304 TIMBERS AND THEIE USES 



dimensions of the beam be accurately measured 

 and that the test piece be precisely rectangular 

 in section. 



The beam is then laid upon the supports of 

 the testing machine, and these supports should 

 move in roller bearings as the wood bends. The 

 distance from centre to centre of the supports 

 should be accurately noted, the distance which 

 the beam projects beyond them, or, in other 

 words, the length of the beam, provided it is 

 longer than the distance between the supports, 

 is unimportant. A load of known weight is 

 then gradually applied to the point on the beam 

 mid-way between the supports ; with each 

 additional increment of load the deflection of 

 the beam is accurately measured. The addition 

 of load is continued till the beam breaks, and 

 from the data obtained it is possible to deduce 

 the elastic limit of the timber and also its 

 ultimate extreme fibre stress or breaking load. 

 Some explanation of the term elastic limit may 

 be necessary. As each additional load is added 

 to the beam it will be found that, in the initial 

 stages, the removal of the load will result in 

 the wood resuming its original shape. Then 

 comes a point, however, beyond which any 

 additional load will cause the wood to be per- 

 manently deflected. The point at which this 

 occurs is known as the elastic limit. It may 



