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The Beam-Compass

The beam-compass, like the calipers, is an instrument for measuring lengths, and is very similar to them in con-ruction, consisting essentially of a long graduated beam with one steel compass-point fixed at one end of it, and another attached to a sliding piece provided with a fiducial mark and vernier.
These compass-points take the place of the jaws of the calipers. It differs from them however in this, that while the calipers are adapted for end-measures such as the distance between the two flat ends of a cylinder, the beam-compass is intended to find the distance between two marks on a flat surface. For example, in certain experiments a paper scale pasted on a board has been taken to represent truly the centimetres, millimetres, &c. marked upon it. We now want to know what error, if any, there is in the divisions. For this purpose the beam-compass is placed with its scale parallel to the paper scale, and with the two compass points lying in a convenient manner upon the divisions. It will be found that the beam-compass must be raised by blocks of wood a little above the level of the paper scale, and slightly tilted over till the points rest either just in contact with, or just above, the paper divisions.

One of the two points is fixed to the beam of the compass; we will call this A. The other, B, is attached to a sliding piece, which can be clamped by a small screw on a second sliding piece. First unclamp this screw, and slide the point B along, till the distance A B is roughly equal to the distance to be measured. Then clamp B, and place the point A (fig. 2) exactly on one of the marks. This is best effected by gentle taps at the end of the beam with a small mallet.

It is the inside edge of the compass-point which has to be brought into coincidence with the mark. Now observe that,- although B is clamped it is capable of a slow motion by means of a second screw called a 'tangent screw' whose axis is parallel to the beam. Move this screw, with so light a touch as not to disturb the position of the beam-compass, until the point B is on the other mark, i.e. the inside edge of B coincides with the division in question. Suppose that the point A is on the right-hand edge of the paper scale division, then B should also be on the right-hand edge of the corresponding division. To ensure accuracy in the coincidence of the edges you must use a magnifying-glass.

You have now only to read the distance on the beam-scale. To do this observe what are the divisions between which the arrowhead of the vernier falls. Then the reading required is the reading of the lower of these divisions plus the reading of the vernier. The divisions are each 1 millimetre. Hence, if the arrowhead falls between the 125th and 126th, the reading is 125 mm. plus the reading of the vernier.

Observe which division of the vernier is in the same straight line with a division of the scale. Suppose the yth to be so situated. Then the reading of the vernier is 7/10 mm. and the distance between the points is 125.7 mm.

Repeat the observation twice, and suppose that 125.6 and 125.7 are the readings obtained, the mean of the three will be 125.66, which may be taken as the true distance between the marks in question.

Suppose that on the paper scale this is indicated by 126 mm., then to make the scale true we must reduce the reading by 0.34 mm. This is the scale correction for this division.

Experiment. - Check by means of the beam-compass the accuracy of the divisions of the given centimetre scale.

Enter results thus:
Division of scale
at which A is placed
Division of scale
at which B is placed
Vernier readings
(mean of 3 obs.)
0 1 cm 1.005 cm
0 2 cm 2.010 cm
0 3 cm 3.010 cm
0 4 cm 4.015 cm
0 5 cm 5.015 cm



Last Update: 2011-03-27