The material must also be a good conductor like copper - that is, it must transmit a charge easily. Current electricity gets its name because people once thought it flowed like water. In any good conductor, some electrons drift freely. When the circuit is switched on, these free electrons all move the same way a short distance, knocking into each other and passing on the charge. This difference, called the potential difference, is created by the battery or generator and is measured in volts.
Resistance is measured in ohms.
In 1819, Danish scientist Hans Oersted noticed that an electric current creates a magnetic field just like a magnet. This discovery paved the way for the invention of generators and electric motors. Since then, it has become clear that the link between electricity and magnetism, called electromagnetism, is among the most important in science. This is sometimes called an accumulator because it can be recharged. The electrodes are big plates, often of lead, while the electrolyte is weak sulphuric acid.