Arbor Scientific
PO Box 2750
Ann Arbor, MI 48106
Variable Resistance
Suggested Time: One Class Period
Introduction
This chapter uses the Genecon and a few pieces of basic equipment to study the phenomenon of
electrical resistance, how this resistance can be varied, and some specific applications for devices which
operate by varying resistance to an electrical circuit or parts of that circuit.
The Basics
Devices which are able to alter the resistance (and thus alter the amount of current flowing) through a
certain part of a circuit or across one or more components of a circuit are called variable resistors.
Variable resistors can be found almost everywhere in electronics. The volume and tone controls of your
TV or stereo, for example, are variable resistors. Part of the circuitry in some dimmer switches, which
can vary the light intensity of a whole room, includes variable resistors.
In your kit you will find a length of nichrome wire, a special kind of metal which has relatively high
resistance to current flow. Variations in current will be obtained by passing current through different
lengths of this wire. Alternately, you may use a thick pencil lead (graphite or carbon), either by carefully
dissecting a pencil or using thick replacement leads for a mechanical pencil. Thin leads are too fragile
and will probably break if used.
The following two activities explore the applications of two types of variable resistors - rheostats and
potentiometers. Be sure to complete your discussions of series and parallel circuits before beginning
these exercises.
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Arbor Scientific
PO Box 2750
Ann Arbor, MI 48106
Suggested Activities
Rheostats
A rheostat varies current flow by varying the resistance of an entire circuit. In a rheostat, the variable
resistance is connected in series with the load. To set up a simple rheostat, arrange your Genecon, a
bulb base, jumper wires, and length of nichrome wire as illustrated on page 12. Turn the handle of the
Genecon at as constant a rate as possible (practice!) while your lab partner slides the connector labeled
“A” up or down the nichrome wire. As you might expect, the resistance increases as the length of the
nichrome wire used increases. What happens to the brightness of the bulb during these changes in wire
length? Where is it easiest to turn the Genecon handle? Where is it the hardest?
Potentiometers
A potentiometer uses a variable resistor to change the voltage across a load, that is, the connection of
the resistor is in parallel with the load. Arrange the components of your circuit as shown below. Note
that the bulb, connected in series for the rheostat, is now connected in parallel. Again crank the
Genecon as uniformly as possible while a partner slides the clip lead “A” up or down the nichrome wire
or pencil lead. Where is the bulb brightest - with a short or long length of resistance? Where is it easiest
to turn the handle? Hardest? What affects the voltage across the bulb? Can you determine at which wire
position the energy being used by the bulb is greatest?
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