The Lucas cell was investigated as an a-radiation detector for continuos measurement of radon concentration in the air with air sampling by means of a pump. The investigations indicate that all the short lived radon decay products produced inside the Lucas cell f 54x74 mm (0.17 l) are attached to the internal walls of the cell and are not removed when the cell is flushed with some fresh air. This effect, and the well known effect of increase of the radon daughters concentration when only radon is introduced into the cell, result in a delay of response of the gauge to variations of the radon concentration. The response of the gauge to step variation of radon concentration is computer simulated and measuring errors of the gauge are estimated. Multivariate processing of the measured signal using Principal Component Analysis technique can improve accuracy of the gauge.