Think of a voltmeter like a water gauge. Just as a gauge tells you how much water is flowing, a voltmeter tells you the electric “pressure” (potential difference) between two points. The two main types of gauges are analogue (with a needle) and digital (with a screen). Both work on the same principle but they look and read differently.
An analogue voltmeter has a moving needle that points to a scale. It is useful when you need to see the trend of a changing voltage quickly. The needle’s position is proportional to the voltage, but you must read the scale carefully.
A digital voltmeter displays the voltage as a number on a screen. It is more precise and easier to read, especially for small or rapidly changing voltages.
Both analogue and digital voltmeters have multiple ranges. Selecting the correct range is crucial:
Use the “range selector” (a dial or button) to set the meter to a range that is just above the expected voltage.
| Range | Maximum Voltage | Resolution |
|---|---|---|
| 200 mV | 0.200 V | 0.001 V |
| 2 V | 2.000 V | 0.001 V |
| 20 V | 20.000 V | 0.001 V |
| 200 V | 200.000 V | 0.001 V |
| 2000 V | 2000.000 V | 0.001 V |
Exam Tip: When you see a question about “measuring a voltage of 1.5 V with a voltmeter”, always pick the 2 V range (or the next higher range) to maximise accuracy. Remember that the reading will be the same on both analogue and digital meters if the range is chosen correctly.
Exam Tip: If a problem asks for the error due to internal resistance of a voltmeter, use the formula:
\$\Delta V = I \times R_{\text{int}}\$
where \$I\$ is the circuit current and \$R_{\text{int}}\$ is the internal resistance (typically ≥10 MΩ). For most practical circuits, this error is negligible, but it can be significant in very low‑current measurements.
🔧 Remember: Always connect the voltmeter across the component you want to measure (parallel connection). This ensures you are measuring the potential difference correctly, just like checking the water level across a pipe.