The pressure tube anemometer (fig.
Is as great as on small surfaces, such as anemometer plates.
The pressure differences on which the action depends are very small, and special means are required to register them, but in the ordinary form of recording anemometer (fig.
In the tube anemometer also it is really the pressure that is measured, although the scale is usually graduated as a velocity scale.
The two great merits of this anemometer are its simplicity and the absence of a wind vane; on the other hand it is not well adapted to leaving a record on paper of the actual velocity at any definite instant, and hence it leaves a short but violent gust unrecorded.
The intensity of pressure was less than on a similiarly exposed anemometer plate.
The great advantage of the tube anemometer lies in the fact that the exposed part can be mounted on a high pole, and requires no oiling or attention for years; and the registering part can be placed in any convenient position, no matter how far from the external part.
The other forms of velocity anemometer may be described as belonging to the windmill type.
Anemometers may be divided into two classes, (1) those that measure the velocity, (2) those that measure the pressure of the wind, but inasmuch as there is a close connexion between the pressure and the velocity, a suitable anemometer of either class will give information about both these quantities.
As to anemometer pressures, it should be observed that the recorded pressure is made up of a positive front and negative (vacuum) back pressure, but in structures the latter must be absent or only partially developed.