12 



H. T. BOVEY ON THE STEENGTH OF 



gradually. As the pressure increases the two ends of the beam are gradually forced down, 

 and the operator is enabled by a very simple device to keep the pressure on each end 

 exactly the same. A thin inextensible thread or wire is attached at each end of the beam, 

 and passing upwards and over frictionless pulleys, is brought down in front of the operator 

 and connected with an indicator. The slightest variation in pressure causes a movement of 

 the needle of the indicator either to the right or to the left, and the operator is thus 

 enabled to diminish or increase the pressure upon either end of the beam so as to bring 

 back the needle to the zero point. The amount of the load is directly read from an indi- 

 cator on the testing machine lever when floating. The principle by which this load is 

 estimated is precisely the same as in the case of an ordinary balance. The lever of the 

 machine is merely a heavy iron girder or beam supported upon a knife-edge at D. The 

 saddle upon which the centre of the beam is supported is suspended directly from the 

 knife-edge C. A jockey weight W runs from A to B. As the pressure upon the two ends 

 of the test-beam increases, it is transmitted to the lever through the rods suspended from 

 C, and the end B rises. In order to ascertain the exact load, the balance is kept perfectly 

 true and the lever floating, or truly horizontal, by running the jockey weight of 200 lbs. 

 forward to the required distance. Thus, if P be the load on the test-beam when the jockey 

 weight is at a distance x from the knife-edge, the value of P in pounds is shown on the 

 indicator in front of the weight as given by 



4 P = M T 2000 X, 

 M being the monient with respect to the knife-edge in pounds, of the beam and of any 

 additional weight at B. 



As the load on the beam increases the beam assumes a curved form, and by means of a 

 cathetometer, or by carefully graduated scales affixed to the test-beam, measurements are 

 taken showing the deviation of the so-called neutral axis of the beam from the original 

 position. 



1 



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WIT^ C KXPt 



In order to do this, a fine wire is fixed to a pin at a point in the neutral surface ver- 

 tically above the centre of the support at one end of the beam, and passing in front of the 

 graduated scales over a pulley fixed in the neutral surface vertically above the centre of 

 the support at the other end, is kept stretched by means of a weight as shown in the sketch. 



These deviations can be easily measured to within l-500th of an inch, and a much finer 

 degree of accuracy is possible if the cathetometer is used. 



The following three tables give the values of the skin-stress (/) at the point of failure, 

 and the coefficient of elasticity (E,) as determined by the progressive loading. 



