How fuses work
Model a fuse using fine strands of steel wool. Too large a current causes the steel fibres to heat and melt, breaking the circuit.
Apparatus and materials
Cells, 1.5 V, with holders, 3
Lamp with holder
Leads, 4 mm, 7
Variable resistor or rheostat
Crocodile clips, 4
Steel wool (wire wool), grade 2
Ammeter, 0-1 A
Health & Safety and Technical notes
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 simple circuit as shown, with three cells, a lamp, an ammeter and a variable resistor in series. Include two crocodile clips.
b Before completing the circuit, connect a 5 – 8 cm length of one strand of the steel wool between the clips. Place this part of the circuit on a heat-resistant mat.
c Complete the circuit. The current through the wool should cause it to heat up; it may even catch fire. Reduce the variable resistance to increase the current.
d You can also reduce the resistance by removing the bulb from the circuit.
1 If the steel wool does not fuse, it can be used to show in a graphic way the variation in resistance with temperature. Blowing on the wire is sufficient to reduce its resistance and increase the current very considerably.
2 Holding the wire between the fingers has the same effect, and gives the students a clue as to what is happening.
3 By sliding the crocodile clip down the steel wool, a point will be reached when it fuses quite spectacularly. If the lamp is not removed, it may fuse first!
4 You could attach a small lump of wire wool between the two crocodile clips (which are placed together). A small 'firework' display ensues. Take care to do this behind a safety screen.
This experiment was safety-tested in April 2006