Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Crossed wires - electrical fault-finding

Class practical

A problem-solving activity, using the concept of resistance in the context of a concert sound system.

Apparatus and materials

For each student group

Fault-finding board (see technical note)

Ammeter, 0-1 A

Voltmeter, 0-10 V

Power supply, low voltage, DC

Leads, 4 mm, 5

Health & Safety and Technical notes

Read our standard health & safety guidance

The fault-finding board simulates a short-circuit in the lead (a pair of wires) from the amplifier to a remote loudspeaker.

A board about 60 cm long and 10 cm wide carries, on one side, two 45 cm lengths of resistance wire (e.g. Eureka, SWG 34). The resistance of each wire should not be less than 3 Ω.

Each of the wires terminates in terminals, providing electrical access from the other side of the board.

Two-thirds of the way along the wires, a soldered link is provided. (You could make a number of boards with wires of different resistances, crossing at different points.) Since Eureka wire is difficult to solder, it may be helpful to twist the wires together first.

The wires have a resistance of about 3 Ω (end to end) and the 'fault' is so placed that, provided the voltage is less than 2, the maximum current is 1 amp.

Label the ends of the boards ‘stage’, ‘speaker 1’ etc. This will also make it easy to refer to results for different boards.


a Using an ammeter and voltmeter, measure the resistance of the wires from each of the two ends, and hence, determine the position of the fault. 

Fault finding board


Teaching notes

1 The ‘fault-finding board’ has two wires representing long wires connecting the amplifier in a sound system to remote loudspeakers. Unfortunately, they have touched at some point along their path. The students' task is to make suitable measurements which will allow them to deduce the point at which they have crossed. 

This is a problem-solving activity using ammeters and voltmeters and an understanding of resistance. Students can make measurements of resistance at the stage end and at the loudspeaker end, and from the relative values infer how far away the fault is.

2 Some may ask about the obviously more likely and more difficult problem, which is an open break in one of the wires. This kind of break is much harder to locate, but it can be done with capacitance measurements.

This experiment was safety-tested in October 2006