A brief discussion of 2 of those instruments, namely the pendulum and the gramivmeter, are addressed below. The magnitude of g is the acceleration of the Earth. The pendulum was the earliest device used for measuring acceleration in gravity and changes in gravity force. They consist of a measured weight on a spring scale. The weight represents the Earth's gravity, since g is contained in the definition of the weight.
Gravimeters respond only to the vertical component of g of an anomalous mass.
They can measure extremely small variations in the acceleration of gravity and have a sensitivity of about 0.01 milligals . Unfortunately, gravimeters suffer from 'drift', ie. This results from imperfect elasticity of the springs. However, it can be monitored by repeated meter readings at a fixed location throughout the day. Station spacing used in a gravity survey can vary depending on the scale of investigation.