Relativistic electron-beam nose-erosion techniques have been employed to produce an electron beam with variable pulse shape and bremsstrahlung capability (dial a pulse). This capability has been employed to excite a large number of electromagnetic fields inside a canonical cavity. Electron-beam and bremsstrahlung pulse-shape parameters have been varied to produce changes in the electromagntic cavity response. For example, generic cavity test parameters such as displacement currents or conduction currents can be emphasized or de-emphasized. A theoretical interpretation of these electromagnetic excitations is presented.