Fig. 394.—Perspective view of the interior of an aneroid.
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828.—The construction of this aneroid is shown in Fig. 394, which is of a 4½-inch instrument, aneroids made for surveying being of two sizes, 3 inches and 4½ inches. A is a solid plate of metal 1/8 inch in thickness, termed the base plate; B the vacuum chamber, circularly corrugated on both sides, made of thin, hard-rolled German silver containing a large percentage of nickel.
829.—An axis is projected from the lower side of the chamber, of about 1/5 inch diameter. This is tapped with a screw and screwed firmly down into the base plate with a counternut. On the upper side of the vacuum chamber the axis is projected upwards to receive the tension of a strong, very flexible spring D above it, to be described. A bridge-piece EE of steel of strong section strides over the vacuum chamber. This piece has a stout arm-piece projecting from it towards A, which is secured to the base plate by a screw that is left open to a hole indicated near A through the outer case of the instrument, by means of which the bridge-piece can be rocked so as to produce more or less tension of the spring D upon the vacuum chamber for final adjustment. The bridge-piece has two points of rigid support in right line, which form a primary—adjusted when the instrument is made—of the spring contra to the pull of the vacuum chamber. The main spring D is made of fine thin steel, carefully tempered, as broad as the chamber. This spring is constructed so that by its elasticity it may have sensitive movement under the pull of 10 lbs. to 15 lbs. per inch of active surface of the vacuum chamber. It is upon the perfection of this spring as much as upon the construction of the vacuum chamber that the sensitiveness of the instrument depends. The upper axis of the vacuum chamber is secured by a cross cotter pin C which gives an exact point of resistance and yet secures flexibility of the spring at the junction. This cotter pin is placed in the centre of the three points of support of the bridge-piece EE. A lever arm G is fixed to the main spring D upon a stout plate of metal which is in direct connection with the point of tension of the vacuum chamber. It is the small movement of this lever arm (about ·01 inch at the chamber) that gives motion to the indicating apparatus. The lever moves a cranked arm on the axis HK, which communicates through the axis to a second cranked arm placed at right angles to the first I. This pulls a chain Q attached to the arm J. The chain is wound round a small drum fixed upon the axis which carries the hand near R. The drum keeps the hand in one direction contra to the pull of the chain by a hair spring R which is just sufficient to overcome the friction of the axis of the hand F. The hand and drum and their fixings are carried by the plate M, which is a light piece of brass projected from a stiff standard fixed from the base plate K. The compound lever apparatus described moves the point of the hand about five hundred times the amount of movement over the first fulcrum of the lever at the chamber.
830.—Compensation for Temperature.—This is a somewhat difficult matter, which is generally brought about by several modifications of parts. Some ordinary aneroids will move upwards about 1/10 inch of mercury by a rise of temperature of 8° centigrade only. This is caused principally by the increase of temperature softening the spring to render it less rigid, and the softening of the vacuum chamber to render it more flexible or sensitive to atmospheric pressure. Some little difference is also caused by the unequal relative expansion of the lever, arms, spring, and chain, these parts being of steel and brass. Compensation can be made in the lever arm G by making this curved and of two unequally expansive metals, as zinc and steel, so that the curvature increases with increase of temperature and the lever shortens. Compensation can also be partially made by making the base plate in two metals—iron and brass—so as to press the standards fixed through the two metals nearer or further apart with temperature changes. But the whole subject is too technical to be entered upon in our limited space, as it depends so much upon the construction of the instrument, which is modified in various ways by different makers in order to effect this correction.
831.—Dial and Hand.—From the delicacy of the structure of the aneroid it becomes evident that no two instruments can be made to exactly the same rate of movement; therefore each instrument has to be separately graduated when it is intended to measure altitudes with it exactly. However close or open the scale may be, it becomes closer as greater altitudes are ascended, the density of the atmosphere as a gaseous fluid decreasing in geometrical progression as the altitude increases in arithmetical progression. From this we can understand that a vernier to the index hand can only read approximately, although it will act fairly well at a certain point of the scale. The best and possibly only correct method of dividing the scale is to put at first a false scale to the instrument, and to read this scale by the index hand with a microscope under an air-pump, compared at every half-inch of height of the column of the mercury by the gauge attached to the pump. When this is carefully done, a zero point is taken of the position of the index hand at the atmospheric pressure at the time, as indicated on the false scale. The proper scale, as it appears upon the dial, is divided from the position of the readings of the false scale, the two scales being superimposed upon a special dividing machine. The dial is afterwards figured and finished.
832.—The ordinary method of reading the aneroid is to let the index point read over the divisions. The author devised a plan, which he has used for many years, of fixing a small plate of aluminium upon the point of the hand, level with the scale, which is raised on a step to read it upon its inside edge, to a fine line on the aluminium. By this means error of parallax in reading is entirely avoided. The author also places an adjustable magnifier to move over the index for reading. This last improvement is now followed by other makers. A pointer also revolves with the outer rim to show the last reading before ascent or descent.
Instruments made with care in the points just indicated must necessarily become expensive. Where the aneroid is to be used as a weather glass, or even as a travelling companion to judge of approximate heights in climbing mountains, such care is not needed, and the instrument may be produced very cheaply of useful quality. On the other hand, where precision is required, a delicately made aneroid will indicate a movement of 3 feet or less in raising or depressing, when holding the instrument horizontally in the hand and giving a light tap on the glass with the finger-nail before reading, so as to put all motive parts in equilibrium.
833.—The Altitude Scale is generally placed near the periphery of the dial; it is the all-important part to the surveyor. This scale is usually set out from a mean of atmospheric pressure at sea level, taken from Sir George B. Airy's tables, which give the extreme pressure of 31 inches barometric pressure for zero at sea level. With this pressure altitudes are taken at intervals according to the indices tested under the air-pump, and the intermediate divisions are graduated to scale. These index points are shown in the table below for a few points:—