KW TENSION TKSTING. 



fully balanced and the vernier set to zero. That is to say, 

 when the beam is in perfect balance, the reading of the 

 machine should obviously be zero. It is necessary to draw 

 special attention to this fact, because it is one that is often 

 overlooked. Very frequently, when the machine has been 

 altered for some other kind of test, additional tackle has 

 been added to the weight on the upper shackle, and this 

 throws the beam out of balance ; in some cases the weight of 

 the specimen itself may have a disturbing effect. It should, 

 therefore, be taken as an axiom, that previous to every test 

 the machine must be carefully inspected to make sure that 

 it reads zero when there is no loaa on the bar. Another 

 point which should be looked to before a test begins is to 

 see that there is nothing to interfere with the free move- 

 ment of the beam or beams ; in some cases it may be found 

 that the slides are tightened up too much, in which case 

 there will be a certain amount of friction added to the load 

 on the specimen, and which is not accounted for. 



78. Fixing the Specimen in the Testing Machine. This 

 is an operation which, though simple, requires a consider- 

 able amount of care. In a vertical machine, the best plan is 

 to fix the top end of the specimen first, and then get hold 

 of the lower end and pull it tight by screwing down the 

 lower cross-head by hand. Before using the wedge grips, 

 care should be taken that both the socket and the wedges 

 themselves are clean ; otherwise there may be some 

 trouble in getting the wedges out of their position after 

 the completion of the test. A little oil rubbed over the 

 wedges will do good in this direction. When the upper 

 wedges have been placed in position, and the specimen 

 inserted between them, care must be taken that the two 

 wedges are on the same level, and that the specimen comes 

 fully through them. To attach the lower end to the 

 machine, the lower crosshead must be raised to the proper 

 height by means of the hand screw adjustment, and the 

 wedges then placed in position by inserting them with 

 the two hands from the lower side. They will then have 

 to be held in this position until a slight tension is put 

 upon the specimen, so that the wedges are enabled to take 

 a grip of the specimen. This pulling tight is effected by 

 putting a small load, say, one-tenth of a ton, on the 

 machine, and then tightening up by means of the screw 

 gear. If there is any difficulty about the wedges gripping 

 the bar, as is sometimes the case with hard metals, it will 

 help matters and cause the teeth of the wedges to penetrate, 



