Copyright Goodheart-Willcox Co., Inc. 100 Electricity Copyright Goodheart-Willcox Co., Inc. Electricity and Magnetism For many years, scientists believed there was a defi nite relationship between electricity and magnetism. The experiments of Hans Christian Ørsted in 1820 proved that a magnetic fi eld was produced around a conductor when a current was fl owing. You can prove this by performing the simple experiment shown in Figure 12-7, just as Ørsted did. Run an insulated wire through a piece of stiff paper or cardboard. The wire should be connected to the positive and negative terminals of a battery. Place two or more small compasses on the paper close to the wire conductor. As the current fl ows, the magnetic fi eld encircling the conductor will cause the compass needles to line up in the direction of the magnetic fi eld. If you reverse the current by changing the battery terminals, the compass needles will indicate that the direction of the magnetic fi eld has also reversed. To determine the direction of the magnetic fi eld, you can use the left-hand rule for a coil. Grasp the conductor with your left hand. Let your thumb point in the direction of the current. Your fi ngers around the conductor will point in the direction of the magnetic fi eld around the wire. Were you to wind an insulated copper conductor into a coil of several turns (if using non-insulated wire, be sure each loop does not touch), you would fi nd that the fi elds around the wire combine to form a magnet. Such a coil can be used as a solenoid. A solenoid uses a magnetic fi eld to cause physical movement. The actual strength of the magnetism produced by such a coil depends on the number of turns of wire and the strength of the current fl owing through the coil. The product of the number of turns of wire multiplied by the amperes is called the ampere-turns of the coil. Ampere-turns is a measurement of the magnetic fi eld strength of the coil. Electromagnets To improve the magnetic fi eld strength of a solenoid, an iron core can be inserted within the windings of the coil. Because iron provides a better path (higher permeability) for the magnetic lines of force than air, the strength of the magnetism is much greater, Figure 12-8. Such a device is known as an electromagnet and is used extensively in electrical/electronic equipment. Remember, the coil wire must be insulated from the conductive core. You can use electromagnets in the construction of relays, doorbells, buzzers, and circuit breakers. pippeeContributer/Shutterstock.com Figure 12-6. Magnetic lines of force are shown through iron fi lings. Cardboard Goodheart-Willcox Publisher Figure 12-7. Current fl owing in a conductor creates a magnetic fi eld around the conductor. Iron core S N Goodheart-Willcox Publisher Figure 12-8. An iron core increases the strength of the electromagnet.