Many occasions occur when it is required to make recordings of one or more fluctuating physical quantities. When the measurements to be made, either of the same quantity at a number of positions or of various quantities at one position, .exceed quite a small number, continuous recording is not a practical proposition. In any event it may not be necessary. The frequency with which it would be desirable to record a particular measurement will depend on: (a) the expected rate of fluctuation of the quantity concerned having regard to smoothing that may be introduced by the primary sensing device and the recorder in use; (b) the magnitude of the expected fluctuations, again almost certainly modified by the apparatus; (c) the resolution available in a single measurement; (d) the accuracy to which a time mean value is required together with the time over which the mean is to be taken.