All conductors show some opposition to electric current.
This opposition to current is called resistance.
A good conductor has low resistance.
A poor conductor, or insulator, has high resistance.
The two main ways of increasing the current in an electrical circuit are by increasing the voltage or by decreasing the resistance.
If you increase the voltage across a component, there will be more current in the component.
Too high a voltage and the lamp will break.
If you increase the number of lamps in a series circuit, there will be less current.
The lamps resist the current, so if you put more lamps into the circuit, there is more resistance.
You could increase or decrease the resistance in a circuit by using a variable resistor (also known as a rheostat).