ANEMOMETER, an-e-mom'e-ter, an in strument for measuring the velocity of moving air or other gaseous substance. It is almost always combined with a recording device and thus becomes strictly speaking an anemograph, though it is popularly known by the former name. The instruments in use as anemometers are of five distinct types: (1) The pendulum or Brooke anemometer; (2) the cup or Robinson anemometer; (3) the °windmill° anemometer; (4) the U-tube or Dines anemometer; and (5) the electrical anemometer.
The first consists essentially of a flat plate of definite area, usually one square foot, ar ranged so as to swing after the manner of a pendulum, by the pressure of the wind upon it. It is used chiefly in the determination of wind pressure upon buildings, bridges and other structures, although this pressure is translated into velocity by a suitable scale. The Robinson anemometer is described at length below. The °windmill° type is simply a minia ture windmill very lightly built, with small 'sails° at the end of comparatively long arms, and held in a position to face the wind by a vane. This instrument is much more accurate in registering sudden gusts and lulls than the cup anemometer, but has the disadvantage of having to shift its position with a change of direction in the wind, and this occupies an ap preciable length of time in which the record is incorrect. The U-tube anemometer is regarded as the most accurate one in common use. Its basic feature is a U-shaped tube containing a mobile liquid in the bend of the U. One of the top ends of the tube is bent so that the opening faces the draught whose velocity is to be i easured; the other end is turned so that at the same time the passing draught exerts a sucking effect upon it. Under the in fluence of these two forces the liquid moves in the tube on which a scale is fastened. In a modification of this instrument called the Seger anemometer, two non-miscible liquids are used, generally paraffin oil and tinted alcohol. The Fletcher anemometer is also of this type, for use under trying conditions, like measuring the velocity of draughts in industrial chimneys.
The liquid used in the Fletcher anemometer is ether. The instrument is at some distance from the chimney, and the arms of the U-tube are greatly extended. The electrical anemome ter, also known as the Linear Hot-Wire ane mometer, depends for its action upon the cool ing by the passing draught of a hot platinum wire. The wire is heated to a dull red phase by its resistance to an electric current passing through it. As the draught cools the wire its resistance is increased, thus disturbing the elec trical balance in a Wheatstone bridge. The additional current necessary to restore this balance measures the velocity of the current blowing upon the platinum wire. This instru ment is extremely sensitive to low velocities, and may be used for velocities between two feet per second and 35 feet per second.
In commercial use anemometers are em ployed in measuring the velocities of gas or steam flowing through pipes; the delivery of hot air in heating schoolrooms and other pub lic apartments, and also of cool air in ventilat ing them; the regulation of the flow of gases in some industries — as in the manufacture of sulphuric acid from pyrites; and in by far the larger numbers in determining and recording the velocity and direction of winds in meteoro logical and climatological observations.
The Robinson cup anemometer is used by the United States Weather Bureau. In this apparatus the hollow cups are made of thin aluminum or copper and are as nearly hemi spherical as possible. These are securely fastened to small square steel arms set with their diagonals horizontal and vertical, respec tively, so as to offer the greatest resistance to the bending action of the wind pressure upon the cups and as little resistance as possible to the wind itself. Copper cups should always be used for exposures where the aluminum does not well withstand the corrosive action of salt air or the acid fumes from smokestacks, etc. In the American standard instrument the cups are four inches in diameter and the arms 6.72 inches long from the axis to the centre of the cups.