Table of Contents
In this article, we will discuss the Abney level.
1. Introduction
Abney level is one of the various forms of clinometers used for the measurement of slopes, taking cross-sections, tracking contours, setting grades and all other rough levelling operations.
It is a light, compact and hand instrument with low precision as compared to the engineer’s level.
The Abney level consists of the following.
(1) A square sighting tube having a peephole or eye-piece at one end and a cross-wire at the other end. Near the objective end, a mirror is placed at an angle of 45 inside the tube and occupying half the width, as in the hand level.
Immediately above the mirror, an opening is provided to receive rays from the bubble tube placed above it. The line of sight is defined by the line joining the peephole and the cross-wire.
(2) A small bubble tube, placed immediately above the openings attached to a vernier arm which can be rotated by means of a milled headed screw or by rack and pinion arrangement. The image of the bubble is visible in the mirror.
When the line of sight is at any inclination, the milled-screw is operated till the bubble is bisected by the cross-wire. The vernier is thus moved from its zero position the amount of movement being equal to the inclination of the line of sight.
(3) A semicircular graduated arc is fixed in position. The zero marks of the graduations coincide with the zero of the vernier. The reading increases from 0° to 60°( or 90° ) in both directions, one giving the angles of elevation and the other angles of depression.
In some instruments, the values of the slopes, corresponding to the angles, are also marked. The vernier is of the extended type having the least count of 5′ or 10′.
If the instrument is to be used as a hand level, the vernier is set to read zero on the graduated arc and the level is then used as an ordinary hand level.
2. Uses of Abney Level
The Abney Level can be used for,
(i) Measuring Vertical angle
1. Keep the instrument at eye level and direct it to the object till the line of sight passes through it.
2. Since the line of sight is inclined, the bubble will go out of the centre. Bring the bubble to the centre of its run by the milled screw. When the bubble is central, the line of sight must pass through the object.
3. Read the angle on the arc by means of the vernier.
(ii) Measurement of the slope of the ground
1. Take a target, having cross-marks, at the observer’s eye height and keep it at the other end of the line.
2. Hold the instrument at one end and direct the instrument towards the target till the horizontal wire coincides with the horizontal line of the target.
3. Bring the bubble in the centre of its run.
4. Read the angle on the arc by means of the vernier.
3. Testing and Adjustment of Abney level
(1) Fix two rods, having marks at equal heights h (preferably at height of observers eye), at two points P and Q, about 20 to 50 metres apart.
(2) Keep the Abney level at point A aginst the rod at P and measure the angle of elevation towards point B of the rod Q.
(3) Shift the instrument to Q, bold it against B and sight A. Measure angle of depression α2.
(4) If α1 and α2. are equal, the instrument is in adjustment i.e., the line of sight is parallel to the axis of the bubble when it is central and when vernier reads zero.
(5) If not, turn the. screw so that the vernier reads the mean reading.
(α1+ α2)/2
The bubble will no longer be central.
Bring the bubble to the centre of its run by means of its adjusting saws. Repeat the test till correct.
Note. If the adjustment is not done, the index error, equal (α1- α2)/2 may be noted and the correction may be applied to all observed readings.
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