DEFLECTOMETERS. 177 



this frame is a plate D, upon which rests the end knife 

 edge of a multiplying lever E ; the second knife edge of this 

 lever is pressed upon by a stirrup F, attached to the centre 

 of the beam, also by a pair of screws. These two sets of 

 attachments must all be made on the neutral plane of the 

 beam, and those which support the frame should be verti- 

 cally above the supports of the beam. The end of the 

 long arm of the lever forms a pointer which moves across 

 the graduated scale G. Readings to one-thousandth of an 

 inch can easily be obtained. A gear of this kind is 

 exceedingly useful for deflection measurements of all small 

 beams, and for the elastic deflections of cast-iron beams. 

 The frame A should be pivoted to the gripping screws, C, 

 at one end, and at the other simply rest upon the screws, 

 so that the shortening of the distance C C during deflec- 

 tion will not distort the frame and so cause errors. 



Each material used in the construction of beams has its 

 own characteristic behaviour under stress. Cast iron 

 invariably breaks, and the strains before failure are com- 

 paratively small. Wrought iron and steel cannot as a rule 

 be broken, but take a large amount of permanent set. 

 Fracture does occur occasionally in steel beams when the 

 metal is too hard. Timber fails by first crushing on the 

 compression side, and then actual rupture takes place by 

 the tearing apart of the fibres on the tension side. For the 

 purpose of measuring the deflections on a large timber 

 beam, a fine wire may be stretched tightly across between 

 the two ends of the neutral axis, and measurements taken 

 from this to the line denoting the neutral axis, which has 

 been previously marked upon the surface of the beam. 



FIG. 87. 



This is shown on Fig. 87. The wire itself may be attached 

 at one end to a nail driven into the beam and kept tight 

 by being made to pass, at its other end, over a second nail 

 or a small pulley, and -carrying a weight. This method 



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