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point may be said to be fully eliaracterised by its distance from the 

 zero line and the values of v and q. 



The distance from the zero line may very easily be determined 

 by the presence of the sqnare millimetre net, while iji a preceding 

 chapter it was pointed out how r is measured. So we have only 

 to describe the best way of ascertaining the value of the radius of 

 curvature q. 



Three ditferent methods were tried for measuring q of which one 

 onlv proved practicable. The other two will only be briefly mentioned. 

 First a reduced diapositive was made photographically of a large 

 drawing on which a number of circles with ditferent, accurately 

 known radii were represented. ( )n the diapositive the radii vary 

 systematically from 0,5 mm. to x. It must be so laid on the curve 

 to be measured that one of the circles coincides with the curve in 

 any point of this latter. By direct comparison the value of q in that 

 point will then be known. 



In the second method three points of the curve are measured, 

 situated at small but mutually equal distances. Calling /• the distance 

 of the two extreme points and p the distance of the middle point 

 from the straight line that joins the two extreme points, the radius 

 ot curvature at the spot where the measurement is made, is 



o 



Sp 



Here k represents the chord and p the height of the circular arc 

 under measurement. 



Fig. 2. 



