The Venturimeter (see figure) is a device used to calculate the flow speed of a fluid in a pipe. At the throat, the area is decreased from A to a and the velocity is increased from v to V. Noticed that at the throat, where the velocity is greatest, the pressure is least, as Bernoulli's equation predicts. This seems quit proper, due to the pressure difference is in the correct direction to speed up the flow. That is, a particle of fluid entering the throat from the left will be accelerated to the right by the pressure difference among the tube and the throat.