

Circuit with extra resistance

Class practical

Introduces the idea of a variable resistor, or rheostat.

Apparatus and materials

For each student group

Crocodile clips, 2

Cells, 1.5 V, with holders, 2

Lamp with holder

Ammeter (0 - 1 amp), DC

Carbon resistor e.g. 1 W 3.9 ohms

Variable resistor or rheostat e.g. 3 W 25 ohms

Wire-wound resistor e.g. 3.9 ohm 3 W or 5 W

Eureka wire 34 SWG, 1 m length

Leads, 4 mm, 5

Diode or LED

Health & Safety and Technical notes

Read our standard health & safety guidance

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator, causing a 'short circuit'. This discharges the cell rapidly and can make it explode. The risk is reduced by using 'low power', zinc chloride cells not 'high power', alkaline manganese ones.


a Set up a circuit of two cells and a lamp.

Circuit of two cells and a lamp

b Make a gap in the circuit, and connect a length of Eureka wire between two crocodile clips in the gap.

c Change the length of wire between the clips and observe the effect on the lamp.

Change the length of wire

d In place of the wire, connect first the fixed resistor and then the variable resistor. Note what you observe. Reverse the connections to the resistor. Can you feel any difference between the two resistors?


e With the variable resistor, make the lamp both brighter and dimmer.

f In place of the resistor, insert a diode and note what happens when the connections to the diode are reversed. Also try placing the diode at different positions in the circuit.


Teaching notes

1 This is an opportunity to introduce students to the concept of resistance, though perhaps not yet as the ratio V/I. 

2 The variable resistor, or rheostat, shows how the brightness of a lamp can be varied. It is helpful if you indicate the path of the current through the resistance wire. (This may involve dismantling a rotary rheostat.)

3 Resistance is a word that comes from the water-flow analogy. That idea was so strong in Ohm’s mind when he started his researches that he said that he was looking for ‘electrical resistance’ and trying to find its properties. When it was found that metal wires give a constant ratio for V/I, that constant was given the name resistance which Ohm had ready for it.

In contrast with this case, the order of events in most physical discoveries is the other way round. Scientists first discover experimentally that some ratio has a constant value and then coin a name for it because it is constant (e.g. stress/strain = the Young Modulus).

In class, this logical order sometimes gets obscured. Some students grab the name of the constant and take it for granted that the name itself assures the constancy, and thus takes away any need for experimental investigation. Then the practical experiment becomes a scheme for making one accurate measurement of that assured constant, instead of an interesting investigation to see what relationship is there. Of course there is nothing wrong in making these measurements: each has its own importance in physics but an organized series of such experiments can give beginners a wrong headed picture of science.

4 In fact, when you compare a wide range of materials, metals are remarkable for their conductance (how easily current flows through them). Conductance is the inverse of resistance.

5 The carbon resistor allows a current to flow in the circuit whichever way it is connected into the circuit. The diode only allows the current to pass in one direction. (LEDs light up and are more fun.)

6 Students could repeat all the experiments with an ammeter in the circuit and observe how it steadily changes as the resistances are altered.

This experiment was safety-tested in April 2006


Related guidance

Working with simple electrical components


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