Results are presented for the design and testing of an electromagnetic device to convert ambient mechanical vibration into electricity. The design of the device is based on an L-shaped beam structure which is tuned so that the first two (bending) natural frequencies have a (near) two-to-one ratio. This creates an internal resonance or autoparmetic condition that can result in a nonlinear dynamic response to a sinusoidal (base) displacement excitation over an extended frequency range. This is in contrast to single degree-of-freedom, linear-dynamics based vibration harvesters which convert energy in a very narrow frequency band. Representative measurements of displacement and power generated are presented. The problem of fatigue failure in the devices presents limits to their long-term operation.