312 Profs. W. E. Ayrton and J. Perry. [Feb. 14, 



When the spring is made of round wire, so that a and b are equal 

 to one another, we have from equation (11) 



B -2 



5i + 2tan2 a 



jst 



let — be called cot /3, then it can be shown that 

 d 



E 



— = —2 tan a tan (a+ 

 N 



so that if is 45° 



E tan/3 + 1 



N" tan, 



(22). 



In order to measure tan|S most conveniently, we may employ 

 (fig. 6) a pair of cylindrio scales, the distance apart of which. 



close to the wire of the spring, is a inches. A point, P, on the wire 

 is observed in its position in the upper scale, the reading being b. 

 Now the spring is elongated by an axial force until the point, P, 

 comes opposite a point on the lower scale, and the reading is now c. 

 As the two scales are similar and parallel to one another, and as a 

 spirit-level has been employed to make the scales horizontal, it is 

 obvious that as the axis is vertical 



