c the determination of whether or not the bottle 10 is defective will be described with reference to fig3 a generally vertical line r serves as a reference for measurements which are made by the lvdt 39 s 14 16 and 18 i e the outputs of the lvdt 39 s are zero when their feelers are at the line r the feelers of the lvdt 39 s 14 16 and 18 contact the bottle 10 at points a b and c respectively the lvdt 39 s 14 16 and 18 intersect the line r at points x y and z respectively the outputs of the lvdt 39 s 14 16 and 18 represent the distances xa yb and zc respectively these measurements may be utilized to determine whether or not a bottle 10 is defective a line from point a to point c passes through the line yb at a point d if the sidewall 11 of the bottle 10 were flat the points b and d would coincide by determining the distance db the variation of flatness of the sidewall 11 may be determined by triangulation the distance db may be calculated from the equation where m xz xy in the preferred embodiment of the invention the position of the lvdt 16 is adjustable and may be set to measure the bottle 10 at the most convenient location the adjustment feature is particularly useful if different types of containers are to.