804 BELL SYSTEM TECHNICAL JOURNAL 



approximate the acceptance limits. The milled specimens (see Standard Specimen 

 above) are to be regarded as standard and are to be used where good breaks are 

 not obtained with these wider specimens and as referee test specimens where the 

 strength of the material approximates the acceptance limits. 



4. Cleaning of Specimens. — Before being measured, each test specimen shall be 

 thoroughly cleaned with carbon tetrachloride applied by rubbing with a soft cloth. 

 When being measured, each specimen shall be wiped clear of dust immediately 

 before being inserted in the measuring instrument. 



5. Measurement of Specimens. — The dimensions of the specimen shall be measured 

 between flat parallel surfaces of hardened steel, or other appropriate material, 

 approximately rs i"- in diameter which shall be applied in such a manner as to 

 exert on the specimen, while the measurement is being taken, a normal pressure 

 of approximately 16 oz. 



6. Calibration of Measuring Instrument. — The measuring mstrument shall be 

 suitably adjusted and calibrated to indicate the distance between the measuring 

 surfaces of the mandrels when exerting a normal pressure of approximately 16 oz. 

 or 35 lb. per sq. in. of mandrel end area with an error not greater than ± 0.00005 in., 

 i.e., to the nearest 0.0001 in. 



Notes 



1. When measuring very thin materials, the micrometer or measuring Instrument 

 should be adjusted and calibrated with the greatest practicable accuracy. 



2. The measuring instrument should be calibrated preferably by means of a 

 hardened steel gage block having flat parallel surfaces and of a thickness approxi- 

 mately equal to that of the material to be measured. 



3. In order to obtain an accurate check, it is essential that the mandrel surfaces 

 and faces of the gage be freed from dust immediately before being brought together. 



4. If a barrel type micrometer indicating 0.0001 in. per scale division is used, 

 care should be taken to have the instrument in good operating condition. The 

 instrument can then be adjusted to give the first click of the ratchet at a mandrel 

 pressure of 16 oz. ± 4 oz. by suitable adjustment of the ratchet spring, measuring 

 the mandrel pressure by means of a small spring balance, having flat parallel contact 

 surfaces, inserted between the mandrels. The micrometer screw and nut shall be 

 so adjusted that no end play or shake of the spindle may be felt when the spindle 

 is gently pushed back and forth in the direction of the axis of the screw. 



5. In calibrating the micrometer against a standard gage block, or in measuring 

 a specimen, the spindle shall be rotated slowly into contact with the work by means 

 of the ratchet until the ratchet clicks once, be retracted about 0.002 in. and again 

 slowly rotated into contact until the ratchet clicks once before reading is taken. 

 In order to avoid exceeding the normal pressure of the mandrels in operating the 

 micrometer, the mandrel shall be brought against the work at a rate not exceeding 

 0.001 in. per second at both first and second contacts given above. This rate is 

 approximately the maximum rate at which the operator is able to count the half 

 thousandths marks on the barrel as they pass the zero line on the frame. 



7. Method of Computing Cross- Section. — 



(a) Material \ in. or Less in Width. — For strip material of fiat wire 5 in. or less 

 in width, the width and thickness shall be measured along the major axis of the 

 specimen at the middle point and at distances of approxirnately 2 and 4 in., respec- 

 tively, at each side of the middle point. The cross-section of strip and flat wire 

 material shall be assumed to be a rectangle. The area of cross-section to be used 

 in computing the tensile strength of the material shall be the product of the average 

 of the five thickness readings and the average of the five width readings. 



{b) Cross-Section of Standard Test Specimen for Material Over ^ in. in Width.^ 

 For the standard test specimen described in Section 3 {b), the width shall be measured 

 at the point of minimum width as determined by trial measurements. The thickness 

 shall be measured in a plane normal to the major axis of the specimen at the point 

 of minimum width, at the middle and at each edge of the specimen. When meas- 

 uring the thickness at the edge of the specimen, the outer edges of the contact 

 surfaces of the measuring instrument shall be approximately ^ in. inside of the 

 edge of the material. The area of cross-section shall be taken as the product of 

 the minimum width and the average of the three thickness readings above specified. 



