18. Operating Point on the Characteristic Curve By operating point on the characteristic curve is meant the point on the E c — I b curve of the audion about which the grid potential and space current vary. E c is the potential of the grid with respect to the filament. If a continuous negative potential such as Q Fig. 36 is applied, the point about which the variations occur is B. There are several reasons why it is preferable to have B as the operating point rather than A. If A is the operating point, the alternating voltage on the grid makes the grid become positive with respect to the filament during one-halt of the cycle, and during that time current flows from the grid to the filament within the audion. This current heats the grid and absorbs power. If the operating point is B, the grid does not become positive with respect to the filament at any time and it absorbs no power in varying the grid potential. By using a negative potential on the grid the audion requires no power for varying the grid potential.