Tensile stress σ is given by:
σ = T/A
where T is the load and A is the cross-sectional area.The unit of tensile stress is the pascal (Pa) which is also equal to a Nm-2.The extension of a wire can be measured using Searle's apparatus or a similar apparatus using a vernier scale.In an experiment, two vertical wires P and Q are suspended from a fixed support.The fixed part of a vernier scale is attached to P and the moving part of the scale is attached to Q.The divisions on the fixed part of the scale are in mm.An empty mass hanger is attached to Q and the scale is set to zero.A load is added to the mass hanger so that the extension of Q can be measured as shown in the diagram.