The space charge grid tube was the first type of tetrode to appear. In the course of his research into the action of the "audion" triode tube of Lee de Forest, Irving Langmuir found that the action of the heated thermionic cathode was to create a space charge, or cloud of electrons, around the cathode. This cloud acted as a virtual cathode. With low applied anode voltage, many of the electrons in the space charge returned to the cathode, and did not contribute to the anode current; only those at its outer limit would be affected by the electric field due to the anode, and would be accelerated towards it. However, if a grid bearing a low positive applied potential (about 10V) were inserted between the cathode and the control grid, the space charge could be made to extend further away from the cathode. This had two advantageous effects, both related to the influence of the electric fields of the other electrodes (anode and control grid) on the electrons of the space charge. First, a significant increase in anode current could be achieved with low anode voltage; the valve could be made to work well with lower applied anode voltage. Second, the transconductance (rate of change of anode current with respect to control grid voltage) of the tube was increased. The latter effect was particularly important since it increased the voltage gain available from the valve.