TENSION TESTING. 137 



if the wider ends of the wedges are tapped with a hammer. 

 When once the grips have taken hold and the test has begun, 

 in most cases it will be found that they will retain their grip 

 to the end. In horizontal machines there is less trouble, 

 as a rule, chiefly on account of the fact that the wedges 

 have no tendency to fall out of their places by their 

 own weight. The specimen is now ready for testing. 



79. Fixing the Extensometer. If the modulus of elasti- 

 city is to be found, it will be necessary to attach the 

 extensometer with which the elastic measurements are to 

 be taken. When putting the bar in the machine, the 

 centre-punch marks should be so placed that they can be 

 easily seen and approached for purposes of measurement 

 during the test. In the case of some extensometers, it will 

 be necessary to provide two pairs of centre punch marks, 

 ten inches apart, upon which to fix the instrument. 

 Generally, however, in most modern extensometers, the 

 attachment screws themselves penetrate the metal slightly, 

 and so make their own marks. The extensometer should 

 be applied to the bar, so that the gripping screws are 

 opposite the axis of the specimen. There is no need to 

 screu* them up tight ; it is quite sufficient if they are made 

 to grip the bar lightly, so long as they are really firm and 

 are not likely to slip. In some instruments an attach- 

 ment is provided for holding the screw clips exactly ten 

 inches apart, and perfectly rigid until the instrument is 

 fixed upon the bar, when the clamp can be removed. 

 This is the case with Professor Ewing's instrument, where 

 a clamp or distance-piece holds the screw clips at exactly 

 ten inches apart, and also maintains the gripping screws 

 quite parallel. This is kept on until the instrument is in 

 place, when it can be removed, leaving the extensometer 

 in the proper position. By some such arrangement as this 

 the instrument can be very quickly and accurately placed 

 in position, and much time and trouble are saved. 



The extensometer being now on the bar, the beam of 

 the machine must be got into a floating position, with 

 either no load on the specimen or a definite small load, 

 say, a tenth of a ton. The latter arrangement is often 

 convenient, especially on a vertical machine, where the 

 wedges tend to become loose, and drop out of their places 

 if there is no load on the bar. The pointer must now be 

 set to zero, ready for the first reading. 



80. Booking the Results. In all testing operations it is 

 most important that a careful and accurate system of book- 



