Next we have the arm. This writes and reads data onto the disc. It stretches out over the platter and moves over it from centre to edge reading and writing data to the platter through its tiny heads which hover just over the platter. The arm, on the average domestic drives can oscillate around 50 times per second. On many high-spec machines and those used for complex calculations this figure can rise into the thousands. Hard drives use magnetism to store information just like on old cassette tapes. For that reason, copper heads are used as they are easy to magnetise and demagnetise using electricity.