The bevel protractor can be used to measure both internal and external angles of objects. It has a protractor dial with degree divisions and an attached Vernier scale to allow for precise measurements. To take a measurement, the object is placed between the protractor's sliding and fixed blades. Readings less than 90 degrees are read directly from the dial, while those over 90 degrees require subtracting the dial reading from 180 degrees. The Vernier scale allows measurements to be made with a least count of 5 minutes. Possible sources of error include damage to the instrument, parallax effects, and observer carelessness.

2. Lab session # 5:
To measure internal and external angle of given object with the help of Bevel
Protractor

3. BEVEL PROTECTOR:
The angles can be measured by precision instrument that is known as universal bevel protractor. The
Vernier scale is attached on base of this protractor. Base circular section contains a protractor dial that
shows divisions in degrees.
Dial also possesses the fitted sliding blade that has the capability of extending in any direction. The dial
and blade rotate together. A small pinion provides fine adjustments and this pinion makes contact with
the blade gear.
Adjustment knob
TURRET

4. Least Count
On every side of zero line, the dividing of Vernier scale is done into the 12 divisions. The space of one
Vernier division is five minute. If Vernier scale zero matches with main scale line then the reading that
we will achieve will be in degrees only. If line of other than zero of Vernier scale matches with main
scale line then five will be multiplied with the divisions number on Vernier scale that are beyond zero
and the number which will obtain will be further added into indicated full degrees number on the
protractor dial.
Least Count = 1 degree / 12 div
= 60 minutes / 12 div
= 5minutes

5. WORKING

6. How to Read a Bevel Protector?
On swivel plate, the line position provides the reading directly with respect to divisions on dial for angle
measurements equal to ninety degrees or less than ninety degrees but for angle measurement of greater than
ninety degrees then the degrees number that is shown on the dial will be subtracted from 180 degrees.
Check the line of other than zero of Vernier scale that matches with main scale line then five will be
multiplied with the divisions number on Vernier scale that are beyond zero and the number which will obtain
will be further added into indicated full degrees number on the protractor dial.
Clockwise
Vernier scale
Anti-clockwise
Vernier scale

7. PROCEDURE
Firstly, the object is grasped in between the bevel protractors sliding and fixed blade. On swivel plate,
the line position provides the reading directly with respect to divisions on dial for angle measurements
equal to ninety degrees or less than ninety degrees but for angle measurement of greater than ninety
degrees than the degrees number that is shown on the dial will be subtracted from one eighty degrees.
Then check the line of other than zero of Vernier scale that matches with main scale line then five will
be multiplied with the divisions number on Vernier scale that are beyond zero and the number which
will obtain will be further added into indicated full degrees number on the protractor dial.

8. PROCEDURE

9. TABLE

10. POSSIBLE ERRORS
• Angle cannot be measured in decimal points.
• The error can happen due to damaged instrument
• Parallax error, personal or instrumental error.
• Error might occur because of scratches on magnifying glass.
• Observer carelessness can also be the cause of error.

11. THANK YOU