3. EQUIPMENT AND INSTRUMENTS 



51 



edge. The extension arms should be at- 

 tached while this test is being made. An 

 arm may also be tested by drawing a fine 

 line along the fiducial edge and then revers- 

 ing the arm to the opposite side of the line 

 (see 3-140). If the fiducial edge still coin- 

 cides with the pencil line, the arm is straight. 



After the arms and centerpieces have 

 been tested, the protractor should be placed 

 on the test plate and centered precisely at 

 the intersection of the two lines at right 

 angles. The fixed arm should be moved to 

 coincide with the central line and the right 

 and left arms moved to coincide precisely 

 with the lines at 90° to the central line. 

 The movable arms should be clamped in this 

 position. If the angles read exactly 90° the 

 verniers are in adjustment; if not, the ver- 

 niers should be moved to read exactly 90° 

 and the test repeated. 



After making this standard test, the arms 

 should be moved to a closed position and the 

 vernier settings at this position recorded on 

 the inside of the storage box. A frequent 

 check on the vernier adjustment is then pos- 

 sible simply by comparing the closed arm 

 vernier setting with the recorded values. This 

 is a safeguard against loss of vernier adjust- 

 ment, but does not alter the requirement for 

 periodic standard tests. 



3-19 Manipulation of metal protractor. — 



To plot a three-point fix, the left and right 

 angles are set on the graduated circle, the 

 final setting being made by the tangent 

 screw. The instrument should be grasped by 

 the metal circle and the fixed arm, and cen- 

 tered at the approximate location of the fix. 

 With the fingers of one hand used as a guide 

 to keep the fixed arm bisecting the center 

 station, the protractor is moved toward or 

 away from the other stations until each arm 

 bisects its respective station. A prick mark, 

 or a pencil mark, is then made at the center 

 of the protractor representing the location 

 of the observers at the time the angles were 

 taken. A movable arm should not be used 

 to guide a protractor into position. 



3-20 Plastic protractors. — A transparent 

 three-arm protractor (Fig. 10) is con- 



structed of clear non-flammable plastic, with 

 a solid disk about 12 inches in diameter con- 

 taining a circle graduated in degrees, and 

 one fixed and two movable arms each con- 

 taining an etched line radial with the center 

 of the protractor. Each movable arm con- 

 tains a vernier graduated in two-minute 

 intervals. There are two sizes of plastic pro- 

 tractors — one has arms about 13 1/2 inches 

 long and the second is constructed of heavier 

 gage material with arms 24 inches long. 



Although slightly less accurate than the 

 metal protractor, the plastic protractor has 

 several advantages. Its transparency makes 

 it possible to plot positions close to control 

 stations. It is lighter in weight, more easily 

 read, and can be more quickly set than the 

 metal protractor. 



3-21 Testing three-arm plastic protrac- 

 tors. — A plastic protractor cannot be ad- 

 justed. It should be tested to see that the 

 plastic has not warped and to determine the 

 index correction. The radial lines should be 

 checked for straightness by superimposing 

 the lines over a steel straightedge. Place the 

 protractor on the test plate, match the etched 

 lines with the lines on the test plate at 30°, 

 60°, and 90° and read the angles on the 

 protractor. The differences, if any, repre- 

 sent the index corrections of the respective 

 arms. 



Plastic protractors used for smooth plot- 

 ting (see 6-48) shall be calibrated as fol- 

 lows : On a section of metal mounted drawing 

 paper draw a straight line and carefully 

 construct a perpendicular, (Fig. 11). From 

 Point A measure off 50 centimeters to Point 

 B on the first line. From the same point 

 measure 18.2 centimeters (50 cm X tan. 

 20°) on each perpendicular to points C and 

 C. With the center of the protractor at 

 position B and the fixed arm through A, the 

 angles to left and right through C and C 

 should be exactly 20°. Differences there- 

 from may be used as index corrections being 

 sure to apply them with the proper sign. 

 Plastic protractors used for smooth plotting 

 should be compared with the standard daily. 



3-22 Odessy protractors. — When hydrog- 



