Via di Pietra Papa 00146 Roma
Currie Electronics
P.O. Box 3
Patten
Essex
SS2 3MQ
UK
17 April 19___
Dear Sir
Please could you supply the following components: Description Quantity Price each Total £/p
BD140 transistors 8 26p £2.08
dielectric trimmer
SW trim 10 pP 1 £6.45 £6.45
Handling charge £1.00
TOTAL £9.53
I enclose a bank draft(no.1563526)for nine pounds and fifty-three pence.
lours faithfully
Rino Rumiati
Now write your own letter ordering transistors, using the information given in Table 1 on page 63.
12
Metal detector
Task 1
Tuning-in
Study this newspaper headline. What do you think the story will be about?
£1/2 M TREASURE FOUND BURIED IN SCOTTISH FIELD
Task 2
Task3
1
2 3 4 5 6 7
1 2
Now listen to a radio news item which refers to the same story and answer these questions:
How did Mr Swanston find the coins?
How does he know when his detector has found something made of metal?
Why did he ask his friends to help?
How many coins did they find?
How old are the coins?
What will happen to the coins?
Who will benefit from this?
Read the short text below, then discuss these questions:
How can you create a changing magnetic field?
How can you detect a voltage created in a buried object?
Metal detectors, despite their technical complexity, are based on a few very simple principles. The most important is that of electromagnetic induction. This means that if an object is placed in a changing magnetic field, an electrical voltage is created in the object.
Task 4
Check your answers to Task 3 with the help of the text and diagrams.
10
The current induced in the metal object produces its own magnetic field, which in turn induces a voltage in the search coil, as the alternating current changes direction.
The circuitry in the control box senses this reaction and converts the voltage into an audible note, which is sent to the headset. As the metal object is approached, the sound in the headset becomes louder, or changes pitch.
Task 5
Label each step in this flowchart with the correct letter from the list below. The first one is done for you.
2
3 I
4
a Magnetic field around the object
b AC voltage in the search coil
с Electric current in the metal object
d Induced voltage in the search coil
e Note heard in headset
f Magnetic field around the coil
Reading Linking what you read with what you know
Not everything in a text is clearly stated. When reading, we have to make links between 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 6 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.
Any metal detector can discriminate between gold and other metals.
Gold necklaces are found quite often.
The search coil is connected directly to a battery.
Metal detectors require a changing magnetic field.
The metal detector can only locate metals which contain iron or are magnetic.
Metal detectors are only used by treasure hunters.
Passing a current through the search coil and then switching it off, creates a pulse of magnetism round the coil.
All metal detectors are fitted with a flashing light to show when an object has been found.
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.
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 5 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 10 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 15 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.
20 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