268 A. BOYD. 
the expansion of the rod due to the centrifugal force of M, 
is M, @2 R? 
32 AH 
The total extension is the sum of these values. 
The distance rods were made of tool steel whose density 
and modulus of elasticity were very carefully measured. 
Samples of the same steel bar from which the rods were 
cut were tested in tension and their elastic deformations 
determined by means of Martens’ mirror apparatus. The 
following were the average values :— 
Coefficient of elasticity 28,747,800 tbs. per square inch. 
Weight of steel per cubic inch 0°2798 Ibs. 
Applying these values to the distance rod used in these 
measurements we have R = 0°97 feet. 
M = 0°2798 x 12° Ibs. 
M,= 0°0932 tbs. 
R, = 0°94 feet. 
The rod was of almost uniform cross section throughout, 
the average area being 0°180 square inch. 
For a speed of one revolution per second the extension 
was found to be 0°0000007544 inch. The extensions vary 
as the square of the speed. These values reduced to scale 
divisions are plotted in the diagram (fig. 7). 
In the case of the aluminium rod the coefficient of elas- 
ticity was taken to be 10,000,000 tbs. per sq. inch, and the 
weight per cubic foot 162'5 Ibs. 
The general arrangement of one of the flywheels with its 
frame is clearly shown in the accompanying photograph 
(fig. 2). The flywheels tested by the author in these 
experiments were six in number :— 
1. Three armed wheel with straight arms. 
2. Four armed wheel with straight arms. 
3. Four armed wheel with curved arms. 
