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Aerial. Used to help receive radio waves. |
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Ammeter. A device used to measure the current
flowing through a certain point in a circuit.
See Measuring
Circuits for information on how to use an ammeter. |

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Battery. A collection of cells.
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Capacitor. A device with two plates separated
by an insulting layer. One simple kind is aluminium foil separated
by waxed paper. The plates build up a charge when electricity flows,
but when the plates are fully charged the capacitor prevents any further
flow of electricity.
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Some capacitors are polarised, and have to be connected the right
way around.
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Some capacitors are variable.
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Cell. A single unit of a battery. The positive
terminal is the longer line. The negative terminal is normally drawn thicker than the positive (unless drawn by hand).
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Diode. A semiconductor device that lets current
through in one direction but not the other. Also see light emitting diode.
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Light emitting diodes (LEDs) are a special kind of diode that
emit light when operating.
Diodes have a higher forward voltage than standard silicon diodes, and the shorter the wavelength of light they produce, the more voltage they require to make light. Hence, blue (and white) LEDs need more voltage than red LEDs. Also see LEDs. |
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Earth. Also called ground. The symbol
saves having to draw in the negative terminal or rail of a circuit,
meaning many diagrams can be drawn much simpler.
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These two circuits are identical. Car wiring diagrams in particular
use this method, since the whole body of the car is the earth
and is connected straight to the negative terminal of the car battery
with a thick wire.
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Fuse. A wire (normally) which will burn out
if too much current is passed through it. Each fuse has a rated current,
which if exceeded will melt, breaking the circuit. There are several
ways of drawing fuses.
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Inductor. Also called a coil. If a current starts
to flow it tries to resist its flow. If a current stops flowing it
tries to keep it going. Thus it tries to maintain the status quo.
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Lamp – indicator. A light bulb simply used and
an indicator if something is working.
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Lamp – lighting. A light bulb used to give illumination.
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LED. Acronym for light emitting diode, a particular kind of diode.
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Loudspeaker. A device which creates sound from
an input electrical signal.
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Ohmmeter. A device used to measure the resistance
of a component (in isolation).
See Measuring
Circuits for information on how to use an ohmmeter. |
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Relay. A kind of switch operated by an
electromagnet. Contacts on the switch side are normally open, common, and normally closed.
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A relay can switch just one thing on, or switch between two things – called a crossover relay. |
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A relay can switch between lots of things at the same time. This one is a double pole double throw relay. |
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Resistor. Something that resists the flow of
electrons. This resistance is dissipated as heat. A light bulb is
a resistor that gets so hot it glows. Also, the resistance of a light
bulb increases as it gets hotter.
The traditional symbol was a zig-zag
line, which dates back to the days when a resistor was made from a
long piece of wire wrapped in such a manner as to not produce inductance
(which would have made it a coil, or inductor). |
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Wirewound resistors are now used only in high-power applications,
with smaller resistors being cast from carbon composition (a mixture
of carbon and filler) or fabricated as an insulating tube or chip
coated with a metal film. To illustrate this, European circuit
diagrams have replaced the zig-zag symbol by a simple oblong,
sometimes with the value in ohms written inside. |
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A resistor can be variable. There are different ways of drawing
this, with either two or three connections. |
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A three connection
variable resistor is called a potentiometer. |
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A light dependent resistor is sensitive to light. Its resistance decreases when light falls on it because the light triggers the release of electrons, which carry current. The more light, the more electrons are available for carrying current, hence the lower its resistance. |
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Solenoid. A long thin inductor (coil) which uses the
magnetic field to move an iron rod in the middle of it. They can be
used to steer radio controlled cars or ring doorbells, and lots of
other things. Compare with a transformer or a relay.
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Switch. A mechanical device to turn power on
and off. There are lots of different sorts of switches. The simplest
are push button (momentary) and on-off (single pole, single throw)
switches.
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An example of a more complex switch is a double pole, double
throw, centre off switch, which I used at one point on my caving headlamp. It
switches two things at the same time, which each have two on positions
and one off position.
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Thermistor. A resistor which changes resistance
depending on temperature. Ordinary
resistors are designed to change resistance as little as possible
with changes in temperature.
A thermistor may have a positive temperature
coefficient (PTC) or a negative temperature coefficient (NTC). NTC thermistors are the most commonly used in the Cambridge Exams syllabus. Their resistance decreases as they get hotter. |
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Transformer. Two coils wrapped around an iron
rod or core. A current through one coil causes a current to flow through
the other due to the magnetic field created by the first current.
Different numbers of loops in the two coils mean that a different
voltage can be obtained in the second coil. |
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Transistor. A semiconductor device with
three leads. It allows a large current to flow across two of its leads
when a small current flows across a different two leads (one shared,
obviously).
They are very useful for amplifying small signals into
large signals, for example in a radio receiver amplifying the radio
signal or in an audio amplifier making a sound signal louder.
Transistors
are either NPN (with the arrow pointing out) or PNP (with the arrow
pointing in). The arrow helps show which way current flows through
the transistor. |
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Voltmeter. A device used to measure the potential
difference (voltage) between two points in a circuit.
See Measuring
Circuits for information on how to use a voltmeter. |