The items in this chapter are useful (but not essential) for aligning the instruments in these notes. Other useful test items are plumb bobs, chapter 2, and targets, chapter 7.



Knife Edge Stands are an accurate way of testing levelling instruments. A set of two is needed. They can be used for:-
1. Making end for end tests on bubble levels, Try-alls (Chapter 2), and any levelling instrument with a straight horizontal base. (They are unsuitable for testing an A frame or a Dumpy Plummet).
2. Measuring the sensitivity of levelling instruments - that is, measuring the deflection of a level indicator like a bubble or a pendulum, for a precise change in height of one of the knife edges.

Each stand is a rectangular flat plate standing on one screwed and two fixed feet. A horizontal knife edge and end stop are attached to the deck of each plate. In the picture,the plate on the left has the screw foot at one side, to correct any difference in slope between the knife edges.
The right hand plate has the screw foot at one end, for adjusting the height of the right hand knife edge. This screw is fitted with a knob, to give fine height adjustment, and a mark on the plate allows a count to be kept of how many turns of the knob have been made. Like this:-

Knife Edge Stands. Sizes in Millimetres.


The screw feet are made from 6 mm. (1/4 inch) Whitworth metal screws, with a pitch of 20 threads per inch. The proportions of the height adjustment stand allow a knife edge height change of half a millimeter per one turn of the screw leg:-
Distance from front legs to back leg = 127 mm..
Distance from front legs to knife edge = 50 mm..
Then change in height S of the back of the plate for one turn of the screw is:-
millimetres..
And change in height K of the knife edge for one turn of the screw is :-
millimetres for each turn of the screw.

In use, the stands are placed on a bench; the instrument is placed between them on the the knife edges, with its ends up against the end stops. The left hand screw foot is then adjusted to take out any wobble in the instrument. The right hand screw is then adjusted to bring the instrument indicator to level. The instrument is then turned end for end to check whether the indicator still shows level; if not it needs adjustment.
The instrument can be tested for sensitivity by turning the right hand screw and observing the deflection of its indicator to a known change in K.



The Calibrated Plumb Line Target is a white board mounted vertically on a base, with a black plumb line and bob suspended from a positioning screw and hanging down in front of the board.

The positioning screw is a 1/4 inch (6 mm.) Whitworth metal thread screw mounted horizontally between two brackets about 50 mm. apart. There is a knob on one end to turn the screw, and a 180º protractor fitted to the adjacent bracket measures fractions of a turn. (Two pointers fitted to the screw shaft, 180º apart, allow readings over 360º ). Locknuts on each end of the screw thread are adjusted to stop lateral movement of the screw, while still allowing it to be turned easily.
One end of the plumb line - a black cotton thread 1.03 mm. in diameter, sold for threading beads - is fastened to the top edge of the board; the line is wound one turn around the positioning screw, then drops vertically to the bob weight, which is an open inverted tin can suspended in water in a larger tin can..(See ‘Re-Thinking the Plumb Line’ in the Theory Notes).

Turning the screw causes the line and bob to move horizontally at the pitch of the screw - 20 turns/inch; the line stays vertical provided the knob is turned slowly to allow the bob time to follow the movement, until it touches the side of the water container. The clearance between the bob weight and the water container is sufficient to allow enough movement for all ordinary measurements.
Like this:-

Because the bob is light in weight, the restoring force is small, and several undesirable effects, leading to unwanted deflection of the bob, have been identified. They are:-
a. Surface tension of the water This efffect can be removed by making sure the bob is completely immersed
( i.e. the water level is higher than the top of the bob ). Strangely, adding soap or detergent to the water to lower the surface tension had no effect.

b. Deflection from external magnetic fields. This only showed up when a non magnetic water container was used - e.g. a glass jar. Using a tin can for the container shields the bob from external fields. An aluminium can, like the one used in the Dumpy Plummet, was tried but found to be too small for this purpose.

c. Deflection from electrostatic charge, and buoyancy. Both these effects seemed to occur together, when the bob was made from a plastic pill bottle, and can be avoided by not using plastic.

The target can be used :-

1.As a reference for aligning an instrument when it is mounted vertically.

2.To align tall sights when an instrument is mounted horizontally.

3. To measure the optical resolution of open sights - i.e to find the smallest movement of the line which can be detected by an observer.

4 To check the measuring accuracy of instruments fitted with scales.

5. To measure the parallax error of an instrument.

For 3, the resolution is measured in arcminutes, so we must know the focal length F.L.. - the distance from the instrument backsight to the plumb line. The graph gives the required focal length in metres, for integral values of resolution in arcminutes per single turn of the screw, or it can be found from :-

M., where ø is the selected number of arcminutes per turn.


For example, for a plumb line position change of 3 arcminutes per screw turn, the required focal length is 1.5 M..

To carry out 4 and 5, it is better to find how far the line has moved for a given rotation of the screw, in millimetres.. One complete turn moves the line 1.27 mm., so the line movement LM for part of a turn is given by:-



, where Nº is the number of degrees of rotation read from the protractor scale. The equation also holds for multiple turns of the screw if 360º is added to the value of Nº for each turn.



Line Strainers. These are two mounting brackets with line clamping and straining ability, for use with the end for centre alignment test. The line clamping washers will hold a 30 kgm. breaking strain nylon fishing line tight enough to prevent any discernible sag over a distance of 10 Metres, when looking along the line from one end. The bolt and washer assembly makes a good optical target if something dark is placed behind it as a background. The diagram is self explanatory.


This is all the test apparatus. The last chapter, no.9, deals with the theory of the surveying instruments, and is included for interest only; it is not necessary to read chapter 9 to make and use any of the instruments.