Currents and conductors

Class practical

An opportunity for students to discover materials which will control the current.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 3
• Lamp with holder
• Crocodile clips, 2
• Leads, 4 mm, 5
• Miscellaneous materials

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.

Read our standard health & safety guidance

Miscellaneous materials could include:

• Stick of wood
• Strip of paper
• Strip of copper
• Thread of nylon
• Aluminium foil
• Pencil lead (preferably a soft grade)
• Bits and pieces from students’ pockets

Procedure

1. Connect up a circuit as shown.

2. In turn, insert samples of different materials in the circuit by clipping them between the crocodile clips.
3. Make lists of those which will carry current, and those which will not.

Teaching Notes

• This is an opportunity for you to offer your students many different components for them to investigate. Besides the variety of materials listed, it is possible to use diodes, light emitting diodes (LEDs), capacitors, motors and even transistors. However, it is wise to test the available components first: a determined class could destroy the stock of low-current diodes very quickly!
• The current passing through the material or component can be indicated by the brightness of a lamp or it can be measured with an ammeter. The variation in the conductance of the materials can be recorded. Include materials which have such a low conductance that they may be loosely termed insulators, at least for the voltage and sensitivity of the ammeter.
• You could raise the question of ‘fair testing’. Does this method give a fair comparison between different materials? (No; the samples should have the same dimensions for a fair comparison.)

This experiment was safety-tested in February 2005