ANEROID (containing no liquid, from Gk. 6. a, priv. wip6c, Oros, liquid + eldos, eidos, form). A barometer first made in serviceable forum by M. Vidi. of Paris, in 1848, :n which tile pressure of the air is measured by the change of form undergone by an exhausted me box under the influence of the atmospheric pressure. In the diagram, Fig. 2, AA, us a ciren lar metal box which has been nearly exhausted of air and then hermetically sealed. The sides are corrugated in concentric rings, so as to in crease their elasticity, and one of them is fixed to the back of the brass case which contains the whole. The amount of exhaustion is such that if the sides of the box were allowed to take their natural position they would be pressed in upon each other, and to prevent this they are kept dis tended, to a certain extent, by a strong spring, 8, fixed to the case, which acts upon the head of the pillar, 13, attached to the side next the face.
When the pressure of the air increases, there be ing little or no air inside the box to resist it, the corrugated sides arc forced inward, and when it diminishes again, their elas ticity restores them to their former place. the box being extremely sensitive to the varying pressure of the ex ternal atmosphere. Suppos ing the two sides pressed inward, the end of the spring, E, will be drawn toward the back of the case, and carry with it the rod, EG, which is firmly fixed into it. EG, by the link 011, acts on the bent lever, I K 14 which has its axis at K, so that, while the arm, Kul, is pushed to the right, .1.K is moved downward. By this motion a watch-chain, 0, attached at L, is drawn off the little drum, and the index-hand, /'/', which is fixed to it, would nmve from the position repre sented in Fig. 1 to one toward the right. When the contrary motion bikes place, a hair-spring moves toe drum and the hand in the opposite way. By this or similar mechanism a very small motion of the corrugated sides produces a large deviation of the index hand.
The aneroid is graduated to represent the niches or millimeters of the mercurial barometer.
It may be made to agree very closely with such an instrument, but, owing to the imperfect elas ticity of the box and the steel spring, it is quite apt to disagree after a few months or years, and especially when exposed to rapid variations of pressure, being in this respect quite analogous to the ordinary thermometer, whose glass bulb has also a defective elastic reaction. Although the aneroid is very convenient, very sensitive, and unaffected by variations in gravity, yet its defects prevent it from becoming a. very reliable instrument. and it must be frequently compared with the standard mercurial. It is often used in ascertaining altitude, especially by engineers and surveyors, where extreme accuracy is not required. The holosterie aneroid made by Naudet and the box-aneroid made by Goldsehmid bear the highest reputation.
The Bourdon aneroid, or pressure gauge. has about the same advantages and defects as the Vidi aneroid. It consists essentially of a portion of a thin hollow ring whose section is a very flat ellipse. The ring, or curved tube, is made of elastic metal, exhausted of air and then hermet • ically sealed, and is, therefore, a vacuum-cham ber. Changes of pressure alter the curvature of this ring, whose changes of shape are shown on a magnified scale by a delicate pointer.
The aneroid barometer is also easily arranged to be self-registering, and is then known as a barograph. Instruments of this kind. made by Richard & Son, of Paris, are especially applicable for use at sea, and are also widely used at land stations. For fuller details of the construction and use of the aneroid, consult Abbe's Treatise oz Instruments (Washington, 1887), and the works referred to therein. or any of the larger treatises on experimental physics. See BAROMETER.