Fig. 3.
A modification of this is shown in [Figs. 2] and [3]. ([See, also, frontispiece]). This is Naudet’s aneroid, and is the one chiefly employed now. It differs from Vidi’s in the substitution of the thin laminated spring (B in frontispiece) for the spiral spring ([S in Fig. 1]).
Fig. 4.
One of the oldest forms of box barometer and the one to which the name aneroid is restricted by some writers, is represented in [Fig. 4]. A rectangular tube, from which the air has been perfectly exhausted, is sealed hermetically, and, having been bent into the form represented in the figure by cbd, is made fast at the middle point b. The varying pressure of the atmosphere causes the extremities c and d to approach or recede from each other. This motion is converted into a to-and-fro traverse of the index, by a mechanism sufficiently well exhibited by the diagram.
This is known as Bourdon’s form. It is not now employed for delicate work. The forms of Vidi and Naudet are, by some writers, designated the holosteric barometers.
The graduation of these instruments is made to correspond with the height of the mercurial barometer, and is expressed as inches or millimeters.
The difficulties to be met by the maker, in securing accuracy of working, are those which arise chiefly from the varying elasticity of the several metallic elements under change of temperature. Greater simplicity of construction might be presumed to be attended with a smaller liability to a kind of error, for which it is exceedingly difficult to compensate. This is the theory of the Goldschmid Aneroid.
The instrument designed for ordinary engineering use is represented by [Fig. 5]. The size recommended by the present makers for this service is 3⅛ inches in diameter and 2½ inches high.
