Measurement of Young's Modulus. 



405 



The Suspension. — The parallel wires to be stretched consist 

 of a single length of wire with its ends fixed below to the 

 hooks supporting the needle, and passing above over a pulley, 

 P (fig. b'), of diameter equal to the distance between the 



Fig-. 6. 



parallel knife-edges on the needle. A slight groove is made 

 round the pulley to keep the wire in position. After the 

 adjustments are complete the pulley is clamped between the 

 blocks X, Y, fig. 6, which swing down to each side of it, so 

 that the suspending wires are rigidly fixed at the top. 



Adjustment. — To adjust the apparatus the needle is placed 

 with its knife-edges on the hooks, and while the pulley is 

 free to turn the needle is adjusted until it rests in a horizontal 

 position. The centre of gravity of the needle is then in a 

 vertical line midway between the suspending wires. The pulley 

 is then clamped and the equality of tension of the wires 

 tested by seeing that they give the same note when struck. 

 If now the centre of gravity of the needle is at the same 

 level as the knife-edges the pitching oscillation will be due 

 entirely to the stretching and contraction of the wires. If 



