174 



It is essential that the deviation from the rectilinear position of the 

 wire should be ascertained with great nicety, and S' Gravesande''s 

 contrivance effects this in a very neat and satisfactory way. A 

 fine steel chain attaches by a hook to the middle point of the 

 extended wire, the other end being secured to the circumference of a 

 nicely-centred wheel. Another chain attached similarly to the wire 

 of the same wheel, has a scale attached to it, a weight placed in 

 which causes the wheel to revolve, and by means of the first chain 

 and hook, pulls, the wire out of the straight line. A long index 

 fixed to the same axis with the wheel, points out the deviation on a 

 much magnified scale, referred to a divided semicircle of brass. 

 Thus a weight being placed in the scale, the corresponding deviation 

 is instantly shewn. 



Let P be the weight in the scale, D the deviation of the wire, 

 s = half the length of the wire between the vices ; it was proved in 

 this communication that, 



P = 2 T 



7 + ^i \l) 



where M is the modulus of elasticity, measured in grains, which is 

 easily reduced to the equivalent length of a similar wire or lamina, 

 according to Dr Young's definition. This is on the supposition that 

 Hooke's Law of Elasticity (the extension is as the extending force) 



(D\ 3 

 — I be found for 



the same wire practically to vary as the cube of the deviation. The 

 value of M, the modulus, is also at once given by a single observed 

 deviation, the tension being known. 



A small correction in the value of P is to be made, for the weight 

 of the wire deflecting it from a straight Une. This small correction 

 had not escaped the notice of S'Gravesande when he verified Hooke's 

 law. 



Example. — A steel pianoforte wire, tension 50,000 grains = T ; 

 *• =25 inches. 



Values of Values of 



D. P. 



^1 ^'(v)' 



•25 inch 1100 grains 1000 100 



•50 ... 2720 ... 2000 720 



•75 ... 5400 ... 3000 2400 



