ANEMOMETER (from Gr.
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.
The Robinson anemometer, invented (1846) by Dr Thomas Romney Robinson, of Armagh Observatory, is the best-known and most generally used instrument, and belongs to the first of these.
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.
Unfortunately, when Dr Robinson first designed his anemometer, he stated that no matter what the size:of the cups or the length of the arms, the cups always moved with one-third of the velocity of the wind.
The other forms of velocity anemometer may be described as belonging to the windmill type.
Probably a sphere would prove most useful for a pressure anemometer, since owing to its symmetrical shape it would not require a weathercock.
Lind's anemometer, which consists simply of a U tube containing liquid with one end bent into a horizontal direction to face the wind, is perhaps the original form from which the tube class of instrument has sprung.
The pressure tube anemometer (fig.
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.
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.
In the tube anemometer also it is really the pressure that is measured, although the scale is usually graduated as a velocity scale.
Approximately II% should be added to the velocity recorded by a tube anemometer for each 1000 ft.
Robinson published a number of papers in scientific journals, and the Armagh catalogue of stars (Places of 5345 Stars observed from 1828 to 1854 at the Armagh Observatory, Dublin, 1859), but he is best known as the inventor (1846) of the cup-anemometer for registering the velocity of the wind.
Anemometer observations show that pressures of 30 lb per sq.
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.
Is as great as on small surfaces, such as anemometer plates.
The intensity of pressure was less than on a similiarly exposed anemometer plate.