190 WltOUUHT IRON AND STEEL liKAMS. 



I-. -mis. In reckoning the length of span the distance 

 1 ictween the centres of these two plates must be subtracted 

 from the total length. 



Assuming that the specimen has been arranged in such 

 A way that the load will come upon it exactly in its centre, 

 and that there will be no tendency for the beam to be 

 twisted or distorted by the load, the next thing to be pro- 

 vided for is the measurement of the deflections produced 

 by the loads imposed. In the absence of a special instru- 

 ment similar to the one used in the test of a cast-iron beam 

 just described, the best arrangement that can be adopted 

 is that shown on Figs. 84 and 85, page 175. It is 

 easy to construct and not costly, and at the same time may 

 be relied upon to give readings sufficiently accurate. In 

 fixing this apparatus, care should be taken that, in the first 

 place, the end of the short arm of the lever which comes 

 into contact with the under side of the beam is applied 

 beneath the exact centre of the web, and not under one of 

 the flanges, as the curvature of the flanges may not be 

 quite the same as that of the beam as a whole ; and, 

 secondly, the bracket which carries the fulcrum of the 

 multiplying lever should be rigidly and immovably fixed 

 to the plate beneath the middle of the beam, and not 

 allowed to come into contact with any object external to 

 the testing machine. It will be found convenient to have 

 the ratio of the multiplying lever so arranged as to be a 

 whole number, such as 10 or 25. When this is so the 

 scale of instrument can be formed of a steel rule divided 

 into tenths and hundredths of an inch ; or a piece of 

 .squared paper may be used, but care must be taken that 

 the divisions on the paper are accurately what they are 

 represented to be. 



When the specimen has been fixed in the machine and 

 the deflectometer attached, its pointer may be set to zero 

 by means of the adjusting screw, and the test proceeded 

 with, the increment of load employed being selected to 

 suit the case in question. In the present example, read- 

 ings were taken at every ton of load. The elastic readings 

 may be kept well within the limit, or they may be carried 

 just beyond this point, until a definite permanent set has 

 taken place ; or, again, the load may be increased until the 

 maximum load which the beam will carry has been 

 reached. In the present instance the second of these plans 

 was adopted. The following is a record of the complete test. 



