The Young modulus may be measured for a material in the form of a wire
using the apparatus shown in Figure 1.
Two identical wires are hung from a beam; a scale is
fixed to one wire and a mass hung on the end to remove kinks in it. This wire is used as a
reference standard. The other wire has a small load placed on it to straighten it and a vernier
scale which links with the scale on the reference wire.
The original length (L) of the test
wire is measured and its diameter is found for various points along its length and an average
diameter calculated. Hence its mean radius r can be found.
Loads are then placed gently
on the wire and the extension of the wire found for each one. They should not be dropped, as this
would subject the wire to a sudden shock. After each reading the load should be removed to check
that the wire returns to its original length, showing that its elastic limit has not been
exceeded.
A graph is plotted of stress against strain and from this the value of the Young
modulus may be found (this is the gradient of the line i.e. F/A divided by e/L).
The wires
should be long and thin to give as large an extension as possible for a given load while retaining its
elastic properties.
Two wires are used to eliminate errors due to changes of temperature and
sagging of the beam.