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Like capacitors, resistors come in many shapes and sizes
to suit the application. We've shown just a few examples.
Most electronics circuits use small "carbon resistors".
These are cylindrically shaped with wires coming from
either end. Usually they have colored bands which are
a code for what the value of the resistance is. They are
called "carbon" resistors because inside is
a small cylinder of carbon, the size and shape of which
determine the resistance |
| Power Resistors are typically lengths of a thick wire
wound around a ceramic tube in order to allow large currents
to flow without melting the resistor. They can get very
hot. An electric stove or heating element is a type of
power resistor designed to make use of the high heat generated. |
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Wire Wound Resistors, sometimes called "rheostats",
are made of a long length a wire wound into loops and
designed to allow you to choose any value of resistance
up to the maximum by tapping into the total length at
various places. Varying the length of the wire used, like
this, changes the total resistance according to:
Resistance = (resistivity) (LENGTH) Area
These "rheostats" can be round "donuts",
like what is used in volume control for radio (except
smaller), or long tubes with sliding contacts, as we showed
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The carbon plate resistor is a special kind of power
resistor. Its size allows large currents to be passed
through it. Rather than having a long a metallic wire
carrying the current, it is composed of many flat sintered
carbon plates squeezed together in a tray. If you squeeze
harder, you make more contact points between the carbon
plates. It's like taking a damp sponge and placing it
on a dry table --- it leaves a little bit of water behind
where it was touching. But if you press down, it makes
more contact and leaves more water behind. In the case
of the carbon plate resistor, since the length is essentially
constant, you are varying resistance by changing the cross
sectional area of the resistor.
Resistance = (resistivity) (length) / (AREA) |
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If you hook the carbon plate resistor in series with
an audio speaker, the speaker will sound louder or softer
as the resistor is squeezed or released. If squeezed,
the contact area of the plates becomes greater, the resistance
gets lower and the voltage drop across the carbon plate
resistor will decrease, allowing higher voltage to reach
the speaker, hence the sound is louder. This idea is how
a microphone can be made. In fact, Alexander Graham Bell
used this principle in his first telephone --- carbon
powder between two plates would have its resistance change
as sound waves caused the plates to vibrate and squeeze
the carbon in a varying fashion. Clever, huh? This technology
is still used in telephones today, being low cost and
easily mass produced. |
