I might be wrong, but accelerometers measure the force between the body of an accelerometer and some inertial mass. For example, a seismometer is a big lump of

I might be wrong, but accelerometers measure the force between the body of an accelerometer and some inertial mass. For example, a seismometer is a big lump of heavy on springs, and when the Earth moves, there is a force between the heavy and the frame holding it. And indeed, at rest, there is a "force" pushing the heavy mass down (whatever you call it). Drop the seismometer from space, both are falling at the same rate and there is no force and no recorded acceleration - fine. But wouldn't that be true for any force that acts with the same strength per unit mass on both the heavy weight and its frame? If I rig it up so there's an electrical field pushing these in an empty space, and it just so happens the force is imparting equal acceleration on both the weight and its enclosure, and no acceleration will be recorded. Or indeed, set the whole thing in free space, make the heavy weight magnetic, and attach a magnet to the side. There is now a force being recorded, but no acceleration - the whole thing is a rigid body. EDIT: in fact to simplify this even further, if I'm negatively charged, standing on some (non-gravitational) positively charged surface, I will feel a force just the same. If I charge the accelerometer, it will show an acceleration.