Inside a piezoelectric version, the sensing element is a crystal which has the property of emitting a charge when subjected to a compressive force.
In the accelerometer, this crystal is bonded to a mass such that when the accelerometer is subjected to a 'g' force, the mass compresses the crystal which emits a signal. This signal value can be related to the imposed 'g' force.
The sensing element is housed in a suitable sensor body to withstand the environmental conditions of the particular application. Body are usually made in stainless steel with welding of the various parts to prevent the ingress of dust, water, etc.
Electrical connection can be via a sealed cable or a plug/socket arrangement.
Many present accelerometers have internal electronic circuitry to give outputs which can be directed used by the associated acquisition or control systems.
Mechanical fixing of the sensor is important in order to achieve true transfer of the vibration or acceleration. Many fixing methods are used including beeswax, hard glues, threaded stud (male or female), magnetic mounts.
Accelerometers are used in many scientific and industrial applications such as predictive maintenance, aerospace, automotive, medical, process control, etc.