Reading. Linking what you read with what you know

Not everything in a text is clearly stated. When reading, we have to make links what we read and what we already know about the subject.

Read this brief text and try to answer this question:

Can metal detectors be used to trace water pipes under a street?

Metal detectors can find buried metal. They were developed for military purposes to locate hidden explosives. They operate on the principle of electromagnetic induction.

To answer the question, you have to link knowledge from the text and knowledge of your own.

From the text we know:

Metal detectors can find buried metal.

From your own knowledge you know:

Most water pipes are metal. They are buried under streets.

The more knowledge we have about our subject and about the world in general, the easier it is to learn new things when we read.

Task 5. Read the text below, then look at these statements.

Are they true or false? You may need to use your own knowledge

As well as information from the text.

1 Any metal detector can discriminate between gold and other metals.

2 Gold necklaces are found quite often.

3 The search coil is connected directly to a battery.

4 Metal detectors require a changing magnetic field.

5 The metal detector can only locate metals which contain iron or are magnetic.

6 Metal detectors are only used by treasure hunters.

7 Passing a current through the search coil and then switching it off, creates a pulse of magnetism round the coil.

8 All metal detectors are fitted with a flashing light to show when an object has been found.

9 Large objects are easier to find than small objects.

10 A coin horizontal to the surface is more difficult to detect than one vertical to the surface.

Text

A metal detector is essential for today's amateur treasure hunter. But only the most expensive detector can reveal the difference between worthless items, such as pull-ring tops from soft drink cans or silver foil, and a rare find such as the gold necklace discovered by one enthusiast last year.

Electronic metal detectors use the principle of electromagnetic induction. This means that, if an object is placed in a changing magnetic field, an electrical voltage is created in the object. In a metal detector, an electrical current is passed through a coil of wire, called the search coil, to create a magnetic field. An alternating current (AC) generator converts the direct current (DC) from the battery into the AC needed to drive the coil. As AC regularly reverses direction, it produces the necessary ever-changing magnetic field.

Currents are created in a metal object which comes within this magnetic field by a process known as induction. This is because all metals conduct electricity. When a current is induced in a metal object (for example, a buried coin), this 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 is passed from the battery through the search coil and then switched off. The pulse of magnetism causes current to flow in any target objects below the ground. But unlike the current in the search coil, the current in the object cannot be switched off; it has to die away naturally. As it dies, the current in the object reactivates the search coil. This voltage is then amplified to indicate with a sound or a flashing light that an object has been found.

The effectiveness of a metal detector depends on the size and position of the object and how far beneath the ground it is buried. For example, a coin buried edge-on to the search coil is much harder to detect than the same coin buried face up.

Task 6. Copy out unknown words and translate them.