The reference surface that normal heights are measured from is called the quasi-geoid, a representation of " mean sea level " similar to the geoid and close to it, but lacking the physical interpretation of an equipotential surface.
32.
In 1919, the renowned physicist, Albert A . Michelson, noted that the most favorable arrangement to obtain high sensitivity and immunity from temperature perturbations is to use the equipotential surface defined by water in a buried half-filled water pipe.
33.
The gradient of the scalar potential ( and hence also its opposite, as in the case of a vector field with an associated potential field ) is everywhere perpendicular to the equipotential surface, and zero inside a three-dimensional equipotential region.
34.
Specifically, the geoid is the equipotential surface that would coincide with the mean ocean surface of Earth if the oceans and atmosphere were in equilibrium, at rest relative to the rotating Earth, and extended through the continents ( such as with very narrow canals ).
35.
Gravity is perpendicular to the equipotential surfaces of the gravity potential, and in electrostatics and in the case of steady currents the electric field ( and hence the electric current, if any ) is perpendicular to the equipotential surfaces of the electric potential ( voltage ).
36.
Gravity is perpendicular to the equipotential surfaces of the gravity potential, and in electrostatics and in the case of steady currents the electric field ( and hence the electric current, if any ) is perpendicular to the equipotential surfaces of the electric potential ( voltage ).
37.
One can construct a simple model geopotential U that has as one of its equipotential surfaces this reference ellipsoid, with the same model potential U _ 0 as the true potential W _ 0 of the geoid; this model is called a " normal potential ".
38.
Once a clean, smooth geopotential field U has been constructed matching the known GRS80 reference ellipsoid with an equipotential surface ( we call such a field a " normal potential " ) we can subtract it from the true ( measured ) potential W of the real Earth.
39.
The actual shape of the equipotential surface is called the geoid and it is not quite ellipsoidal, because of the unequal distribution of mass of the continents, etc . And even more accurately than the equipotential surface, you really have to describe every single point, to account for every rock, hill, bump on the ground, etc . talk ) 17 : 41, 15 April 2010 ( UTC)
40.
The actual shape of the equipotential surface is called the geoid and it is not quite ellipsoidal, because of the unequal distribution of mass of the continents, etc . And even more accurately than the equipotential surface, you really have to describe every single point, to account for every rock, hill, bump on the ground, etc . talk ) 17 : 41, 15 April 2010 ( UTC)
