98 SENSITIVENESS. 



In all machines the position of the operator should be 

 such that he can work his machine and at the same time 

 have a good view of the specimen, and so be able to note any 

 changes that may be taking place. 



53. Adaptability. In all machines used for general 

 work, provision is made for testing in tension, compression, 

 cross-breaking, and often torsion. It is necessary that the 

 change may be easily made from one kind of test to another. 



In this respect horizontal are somewhat better than 

 vertical machines, because in the former the heavy parts 

 have only to be moved horizontally along the bed ot the 

 machine and not lifted or lowered. What appears to be 

 one of the best machines as regards adaptability is that 

 which was one of the last described, the horizontal Buck- 

 ton-Wicksteed machine, in which changes, both of length 

 and the nature of the stress, can be very rapidly and easily 

 made, and long screwing operations avoided. Next in order 

 come the other horizontal types, and, lastly, the vertical 

 machines. In one respect a vertical machine is always 

 preferable to a horizontal one, and that is when a com- 

 pression test has to be made of some substance, such as 

 stone, which requires to be bedded in plaster of Paris. 



54. Sensitiveness. The measuring apparatus should 

 be capable of indicating very small variations of load. In 

 order that a machine may be sensitive, one essential is that 

 all the knife edges shall be in good condition. If a 

 small load is added to the load on a tension specimen, that 

 increment of load should have the effect of at once moving 

 the lever on which the jockey weight is hung. The 

 resistance which is offered to this motion is almost entirely 

 due to the friction of the knife edges. If these are in 

 good condition, the friction is extremely small, and the 

 sensitiveness of most machines is such that differences of 

 load far less than are required in practical testing cause 

 movements of the lever. But the effect of the continued 

 rolling wear between the knife edges and their plates, and 

 especially of the shocks caused by the fracture of the 

 specimens, is to produce a flattening of the sharp edges 

 and a consequent increase of friction and loss of sensitive- 

 ness. This effect is also brought about by the knife edges 

 losing their straight form and becoming slightly curved, 

 and in this way being virtually widened and the friction 

 increased. 



A great deal depends on the nature of the steel and on 

 the way it has been hardened, and some machines will lose 



