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Flexible pencils, which can be bent into many shapes can be used to demonstrate how a resistance strain gauge works. The pencils are made of elastic graphite covered by elastic plastic casing. The graphite is a composite material made of graphite powder and a binding substance. The resistance of the graphite in a 30 cm long pencil is about a dozen k.
1. Take one flexible pencil. Cut off the rubber mounted in metal together with the sharpened writing end. Then remove 1 cm off half of the plastic outer casing from each end, so that the upper part of the graphite is exposed.
2. Connect the pencil to an ohmmeter with crocodile clips (make sure it is a good connection) and measure the resistance of graphite in a straight, unbent and non-stressed pencil.
3. Stretching, bending and compressing the pencil will result in a change of resistance. For all kinds of deformation, except compression, the ohmmeter shows resistance increasing from a dozen to around twenty k. In the case of compression, a decrease of resistance occurs.
The greater the stress put on the pencil by deformation the greater the level of resistance.
If the deformation and stress are reduced by straightening the pencil the resistance of
graphite returns to its initial value.
Based on an article by
S Bednarek From Physics
Education Volume 40 Issue 1
Themes: Electricity, materials, structures, forces

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