Anemometer use is constructed from a ping-pong ball attached to a string. When the wind blows horizontally, it presses on and moves the ball; because ping-pong balls are very lightweight, they move easily in light winds. Measuring the angle between the string-ball apparatus and the vertical gives an estimate of the wind speed. This type of anemometer is mostly used for middle-school level instruction, which most students make on their own, but a similar device was also flown on Phoenix Mars Lander. The first designs of anemometers that measure the pressure were divided into plate and tube classes. These are the
Gibbs measure property. Measures with this property are sometimes called Markov random fields. More strongly, the converse is also true: "any" positive probability distribution (nonzero density everywhere) having the Markov property can be represented as a Gibbs measure for an appropriate energy function. This is the Hammersley–Clifford theorem. What follows is a formal definition for the special case of a random field on a lattice. The idea of a Gibbs measure is, however, much more general than this. The definition of a Gibbs random field on a lattice requires some terminology: We interpret as the contribution to the total energy (the Hamiltonian)