 Harries is believed to be the first engineer to discover the "critical distance" effect, which maximized the efficiency of a power tetrode by positioning its anode at a distance which is a specific multiple of the screen grid-cathode distance.This design also minimized interference of secondary emission electrons dislodged from the anode.EMI engineers Cabot Bull and Sidney Rodda improved the Harries design with a pair of beam plates, connected to the cathode, which directed the electron streams into two narrow areas, which caused the electron beams to act like a virtual suppressor grid to deflect some secondary electrons back to the anode.