In the first three sections, we have looked at devices whose usefulness is dependent on their form. In the case of the Pirani sensor, it was the dimensions of the microbridge that affected its sensitivity; in the AFM probe, its ability to resolve features on a surface is determined mostly by the form of the last few nanometres of its very tip. With devices the emphasis is not so much on the form of the structure as on how to make it move in the right way and, just as importantly, how to detect motion. This is of relevance to the AFM probes, too (we've already seen some dynamic aspects of this), as the optical method is not the only way to measure the tiny movements of the cantilever. This is just as well, as many of the most sensitive inertial devices are to be found inside family cars – a market that is notorious for demanding the best performance at bargain-basement prices, where carefully vibration-isolated, long-path-length optical arrangements have no place.