1. A variable area capacitor comprising: a. a flexible electrode responsive to a physical effect; b. a rigid electrode with a three-dimensional surface contour facing said flexible electrode; c. a dielectric spacer sandwiched between said flexible electrode and said rigid electrode; d. said dielectric spacer maintains a substantially fixed capacitance spacing between regions of mutually opposed areas of said rigid electrode and said flexible electrode, said regions of said fixed capacitance spacing increasing as said flexible electrode deflects in response to said physical effect; e. said surface contour having a shape that provides a substantially linear change in capacitance of said variable area capacitor in response to said physical effect.