Voltage - also known as a potential difference, is needed across an electrical component is needed to make a current flow through the component.

In small circuits, cells or batteries often provide the voltage needed for the circuit to work. Each circuit has it's own voltage and current requirements, and this is governed by the components you wish to add to the circuit.

For instance if you connect a 3 volt rated filament bulb to a battery giving 1.5 volts, it would not work as intended (it would glow dimly).

Conversely if you connect a 3 volt rated filament bulb to a battery giving 4.5 volts, it would also not work as intended (it may also be damaged).

Measuring DC voltage

Voltage is measured in volts, (V).

The voltage across a component in a circuit can be measured using a voltmeter. This must be connected in parallel with the component in a circuit.

Below is an image taking a voltage reading from a new battery - the multimeter is set to measure DC (Direct Current) Voltage, which is the kind you get from a battery.

If you add a simple circuit to the battery and measure the voltage across each of the components, the voltages measured should add up to the total voltage supplied by the battery.