BULLETIN 532, U. S. DEPARTMENT OF AGRICULTURE. 



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MEASURING APPARATUS. 



Referring to figure 1, it will be noted that to measure the change 

 in length of the specimen, crossbars of steel one-half inch square were 

 cast in the concrete. They were placed 50 inches apart, center to 



center, and this was considered the 



(" — .* j gauge length of the measurements. 



On each end of these bars a plug of 

 steel with a rounded conical point 

 was fastened, and micrometer meas- 

 urements were taken between these 

 points on each side of the specimen. 

 Two readings were taken, so that 

 unequal movements of the two sides 

 g of the specimen could be cared for 

 | by averaging the readings, thus ob- 

 g taining expansion or contraction 

 ° along the center line. 

 § For obtaining the readings a spe- 

 g cial form of instrument (fig. 1) was 

 a designed and constructed in the labo- 

 | ratory. It consisted of a micrometer 

 *> head reading to 0.0001 inch mounted 

 '3 at the end of a steel yoke. This 



in " 



I yoke contained two steel rods five- 

 si . 

 u sixteenths of an inch in diameter 



o 



^ bolted to two end crosspieces, one 

 •g holding the micrometer and the 

 g, other a flat-ended steel pin. In tak- 

 < ing measurements the flat-ended pin 

 h was held in contact with the lower 

 conical point of the specimen, and 

 the micrometer was screwed down 

 to contact with the upper conical 

 point. During the initial readings 

 electrical contact was used in order 

 to read the micrometer to the nearest 

 0.0001 inch. This, however, was dis- 

 carded when it was found that read- 

 ings of sufficient accuracy could be 

 attained without it. In order that 

 any change in the measuring instrument, due to wear or accident, 

 might be detected, readings were taken repeatedly on a steel gauge 

 bar hung from the specimen. Before any reading was made the in- 

 strument was hung alongside the specimen for a sufficient time to 



