Magnet

A magnet is a material or object that produces a magnetic field. This field is invisible, yet it is responsible for the magnet's most notable property. It exerts a force that pulls on ferromagnetic materials, such as iron, steel, nickel, and cobalt. It can also attract or repel other magnets.

While most substances respond weakly to magnetic fields through other types of magnetism, ferromagnetic and ferrimagnetic materials are the only ones strongly attracted to a magnet. These materials are essential for creating the permanent magnets we use in everyday life, like the ones on a refrigerator door.

Scientists categorize ferromagnetic materials into two distinct types: "soft" and "hard." Magnetically soft materials, such as annealed iron, can be magnetized easily. However, they do not tend to stay magnetized for long. In contrast, magnetically hard materials stay magnetized. Permanent magnets are manufactured from these hard materials, such as alnico or ferrite. During manufacturing, these materials undergo special processing in a strong magnetic field. This process aligns their internal microcrystalline structure, making them very difficult to demagnetize.

To understand how a magnet behaves, we can look at two different scientific models. The magnetic pole model is a convenient way to think about magnets. In this view, a magnet has a north pole and a south pole. If you break a bar magnet in half to try and separate these poles, you will not succeed. Instead, you will simply create two smaller magnets, each with its own north and south pole.

Another model is the Ampère model. This model suggests that magnetization comes from microscopic, or atomic, circular currents throughout the material. These are called Ampèrian currents. For a cylindrical bar magnet, these tiny currents make the magnet behave as if a large sheet of electric current is flowing around its surface.

Humanity's relationship with magnetism began with lodestones. These are naturally occurring pieces of magnetite, which is a type of iron ore.

The word "magnet" comes from the Latin *magnetum*, meaning lodestone. This word likely traces back to the Greek term for "stone from Magnesia." While the exact location of Magnesia is debated, it may refer to a region in Greece or Anatolia. Ancient people used suspended lodestones as the very first magnetic compasses. The earliest written descriptions of these properties come from Anatolia, India, and China from approximately 2,500 years ago.

Significant historical developments changed how we use magnetic force. In 11th-century China, researchers discovered that quenching red-hot iron aligned with Earth's magnetic field would leave it permanently magnetized. This led to the navigational compass described in the *Dream Pool Essays* of 1088. Later, in 1743, Daniel Bernoulli invented the horseshoe magnet. This shape prevents a magnet from demagnetizing itself by returning the magnetic field lines to the opposite pole.

In 1820, Hans Christian Ørsted discovered that electric currents could deflect a compass needle. This discovery allowed William Sturgeon to develop an iron-cored electromagnet in 1824. Finally, Joseph Henry advanced the electromagnet into a commercial product by 1830, providing access to strong magnetic fields for the first time.

Magnetism can be measured using several specific values. The overall strength of a magnet is called its magnetic moment. This is a vector that characterizes the magnet's total properties. For a bar magnet, the direction of the magnetic moment points from the south pole to the north pole. The strength of the magnetic field itself, often called the B field, is measured in teslas.

We can also measure magnetization, which is the local value of the magnetic moment per unit volume. For example, a good bar magnet might have a magnetic moment of 0.1 A·m2. Iron can reach a magnetization of around one million amperes per meter, which explains why iron magnets are so effective.

Today, magnetism is integrated into many complex technologies. Electromagnets are a primary example. These are made from a coil of wire that acts as a magnet only when an electric current passes through it. If the current stops, the magnetism stops. These are often wrapped around a "soft" ferromagnetic core, like mild steel, to greatly enhance the field. This principle is used in various tools and devices, ranging from industrial ore separators to the components found inside a hard disk drive.