How a metal detector works

Electronic metal detectors use the principle of electromagnetic induction. This means that, if an object __________ (to place) in a changing magnetic field, an electrical voltage ____________ (to create) in that object. In a metal detector, an electrical current ____________ (to pass) through a coil of wire, called the search coil, to create a magnetic field. The DC from the battery ___________ (to convert) into the AC needed to drive the coil with the help of an AC generator. Since AC regularly reverses the direction, the necessary ever-changing magnetic field ___________ (to produce).

Currents _____________ (to generate) in a metal object that comes within the magnetic field by a process known as induction. When a current __________ (to induce) in a metal object (e.g., a buried coin), it in turn produces its own magnetic fields. These magnetic fields are capable of inducing a small amount of electricity in the detector’s search coil itself.

The simplest kind of metal detector is the pulse induction type. A powerful current _____________ (to pass) from the battery through the search coil and then switched off. The pulse of magnetism causes currents to flow in any target objects below the ground. But, unlike the current in the search coil, the current in the object cannot ___________ (to switch off); it has to die away naturally. As it fades, the current in the object reactivates the search coil. This voltage ________ then _________ (to amplify) to indicate with a sound or a flashing light that an object has been found.

2.8 Answer the questions

1. What does the principle of electromagnetic induction mean?

2. What is an electric current passed through a search coil in a metal detector for?

3. What device helps to transform DC into AC?

4. When is the changing magnetic field produced?

5. How are the currents generated in a metal object? Describe the process.

6. Why is the simplest metal detector called the “pulse induction detector”?

7. When is the search coil reactivated?

8. How can we learn that an object has been found?

2.9 Fill in the scheme with the sentences given below to describe the work of a metal detector

1. Current flows in an object underground. 5. Current leaves the battery.

2. Current in the object reactivates the search coil. 6. Current in an object fades.

3. Current passes through the coil of wire. 7. Voltage increases.

4. A sound or a flashing light indicates the object found. 8. Current is switched off.

SPEAKING

3.1 Put yourself in the position of the guide. Use the diagram, your notes and phrases from below to describe the whole process in your own words.

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