Vol. X, No. 2.] A Demonstration Apparatus. 53 



[N.S.] 



mirror the shift of the spot of light is nearly ' proportional to 

 the descent of the plates, through a fairly wide range of the 

 scale, say from 10 to 90 cms, as shown in Table I. This proves 

 that it "is legitimate to use the apparatus for quantitative 

 measurements so far as constancy of magnification is con- 

 cerned . 



TABLE I. 



Descent of the plate L Position of the 



as measured by the spot of light Magnification, 



micrometer screw. on the scale. 



79 



0484 15-0 1467 



•0968 22-1 146-7 



•1452 292 1467 



•1936 36-3 146-7 



2420 43-4 1467 



•2904 50-55 146*8 



•3388 57-60 146-7 



•3872 648 1469 



•4356 720 1471 



•4840 791 1471 



•5324 86-4 1474 



•5808 93-8 147-8 



Determination of the Magnification. 



In determining the load-elongation curve for a metal and 

 finding the value of Young's Modulus from the curve, the 

 magnification of the instrument was found by a slightly different 

 process to that described in the preceding paragraph. A 

 length of wire is taken such that for the greatest elongation 

 to be plotted the spot of light remains on the scale. The wire 

 is suspended from a heavy rigid stand. At the other end ot 

 the wire is fixed a scale pan to carry weights, and vanes are 

 attached to a length of thick wire which in turn is fastened 

 underneath the scale pan, and dips into a vessel of water witn 

 the object of damping the oscillations of the system, ine 

 apparatus CD is lightly clamped to the wire, as described 

 above, and the rest of the apparatus adjusted. The position 

 of the spot of light is then noted, the plates made to descend 

 through a known distance by sliding them down the wire ana 

 the position of the spot of light again noted. This gives the 

 magnification which can be determined easily to 1 / - ±* 

 plates are then clamped tightly in their former poM^on, ™ 

 readings of the spot on the scale taken for different loads in the 



pan. 



J 5hii « be ejected either by means of a reading microscope or by 



.x *■ nWfh ft i-»flir nf dividers. 



direct measurement with a pair ot divid 



