that was magnetized, so that it could hold on to screws when installing or removingthem from hard-to-reach places.Permanent magnets are best made from materials with high retentivity. Magnetsare made by using a high-retentivity ferromagnetic material as the core of an electro-magnet for an extended period of time. This experiment is not a good one to do at homewith a battery, because there is a risk of battery explosion.If you want to magnetize a screwdriver a little bit so that it will hold onto screws,just stroke the shaft of the screwdriver with the end of a bar magnet several dozentimes. But remember that once you have magnetized a tool, it is difficult to completelydemagnetize it.The solenoidA cylindrical coil, having a movable ferromagnetic core, can be useful for various things.This is a solenoid. Electrical relays, bell ringers, electric “hammers,” and other me-chanical devices make use of the principle of the solenoid.A ringer deviceFigure 8-7 is a simplified diagram of a bell ringer. Its solenoid is an electromagnet, ex-cept that the core is not completely solid, but has a hole going along its axis. The coilhas several layers, but the wire is always wound in the same direction, so that the elec-tromagnet is quite powerful. A movable steel rod runs through the hole in the electro-magnet core.144 Magnetism8-7A solenoid-coil bell ringer.