ators used a needle-shaped piece of this ore as the basis of a crude compass. It was found that when this needle-shaped piece of magnetite was suspended and allowed to move freely, it would always turn to the north. What we are dealing with here are permanent magnets. Remember, we used permanent magnets in the telephone subset to set up a magnetic field in its earpiece (Section 5.3.2). Figure A.9 shows an artist's rendition of the magnetic field around a bar magnet, the most common form of permanent magnet. In the early nineteenth century it was learned that this magnetic property can be artificially induced to a family of iron-related metals by means of an electric current. 4Electrical power, of course, can manifest itself or be useful in other ways besides the generation of heat. For example, it can rotate electric motors (mechanical energy) and it can be used to generate a radio wave. REVIEW OF FUNDAMENTALS OF ELECTRICITY WITH TELECOMMUNICATION APPLICATIONS Figure A.9 An artist's rendition of the magnetic field around a bar magnet. The magnetic field was maintained while current is flowing; it collapsed when the cur- rent stopped. Magnets, as we know them today, are classed as either permanent magnets or electro- magnets. A typical permanent magnet is a hard steel bar that has been magnetized. It can be magnetized by placing the bar in a magnetic field; the more intense the field, the more intense is the induced magnetism in the steel bar. The influence of a magnetic field can be detected in various ways, such as by a conventional compass or iron filings on paper where the magnet is placed below the paper. With this latter approach with the iron filings we develop an arrangement of the filings that will appear almost identical with Figure A.9. The lines that develop are commonly known as lines of magnetic induction. All of the lines as a group are referred to as the flux, which is designated by the symbol . The flux per unit area is known as the flux density and is designated by B. We arbitrarily call one end 