Relative gravimeter; instrumental drift কী?

A relative gravimeter is an instrument that measures differences of gravity between stations: a mass on a spring, kept at a null position by a second, adjustable force whose dial reading, multiplied by a calibration constant, gives gravity in milligals up to an unknown constant. Its instrumental drift is the slow change of its reading at a fixed station, caused by the creep of the spring, temperature and shocks; it is measured by reading again, after a few hours, a station already read, the base station.