A device that responds physically to a change in a parameter (such as vibration, temperature, strain etc.) can be placed so as to retrieve useful information. While it is possible to use many different types of sensor, and transduce the varying parameter into a digital form, the main workhorse of the STRAAM family is a type of accelerometer that is the most sensitive measuring device available. These servo-type accelerometers were developed for military purposes, but are particularly useful for civil engineering because of their sensitivity and precision. This means that even a very small force acting on a structure gives enough movement that the accelerometers will distinguish that movement. In practical terms this means that movements that are much smaller than can be seen are easily measurable.