By current we mean the flow of negative charge, i.e., the flow of electrons. Metals have lots of electrons. Hence, current flows in them, and they are good conductors. Substances such as air and glass are bad conductors. We need a source of power, such as a battery, to bring the electrons in motion so that a bulb can start glowing. The battery has a positive (+) and a negative (-) terminal. There are a lot of electrons at the negative terminal and only a few at the positive one. The flow of electrons from the negative terminal to the positive terminal is the current. It is measured in amperes (A). The greater the difference in charges between the two terminals, the stronger will be the flow of the current. This difference is known as the electrical voltage. It is measured in volts (V).