A permanent magnet is a magnet that is permanent, in contrast to an electromagnet, which only behaves like a magnet when an electric current is flowing through it. Permanent magnets are made out of substances like magnetite (Fe3O4), the most magnetic naturally occurring mineral, or neodymium, a powerfully magnetic synthetic substance. The Earth itself is a hugepermanent magnet, though its magnetic field is quite weak relative to its size. Humans have used the magnetic field of the Earth for navigation since the compass was invented in ancient China. Even the most powerful permanent magnet is not as strong as the stronger electromagnets, so their applications are limited, but they still have many uses. The most mundane would be use as refrigerator magnets, but magnets can be found everywhere, including your hard disk, ATM and credit cards, speakers and microphones, electric motors, and toys. Electric motors work through an interaction between an electromagnet and a permanent magnet. Every permanent magnet generates a magnetic field, just like any other magnet, which circulates around the magnet is a distinct pattern. The size of the magnetic field is related to the size of the magnet and its strength. The easiest way to view a magnetic field generated by a permanent magnet is to scatter iron filings around a bar magnet, which quickly orient themselves along the field lines. Every permanent magnet has two poles, named north and south, though they could have just as easily been called A and B. Similar poles repel while opposite poles attract. It takes a lot of effort to hold repelling poles of a magnet together, while it takes effort to remove attracting poles. The most powerful magnets attract so hard that they can cause injuries by pinching skin between them. For thousands of years, permanent magnets were the only magnets that humans had. The electromagnet was only invented in 1823. Before that, magnets were mostly novelties. Using an electromagnet, it is possible to induce a current in any ferromagnetic materials, such as an iron paperclip. The effect quickly fades, however. Permanent magnets can be a nuisance by threatening to erase data from ATM cards or computer hard drives. A common way of erasing data from a computer hard drive and making sure it is irrecoverable is to run a magnet over it. Otherwise, "deleted" files actually remain on a computer, just hidden. This is because it is more work to have the computer actually delete the data. Rather, the data is designated as a sector of the hard drive that can be written over without concern.