116 



GOODMAN EXTENSOMETER. 



of a rubber band D in a centre punch mark on the surface of 

 the specimen. The measuring lever or pointer, P, has its 

 fulcrum on this rigid frame, and the termination of its 

 short end is a centre point similar to the one which has 

 been mentioned, and which is also held in a centre punch 

 mark by a second rubber band. As the bar alters in length 

 these two points become separated, and the point of the 

 lever is caused to move down the scale. The calibration is 

 effected as in the former case. This instrument may be 

 relied upon to give accurate readings, but the writer has 

 found tnat the chief difficulty attending its use is in the 

 attachment. A possible cause of error in this instrument 



FIG. 46. 



is due to the fact that the readings given only apply to the 

 length variations of one surface of the bar, and not to those 

 of the axis. 



66. Goodman Extensometer. Another instrument of 

 the lever multiplying type is that of Professor Goodman. 

 This instrument is intended for use on vertical test speci- 

 mens, and consists of a simple lever arrangement. It is shown 

 in plan and elevation on Fig. 46. Here A is the specimen 

 in the machine ; B and C are two forked clips, one at the 

 top and the other at the bottom of the specimen ; these 

 serve as the means whereby the extensometer is attached 



