The anode characteristic of a screen-grid valve is thus quite unlike that of a triode. It includes a range of anode voltages, where the anode voltage is less than that of the screen grid, in which there is a distinctive negative resistance characteristic, sometimes called "tetrode kink". This is usually undesirable, although it can be exploited as in the dynatron oscillator (Eastman, p 431). The approximately constant-current region of low slope at high anode voltage is also a distinctive characteristic of the tetrode. This is highly desirable, since it corresponds to a high source resistance in the anode circuit, and greatly enhances the voltage gain which the device can produce. Early screen-grid valves had amplification factors (i.e. the product of transconductance and anode slope resistance) ten times that of comparable small-signal triodes. The high anode resistance (i.e. low slope) in the normal operating range is a consequence of the electrostatic shielding action of the screen grid, since it prevents the electric field due to the anode from penetrating to the control grid region, where it might otherwise influence the passage of electrons, increasing the electron current when the anode voltage is high, reducing it when low.