Electron movement as the basis of magnetism in bar and horseshoe magnets can be explained by assuming that the atoms of iron are so lined up in the magnets that the electrons are circling in the same direction and their individual magnetic lines of force add to produce the magnetic field.

The magnetic field is produced by current flowing in a single loop of wire (fig. 1-19). The magnetic lines of force circle the wire, but here they must follow the

Figure 1-15.- Magnetic lines of force.

Figure 1-16.- Bar and horseshoe magnet.

Figure 1-17.- Effects between magnetic poles.

Figure 1-18.- Electromagnetism.

curve of the wire. If two loops are made in the conductor, the lines of force will circle the two loops. In the area between the adjacent loops, the magnetic lines are going in opposite directions. In such a case, because they are of the same strength (from same amount of current traveling in both loops), they cancel each other out. The lines of force, therefore, circle the two loops almost as though they were a single loop. However, the magnetic field will be twice as strong because the lines of force of the two loops combine.

When many loops of wire are formed into a coil, as shown in figure 1-20, the lines of force of all loops combine into a pattern that greatly resembles the magnetic field surrounding a bar magnet. A coil of this

Figure 1-19.- Electromagnetism in a wire loop.

Figure 1-20.- Electromagnetism in a wire coil.

type is known as an electromagnet or a solenoid. Electromagnets can be in many shapes. The field coils of generators and starters, the primary winding in an ignition coil, the coils in electric gauges, even the windings in a starter armature, can be considered to be electromagnets. All of these components produce magnetism by electrical means.

The North Pole of an electromagnet can be determined, if the direction of current flow (from negative to positive) is known, by use of the left-hand rule (fig. 1-21). The left hand is around the coil with the fingers pointing in the direction of current flow. The thumb will point to the North Pole of the electromagnet. This rule is based on current, or electron, flow from negative to positive.

The left-hand rule also can be used to determine the direction that the lines of force circle a wire-carrying current if the direction of current is known. This is done by circling the wire with the left hand with the thumb pointing in the direction of current flow (negative to positive). The fingers will then point in the direction that the magnetic field circles the wire.

Figure 1-21.- Left-hand rule.

The strength of an electromagnet can be increased greatly by wrapping the loops of wire around an iron core. The iron core passes the lines of force with much greater ease than air. This effect of permitting lines of force to pass through easily is called permeability. Wrought iron is 3,000 times more permeable than air. In other words, it allows 3,000 times as many lines of force to get through. With this great increase in the number of lines of force, the magnetic strength of the electromagnet is greatly increased, even though no more current flows through it. Practically all electromagnets use an iron core of some type.