ANEROID (formed in an anomalous way from Gr..a, priv., and ?zeros, wet), the name given to a barometer invented by 31. Vidi of Paris, in which the pressure of the air is measured without the use of liquid, as in ordinary instruments. The face of the A. barometer, represented in fig. 1, has a diameter of about 5 in., and the case behind, which contains the mechanism, is general idea of which is given in fig. 2, is about 2 in. deep. The pressure of the atmosphere acts upon a circular metal box, AA, about 3 in. in diameter, and f of an inch deep, which has been nearly exhausted of air, and then soldered air-tight. The sides are corrugated in concentric rings, so as to increase 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 Fin. 1.
Fin. 2.
prevent this they are kept distended, to a -..,, certain extent, by a strong spring, CH --"" S fixed to the case, which acts upon the 0,.. go; : 0 Allt4 4,4,4/0,t.„).
head of the stalk. B, attached to the side 1,,,, next the face. NVhen the pressure of the A t l'.: air increases, there being little or no air pg,11,_z--- --„. ,p• .
inside the box to resist it, the corrugated ft _,:-..:--r- I r." sides are forced in wards, and when it c4 .. J ....
diminishes again, their elasticity restores - --- A them to their former place; and thus the ,...,:.".% a little box becomes a spring extremely ca , • 2 ,,, ,. ,A K
sensitive to the varying pressure of the _j_____„ external atmosphere. Supposing the two "Vit„I .1St \`‘‘ • ''W' sides pressed inwards, the end of the i 1 .
0 spring, E, will be drawn towards the back of the case, and carry with it the rod, EG, which is firmly fixed into it. Aneroid barometer.
EG, by the link Gil, acts on the bent , lever, IIKL, which has its axis at K, so that, while the arm, K1L is pushed to the right, LK is moved downwards. By this motion, a watch-chain, 0, attached at L, is drawn off the little drum, 31, and the Index-hand, PP, which is fixed to it, would move from the position represented in fig. 1 to one towards the right. When the contrary motion takes place, a hair-spring moves the drum and the hand in the opposite way. By this mechanism, a very small motion of the corrugated sides produces a large deviation of the index-hand, riff of an inch causing it to turn through 3 inches. The A. barometer is graduated to represent the inches of the mercurial barometer. Both from its small size and construction, it is extremely portable, and consequently a very useful instrument; but from its liability to chatirre from time to time, it must be frequently compared with the mercurial barometer. "metallic barometer" of M. Bourdon is a modifica tion of the A. principle.—See Manual of Barometers, by J. H. Belville.