174 



DEFLKI T>MI-:TERS. 



The supporting and loading of ;i beam under test is not 

 as a rule difficult, but the measuring of the strains requires 

 some special care. In a tension test, measurements can be 

 made along the length of the bar, but in a 1 tending test the 

 strains are in a direction at right angles to this, and the 

 supporting of the measuring apparatus is not so simple as 

 in the former case. 



104. Deflectometers. These are, as their name implies, 

 instruments for measuring the strains on a specimen under 

 a transverse test. As in tension testing, the first and most 

 important condition to obtain is that the instrument used 

 shall be entirely supported by being attached to the speci- 

 men, and no measurements should be taken from parts of 

 the testing machine itself. The reason for this is that the 

 machine parts undergo strain themselves, and there may 

 be small strains of the beam due to the compression of the 



U 



FIG. 83. 



material on the points where it bears on the supports. 

 Strains due to either or both of these causes will be 

 recorded as part of the legitimate bending strain, and are 

 therefore to be avoided. In cases where the span of the 

 beam is great relatively to the depth, and consequently 

 the bending strains are relatively great, and also when the 

 supports are of the rolling or rotating kind, and there is 

 likely to be very little indentation of the specimen, it is some- 

 times permissible to transgress the rule and to measnrr 

 the deflections from a frame attached to the supports near 

 the beam itself, and the likelihood of error will be small. 

 But it is* advisable, when it can possibly be arranged, to have 

 the measuring gear wholly supported by the specimen. 



In the rougher kind of tests, such as those carried out 

 on bars of cast iron, and where it is not required to deter- 

 mine the modulus of elasticity, the measurements of strain 

 may be made with a steel rule divided into hundredths of 

 an inch. The arrangement of this is shown on Fi^. M. 



