GOODMAN EXTENSOMETER. 



117 



to the specimen. The attachment is made by two pairs of 

 screws, which are made to grip the bar in the centre of its 

 width. Two brass tubes, D and E, fixed to the fork clips, 

 serve to carry the scale F, which is divided horizontally 

 into inches and tenths of an inch. A vertical brass rod, 

 H, attached to the clip C, has a small groove on its end 

 which forms the fulcrum upon which the middle knife 

 edge of the pointer, P, rests. At the outer end of the short 

 arm of the lever is a second knife edge, pointing upwards, 

 and upon which rests a second vertical rod G, depending 



from the clip B. The other arm of the lever terminates in 

 a point which moves over the scale F. The leverage is 

 100 to 1. When the instrument is about to be used the 

 screws are made to lightly clip the specimen at points 10 

 inches apart ; the dimensions are so arranged that this 

 brings the lever nearly horizontal and the pointer at the 

 zero of the scale. An exact adjustment to zero can be 

 made by means of a small screw L below the lever. As 

 the bar stretches, G is lifted upwards and H is lowered, 

 and consequently the pointer descends along the scale. 

 As the leverage is 100 to 1, a vertical movement of the 

 pointer of one-tenth of an inch means an extension of the 

 bar of one-hundredth of this, or one-thousandth of an inch ; 

 by estimation readings are easily taken to TTjooffth of an 



