associated gravitational potential. The gradient of this potential is a force. A gravimeter measures this gravitational force. The first gravimeters were vertical accelerometers, specialized for measuring the constant downward acceleration of gravity on the earth's surface. The earth's vertical gravity varies from place to place over the surface of the Earth by about ±0.5%. It varies by about (nanometers per second squared) at any location because of the changing positions of the sun and moon relative to the earth. The change from calling a device an "accelerometer" to calling it a "gravimeter" occurs at approximately the point where it has to make corrections for earth tides. Though similar in design to other accelerometers, gravimeters are typically designed to be much more sensitive. Their first uses were to measure the changes in gravity from the varying densities and distribution of masses inside the earth, from temporal "tidal" variations in the shape and distribution of mass in the oceans, atmosphere and earth. Gravimeters can detect vibrations and gravity changes from human activities. Depending on the interests of the researcher or operator, this might be counteracted by integral vibration isolation and signal processing. The resolution of gravimeters can be increased by averaging samples over longer periods. Fundamental characteristics of gravimeters are the accuracy of a single measurement (a single "sample"), and the sampling rate (samples per second). for example: Gravimeters display their measurements in units of gals (cm/s2), nanometers per second squared, and parts per million, parts per billion, or parts per trillion of the average vertical acceleration with respect to the earth. Some newer units are pm/s2 (picometers per second squared), fm/s2 (femto), am/s2 (atto) for very sensitive instruments. Gravimeters are used for petroleum and mineral prospecting, seismology, geodesy, geophysical surveys and other geophysical research, and for metrology. Their fundamental purpose is to map the gravity field in space and time. Most current work is earth-based, with a few satellites around earth, but gravimeters are also applicable to the moon, sun, planets, asteroids, stars, galaxies and other bodies. Gravitational wave experiments monitor the changes with time in the gravitational potential itself, rather than the gradient of the potential which the gravimeter is tracking. This distinction is somewhat arbitrary. The subsystems of the gravitational radiation experiments are very sensitive to changes in the gradient of the potential. The local gravity signals on earth that interfere with gravitational wave experiments are disparagingly referred to