@DanielTork: because the goal of the lab exercise is to determine that resistors resistance. – PlasmaHH Mar 6 at 22:08
2  
@creagtheleprechaun: With ≥1MΩ the voltmeter’s added parallel resistance is negligible compared to the 180Ω or 240Ω resistors in the circuit. – Michael Mar 6 at 22:39

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Measuring voltage across each resistor. Figure 2. Measuring voltages at various points in a circuit using a common reference.

Make sure you are measuring at shown in Figure 1. Alternatively use the method of Figure 2 and subtract voltages to calculate the voltage across each resistor.

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As Transistor says, you're measuring it wrong. I used a couple different methods to find that the mystery resistor is 330 ohms. So here are the voltages at the various points in the circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

It looks like you measured the voltage across the mystery resistor correctly, then mistakenly measured the voltage across both other resistors. Then you measured the voltage across all three resistors.

What you want to do is measure the voltage from the positive battery terminal to point (a), then from (a) to (b), then from (b) to (c).

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