CHEMICAL AND PHYSICAL PAPERS. 89 



the shaft, M. The casing, K, surrounds a gear of 144 teeth, which 

 makes one revolution for every 2000 pounds increase of load on the 

 test piece. On the shaft, M, is mounted a gear of eighteen teeth, which 

 meshes with the gear of 144 teeth, and thus makes one revolution for 

 every 250 pounds change in load. 



The screw, S, has twenty threads per inch, and the resulting load is, 

 therefore, 250x20, or 5000 pounds per inch. 



The capacity of the testing-machine being 100,000 pounds, and the 

 suitable load scale being 5000 pounds per inch, it would require a 

 drum twenty inches long. Instead of this, the drum is made five 

 inches long, and four pens are arranged so as to have a total move- 

 ment of twenty inches. The pens move from left to right with in- 

 crease of load. 



As pen Pi reaches the end of its travel it presses against the finger 

 Fi, which in turn moves the shifting-rod, H, causing finger F-i to move 

 the pens Po, P3 and P4 to the right, until the pen P2 is meshed with 

 the lead screw, and takes up the curve five inches behind the pen Pi. 

 In a similar manner the four pens supplement each other, either on 

 increase or decrease of load. 



The lever, L, operates a cam which releases the pens, so that the 

 drum may be removed. 



The rack, R, is held in mesh with its pinion by a spring, which 

 allows the rack to slip in case the test piece should break with the 

 recording instrument in place. 



Figures 3 and 4 are two typical curves as drawn by the instrument. 



The instrument is fairly satisfactory as it stands, but might, to ad- 

 vantage, have the lesser work of moving the pens accomplished by a 

 mechanism actuated by the elongation of the test piece. The drum 

 would then be rotated proportionally to the load. This would do 

 away with the considerable strain between the upper and lower clamps 

 caused by the high ratio of gearing and the inertia of the drum. 



