Magnet Mining

A material that you can turn into a magnet is called a ferromagnetic material. Some examples are iron, steel, nickel, and cobalt. Once a ferromagnetic material becomes magnetic, it is able to attract other magnetic materials. A magnet always has two opposite poles, which are referred to as north and south. Similar or "like" poles will repel each other (push each other away). Opposite or "unlike" poles will attract each other (pull towards each other) as shown in Figure 1.

Figure 1. Diagram showing how magnets can attract and repel each other depending on the orientation of their poles. The north and south poles are labeled "N" and "S" respectively.

Each magnet produces a magnetic field, which is the area around the magnet where a magnetic force exists that can act on other magnetic materials. You can make a magnetic field visible by sprinkling iron filings on and around a magnet. The iron filings react to the magnetic forces and line themselves up along the magnetic field lines as shown in Figure 2. Magnetic fields are usually represented by such magnetic field lines. These lines describe the direction of the magnetic force within the magnetic field. You can measure the direction of a magnetic field with a compass. A compass contains a needle shaped magnet that can freely move on a balanced pivot point. When the magnetic needle is exposed to a magnetic field, it will line up with the magnetic field with its south pole pointing toward the north pole of the magnet and its north pole pointing toward the south pole of the magnet.

Figure 2. Picture showing the magnetic field lines of a bar magnet's magnetic field visualized with iron filings.

Magnetism is often used for navigational purposes. This is possible because the Earth itself creates a magnetic field which is caused by the flow of molten iron inside the Earth's outer core. Earth's magnetic field looks similar to that of a bar magnet with its magnetic north pole near the geographic south pole and the magnetic south pole near the Earth's geographic north pole as shown in Figure 3. The Earth's magnetic field acts on all metal objects on the Earth's surface. Thus, a compass will align itself in the Earth's magnetic field and show the way North or South.

Figure 3. Illustration visualizing Earth's magnetic field which resembles the magnetic field of a bar magnet.

However, Earth's magnetism has not been applied for navigational purposes only. In World War II, magnetism was used to create special undersea explosives called magnetic mines. These mines were first developed by the British and German navy after World War I. When the Germans started to employ these magnetic mines in 1939, they turned out to be devastating for the British navy. The key to these mines was that they had a special magnetic trigger mechanism. But why magnetic mines to destroy ships? A battle ship is usually made of lots of steel, which is a ferromagnetic material. This means that such a ship is like a huge floating magnet with a large magnetic field surrounding it. Part of this magnetic field is caused by the ship components that consist of hard iron. During ship construction, these parts become magnetized by the Earth's magnetic field, creating some amount of permanent magnetism in the ship hull. At the same time, all the ship components made of soft iron will become temporarily magnetized by the Earth's magnetic field, which results in an additional induced magnetism of the ship. As the ship moves through the water, its magnetic field will move with it and interact with any other magnetic field close by.

The magnetism of these battle ships was what triggered the magnetic mines. Their ignition mechanism was designed in a way that it only reacted to a magnetic field. One example is the so-called dip needle mechanism. Such a trigger works very much like a compass turned on its side. A magnetized needle is carefully balanced inside the firing circuit. It is aligned horizontally to the Earth's magnetic field and leveled so that it does not make contact with the rest of the circuit. However, as soon as a ships vertical magnetic field acts on this needle, it will dip up or down and make contact to close the loop of the firing circuit which triggers the mine, as shown in Figure 4.

Figure 4. Simplified schematic of the dip needle firing circuit of a magnetic mine.

This magnetic firing mechanism was very effective and responsible for a lot of destructed ships during World War II. This was until each side managed to develop technologies that could either mask the magnetic field of a ship or that allowed to trigger the magnetic mines from a distance. One of these technologies is called "degaussing". Degaussing a ship means reducing or eliminating the ship's magnetic signature by producing an opposite magnetic field. The opposite magnetic field was induced by letting a controlled amount of direct current flow through degaussing coils that were wrapped around the ship. The objective of these coils was to generate a magnetic field of equal magnitude and opposite direction that would cancel the ship's magnetic signature. With no magnetic signature the ships became undetectable by magnetic mines.

In this lesson plan, your students will simulate the search of magnetic mines represented by small ceramic magnets. With the magnets hidden from student's sight, they will use a mobile device with a built-in magnetometer to detect the hidden magnets' positions. A magnetometer is an instrument used for measuring magnetic forces and the strength of a magnetic field. Although such magnetic sensors were initially developed and used for navigation and tracking purposes, nowadays they are built into many consumer electronics such as mobile phones, tablets, or computers. With a built-in magnetometer, these devices can function as electronic compasses and can measure magnetic fields. Some magnetometers use the Hall effect discovered by Dr. Edwin Hall in 1879, to detect and measure magnetic fields. Hall discovered that magnetic fields have the tendency to deflect a moving charge as it flows through a conductor. This creates a voltage that can be measured by the sensor. The magnitude of the Hall voltage is proportional to magnetic field strength. Other magnetometers use so-called magnetoresistors that consist of special metal alloys that change their resistance in response to a magnetic field.

The magnetic field strength is measured in teslas (T). The stronger a magnet, the stronger is its magnetic field. Earth's magnetic field strength ranges from about 25 to 65 microteslas depending on the location on Earth, whereas ceramic magnets typically have a magnetic field strength of 0.5 to 1 tesla. Your students will use a mobile device and a sensor app with a magnetometer to map the location of hidden "mines" with the help of the device's built-in magnetometer. Based on their measurements, students will be able to derive the hidden magnet's magnetic field strength and in a second part of the lesson can explore ways of how to "cloak" their magnetized ships to make them less detectable.