# Modelling a resistive survey

##### Class experiment

This activity models an archaeological survey (as students in the UK might have seen in TV programmes such as Time Team).

They learn that the resistance between two points depends on the shape and resistivity of the material(s) with which they are in electrical contact.

#### Apparatus and materials

For each student group

Model 'field' (see technical notes below)

Resistance meter (or ammeter, voltmeter and low voltage power unit or cell)

Connecting probes (e.g. clean 4 mm plugs), 2

Graph paper (A4 sheets)

Computer running a spreadsheet/graphing package (optional)

#### Health & Safety and Technical notes

To make the model 'field' you need:

• hardboard sheet, about 30 cm square, preferably with a grid of ready-drilled holes with a spacing of about 2 cm (or drill your own grid)
• Teledeltos paper, same size as hardboard
• metal cooking foil strip a few centimetres wide
• cork tile, same size as hardboard or slightly larger
• strips of wood to make frame

Cut a piece out of the Teledeltos paper to represent a buried wall. The exact shape does not matter, but it needs to be at least 5 cm wide in any dimension.

Glue a strip of metal onto the Teledeltos paper to represent a buried waterlogged ditch.

Glue the Teledeltos paper onto the cork tile.

Construct the model as shown in the diagram.

#### Procedure

a Connect the probes to the meter(s).

b Measure the resistance between pairs of holes in the grid. This is best done by a systematic survey, e.g. by moving the pair of probes along two parallel rows of holes.

c Display the results on graph paper and/or using a computer graphing package.

d Identify regions of anomalously high and low resistance and hence locate the hidden wall and ditch. Discuss the factors that determine the resistance measured between a pair of probes.

#### Teaching notes

1 This is intended as a 'fun' introduction to the idea of resistivity and the dependence of resistance on sample dimensions.

You will probably want to follow this activity with further activities or demonstrations, in which students explore the resistance of simple shapes and carry out calculations.

2 The resistance measured between two probes depends on the resistivity and shape of the underlying material. A uniform sample of length l and cross-sectional area A has resistance R such that

R = ρl / A

where ρ is the resistivity of the material.

This experiment comes from Salters Horners Advanced Physics ' University of York Science Education Group.

Diagrams are reproduced by permission of the copyright holders, Heinemann.

This experiment was safety-checked in January 2005