C6 < :>.MI'KKSSION. 



their levers provided with alternative knife edges. The 

 ordinary fulcrum distance on the 100-ton machine is 4 in., 

 with a poise weight of 2 tons, giving a maximum load, 

 with the full movement, of the weight of 100 tons. If, 

 now, the fulcrum distance can be increased, say, to 12 in., 

 where before a movement of 4 in. meant an increase in the 

 load of 2 tons, now a movement of 12 in. gives the same 

 increase of load, and the maximum load is only one-third 

 of what it was before. By this means a much more 

 sensitive machine can be obtained for light testing. In the 

 three machines mentioned above, this plan has been adopted, 

 and two knife edges provided, either of which may be 

 used. The operation of changing from one knife edge to 

 the other is not a lengthy one, and may be performed with 

 comparative ease. 



32. The Knife Edges. The knife edges used in all these 

 machines are made of hardened steel ; they are 20 in. in 

 length, and rest upon flat steel plates, also hardened. The 

 maximum load on the specimen being 100 tons, the load on 

 the main knife edge is this load plus the weight of the 

 poise and beam, or rather more than 



100 _ . . , 



= 5 tons per in. 01 length. 



33. Graduation of the Steelyard. If the fulcrum 

 distance is 4 in. and the poise weight 2 tons, then 

 differences of load of 1 ton will be given by a movement 

 of the poise weight of 



4 

 f -Sm. 



These 2 in. distances are marked upon a steel scale attached 

 to a beam. They are divided into -jVths of a ton; and 

 further, by means of a sliding vernier attached to the poise 

 weight, into hundredths and two-hundreths of a ton. 



34. Compression. So far, the machines which have 

 been described have only been considered as suitable for 

 tensile tests ; but, in addition to these, all testing machines 

 of any importance are provided with means whereby 

 specimens may be tested under compressive and bending 

 stresses. 



The general principle adopted for the carrying out of 

 these tests is the same in all machines, whether horizontal 

 or vertical, single lever or multiple lever. 



On Fig. 25 is shown a side elevation of the standard 

 and holding shackles of a 100-ton Buckton testing 



