[BURTON] YOUNG’S MODULI OF METALS 21 
v 
YQ AR 
2d, + 4D 
But the angle ¢ is also given by W Ê 
2q.Ak 8 
where W is the weight added at the centre of the bar, 1 the distance 
between the knife-edges, q Young’s modulus, and Ak? the moment. of 
inertia of the cross section about a diameter. 
(d, + 2D).WF 
This gives finally q = ————-——__-~ 
In the experiment performed the bar was supported on knife- 
edges which were placed on two stone piers in the basement of the 
Physical Laboratory. The values of the quantities involved in the 
above equation were as follows:— 
71-9941 cms. 
45-300 cms. 
69-9941 cms. 
1,000 grams. 
7-211 cms. (mean of 45 readings). 
Kel de tl 
<£-5- 
AN = af) may be determined from the diameter recorded above, 
1-2675 cms. 
Supplying these numbers in the above formula we obtain 
q = 10-667 X 10" dynes per sq. cm. 
3. Ratio of the Adiabatic and Isothermal Moduli. 
The recorded experimental values of the ratio q’: q is 
q’/q = 1-022. 
The value of 1/q—1/q’ — -0200 X 10-”. 
The value for this difference deduced thermodynamically by Lord 
Kelvin is (see above) 
w? .T /J.K.d. 
