Abstract: An extensometer for measuring dimensional change comprises a pair of adjacent conductors (10) helically wound to form a bifilar one layer coil so as to have substantial elastic deformation. The coil is embedded in dielectric material (13) capable of similar elastic deformation whereby the extensometer has a large dynamic range. Elongation or deformations of the extensometer cause a change in electrical capacitance between the conductors which change in capacitance is monitored to measure the elongation or deformation. The conductors may have a rectangular cross-section with rounded corners to improve linearity of the graph of capacitance versus elongation. The extensometer has a length which is at least an order of magnitude greater than the cross-sectional dimension thereof. A method of constructing the extensometer includes forming the coil on a mandrel (11) removing the mandrel and immersing the coil in a container of liquid dielectric material until the liquid cures.