82 



a more accurate method than that often used, involving the breaking of 

 a half dozen steel bars from one rod at different laboratories and compar- 

 ing the average bi'^akiug load. 



In the work of examining the accuracy of the Purdue University test- 

 ing machines, three nickel steel bars were used; two with a length be- 

 tween shoulders of 12 inches, and one with a similar length of 30 inches. 

 In the case of the latter bar, it was possible to attach two extensometers 

 to the bar in tandem, and by exchanging the position of the extensometers 

 to compare the latter. I'he modulus of elasticity was measured in case 

 of each bar on the machines of the three laboratories using the extenso- 

 meters possessed by the three lalwratories. One of these extensometers 

 was sent from one laboratory to the other. If the extensometers are alike 

 in their graduation and the modulus of elasticity of the bars is found to 

 be equal on the various machines, the latter may lie judged to have no 

 relative errors. The observations at the University of Illinois were taken 

 under the direction of Professor A. N. Talbot. 



Taking the average of three l)ars tested at the three lalioratories it 

 appears (Table I) that the value of Young's [Modulus at the Purdue labo- 

 ratory is (in 100,000 pounds per square inch units) 29.22; at Illinois 

 laboratory, 20.33; at Watertown lalioratory, 28.0(5. Between the Purdue 

 laboratory and the Illinois laljoratory there is thus a relative differ- 

 ence of only about % of one per cent., an accuracy much in excess of 

 that needed in any work for wliich tliese machines are used. 



By interchanging the positions of two extensometers in case of the long 

 bar, an opportunity existed of comijaring the indications of two extenso- 

 meters of different type—the Kiehle extensometer (a screw micrometer) 

 and the .Johnson extensometer (a roller type). In Table II it is seen that 

 the two extensometers yield identical results. 



It is thus assuring to know tlie reliability of the ordinary type of 

 testing machine and extensometer. If the Watertown machine is correct, 

 the other machines yield results aliout two per cent. high. The Water- 

 town extensometer, however, was not compared with the other extenso- 

 meters. 



ELASTIC CHANGES. 



After the work of calibrating apparatus was complete, two of the 

 bars were used in the study of the variability of the proportional and yield 

 limit. 



