Voltmeter
A voltmeter is used to measure potential difference between two points on a circuit.
It is usually in the shape of a small box, with either a needle that moves over a scale, or a digital screen that shows the reading. There are two probe wires from the voltmeter that connects to the two points on the circuit.
When the two probe wires are connected to the circuit, the reading on the voltmeter shows the potential difference between those two points.
It is important to note that the way that the voltmeter connects to a circuit is different from the way an ammeter connects. The voltmeter just connects to the two points. For the ammeter, a wire in the circuit must be cut or replaced by two wires, and the ammeter inserted into the circuit.
The voltmeter would normally be able to measure a limited range of voltage, for example from 0.01 V to 10 V. To measure lower or higher voltages, something inside the voltmeter must be changed. We shall not worry about what actually happens inside for now. In modern digital voltmeter, we can usually change the range by turning a knob.
For example, suppose that the voltmeter connected across a light bulb reads 0.9 V when the knob is set to read volts (V). In this setting, the reading may just be zero of the potential difference is below 0.01 V. If the knob is turned to millivolts (mV), the voltmeter would now be able to read down to 0.01 mV, or 0.00001 V. The 0.9 V reading changes to 900 mV.
