An instrument is described which measures the displacement y of the arm of a pen recorder at regular intervals along the recording chart, whilst the chart is being produced. The instrument computes Σy, and Σy². Its computing section does not contain a squaring device, but produces the square of a given measurement by means of an interconnected system of Post Office relays. The quantities Σy, Σy², and n the number of measurements made, are displayed on Post Office type message registers. The quantities recorded facilitate the calculation of the mean and standard deviation of the pen arm displacement.