274 ON THE RIGIDITY 



the cut ends of these brass tubes and firmly fastened there by- 

 wire twisted around the tubes outside. Cords of considerable 

 length were used, in order that the eHect of the gripping at the 

 ends might be inappreciable. 



The cords were suspended from an iron bracket, moveable 

 on vertical guide-posts which were attached to the wall of the 

 laboratory, and capable of being firmly clamped to these posts 

 at any desired elevation. 



The upper brass tube passed through a wooden socket 

 firmly fixed in the bracket. It was held in this socket by 

 friction, and while it could be rotated by hand, there was no 

 danger of its shifting its position otherwise. The brass tube 

 projected above the socket, and carried a wooden disk, on which 

 was a divided circle. A pointer fixed over the disk indicated 

 the number of degrees through which the cord was twisted. 

 Thus any desired torsion could be given to the cord at the 

 upper end. 



The brass tube at the lower end of the cord carried, in a 

 plane perpendicular to it, a light wooden arm for the application 

 of the twisting force in the static experiments, and which 

 served as a platform for the stretching weights. 



These weights were square leaden plates of about four 

 inches' edge. They had holes of the size of the brass tube cut 

 in the centres, and slits leading to them from the edge, so that 

 they could be easily put on and taken off. 



In applying the kinetic method, the cord was kept fixed at 

 the top, while the lower end, with the plates attached, was 

 twisted through some angle and then let go. The time of 

 oscillation was then determined by means of a stop-watch. 

 This datum, together with other data easily obtained, viz., 

 length and diameter of cord, and moment of inertia of plates,, 

 gave the means of finding the rigidity. In determining the 

 time of oscillation, it was soon noticed that it varied with the 

 angle through which the cord was twisted. Hence the cord 



o * 



was always twisted through known angles. In the static, as in 



