v M is the voltage across the discharge tube, i M is the current flowing through it and n e is the number of conduction electrons. A simple memristance function is R ( n e ) = F n e . α α , β β and F are parameters depending on the dimensions of the tube and the gas fillings. An experimental identification of memristive behaviour is the "pinched hysteresis loop" in the v − − i plane. For an experiment that shows such a characteristic for a common discharge tube, see "A physical memristor Lissajous figure" (YouTube). The video also illustrates how to understand deviations in the pinched hysteresis characteristics of physical memristors.